another ? regarding instant water heater
cliffy
| Posted in Energy, Heating & Insulation on
I am planning on installing a instant water heater sometime in the next little while. Leaning toward the Bosch Aquastar.
I live in a 4 season climate with snow from December to April. Two adults two kids in the house. Heating 2600 square feet, gas stove and drier.
Anybody have good info on what percentage of the gas bill is being burned by my 40 gallon high efficiency hot water tank. I’m thinking 20 percent but it is just a guess. I was thinking I need to analyse how much I’m spending on the present heater to get a handle on how long the new heater needs to be in service to pay for itself. I think 4o gallons heated all day is a big waste.
Have a good day
Cliffy
Replies
Hi,
If you use the typical 20 gallons per person per day, then your water heating part of the gas bill should be about:
(80 gal/day)(8.3 lb/gal)(120F -60F)(1 BTU/lb-F)/(0.62 efic) = 64000 BTU/day, or 0.64 therms per day.
This would be about 235 therms a year. Your gas bill should tell you how many therms a year you use and what you pay per therm.
A therm is 100K BTU, some utilities call a therm a ccf (100 cubic ft).
A tankles heater would increase the efficiency from about 62% for your current heater (maybe worse if its an old one) up to about 85%.
I just put in a Takagi tankless last month -- we like it a lot.
If you look around the internet, you can find some good deals on tankless heaters. Your state may also offer an incentive:
http://www.builditsolar.com/References/energyincentivesrs.htm
Gary
Do those efficiency ratings mean while the units are operating or do they also account for the fact the instantaneuos heater will only run while I'm using hot water and the storage tank runs all day and night all year?
How much did your energy consumption drop after the switch to the Takagi compared to last month?(or do you know yet?)
Have a good day
CLifffy
Hi,
The efficiencies I used are the published "Energy Factors" -- as I understand it, the energy factor test measures efficiency in a way that represents typical use including standby losses.I don't have any way to measure the energy use for the new tankless. Because the burner modulates its output depending on water temp and flow, I can't just time how long the burner is on as I did with my old one. In my case, I'm adding this tankless to the far bathroom, and getting rid of the very wasteful recirculation loop that used to go from the old tank to the far bathroom. By actual measurement this recirc loop was costing me 200 gallons of propane a year!Gary
there is another thread running right now.
It appears that the efficienies are dependent on many factors
It also appears that claims of significant savings may be wrong
check the other thread
Cliffy,
If you are buying a tankless water heater because you think it is instantaneous, you will be sadly surprised. In fact, the delay between when you open a hot water fixture and when you actually get hot water will be increased. It will be increased because when you open the valve, cold water comes out of the tankless water heater until flow is sensed, the fan comes one to purge the combustion chamber, the burner ignites and the heat exchanger heats up. This sequence typically takes 10 to 15 seconds while with a storage water heater, hot water leaves the water heater as soon as you open your hot water fixture. Hence, your delay time is increased and your water useage is increased.
Tankless water heaters are considered by the Department of Energy to be somewhat more efficient than storage water heaters. If you compare a typical new storage water heater to a typical tankless water heater, you will see that the storage water heater will have an Energy Factor of 0.58 while a typical tankless will have an Energy Factor of 0.82. According to the DOE, for an average family, that would represent an annual savings of 76 Therms per year ( a 0.58 storage unit uses 258 T/yr and a 0.82 tankless unit uses 183 T/yr.) If your gas price is $1.22/Therm, then the expected annual savings is $92/year. It is true, that is a 29% savings but it is only $92/yr which will have trouble paying for the annual servicing that most tankless recommend.
I leave it to you to decide if a savings of $92/yr is worth the extra costs of the unit and it's installation.
Bill
I'm pretttty familiar with how the heater works, we had a couple of Bosch's where I used to work. I'm also familiar with waiting for hot water as my present tank is quite a distance from my shower!
Thanks for coming up with a tangible number. I just think I'm spending a fair amount of money heating 42 litres of water 24 hours a day all year.
Have a good day
CLiffy
"I just think I'm spending a fair amount of money heating 42 litres of water 24 hours a day all year."
I'm not sure I buy that arguement. (Although I'm certainly not an expert in this field)
I figure I'm heating my house roughly about 6 months out of the year. any heat that's "lost" from the water heater is inside the house, so it isn't wasted at all. So if I had a tankless water heater, I figure the savings would be zero for half the year.
I don't know if they've figured that into their equations or not when they figure how much tankless heaters should save.
I'm on the road constantly, where the hell is Easy Street?
I guess that is why the utility room is always so cosy!
Good way to look at it. Reminds me of my mother in law. Whenever she is done with the oven she opens the door to let the heat out into the kitchen.
Have a good day
CLiffy
Cliffy,
our best estimates are that a modern water heater looses about 45 to 50 Therms a year in the standby mode. Hence, another way to look at it, is the cost of purchasing and installing a tankless water heater worth about $60/yr?
Your investment decision...does not seem wise to me!
Bill
Hey Bill - I didn't know you were still hanging around here. Glad to see ya posting again.I still appreciate the help with my water heater a couple of years ago.
In the fertile land of promises many have died of hunger. [The Mafia Manager]
BossHog,
I am delighted to be able to help.
Bill
A while back you posted a link to a study that compared real operating cost for a tank to tanksless. IIRC is was a family of 4 in MI.I had just tried searching for it for this thread.http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=101974.1I seems that you have used both Whoover and BillHoover in the past and I searched on Whoover.Do you know the study that I am refering to..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I am not in the office so my resources are limited. I believe the study you want was done by the Davis Energy Group for the California Energy Commission. In that case, they monitored a family of 4 for a month with a storage water heater and then replaced it with a tankless water heater and continued to monitor it for a while. There were some interesting conclusions:
1. In real life, neither the storage or the tankless water heaters were as efficient as when tested in the DOE test.
2. The tankless was about 33% more efficient...consistent with the numbers quoted by the DOE
3. The water use increased singifcantly (almost doubled) when the tankless was installed. Part of that is the ignition delay process which is inherant in all tankless water heaters but much of it was behavioral...look at my new toy, I can take endless showers. How much behavioral was will be associated with tankless water heaters over the long run is highly debated without any good data to support either position.
4. The efficiency of the tankless water heater is very low when the draws are of short duration...logical since the heat exchanger heats up and then cools down. Clearly tankless are more efficient durng long draws (like the 10 gallon draws used in the DOE test).
Sorry I can not provide an exact reference for this work.
Bill
>>The efficiency of the tankless water heater is very low when the draws are of short duration...logical since the heat exchanger heats up and then cools down. Clearly tankless are more efficient durng long draws (like the 10 gallon draws used in the DOE test).<<I can see where the initial heating and subsequent rapid cool-down would work against efficiency when using it in short bursts, compared to tank heaters.How much might that be offset by the standby losses with tank heaters?>>The tankless was about 33% more efficient...consistent with the numbers quoted by the DOE<<Does that mean their actual energy usage dropped by 33 percent?It sure would be good to know of some real-use studies under various usage conditions. I have a job coming up where the client is expressing interest in tankless.Steve
Look at the charts in this test.http://www.energy.ca.gov/title24/2008standards/prerulemaking/documents/2006-05-18_workshop/2006-05-11_GAS_WATER.PDF.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
As I said in a previous post, the DOE would say that a tankless is about 30% more efficient than a low efficiency, new storage water heater...It would save about $95 out of an annual water heating cost of about $300. (The DOE test includes a measure of the standby losses so the tankless advantage in that area is accounted for.) Hence, the question to the purchaser is, Is the additional installed (hardware plus installation) worth it for a savings of less than $100/yr.
I know some more real life test data would be useful, unfortunately, everyone uses their water heaters differently so generalizations are impossible to make. That is why the DOE requires all water heater manufactures to conduct the DOE Energy Factor test for each product. This test is a simulation of actual use with 6 10 gallon draws for each hour and then an 18 hour standby period. The Energy Factor is essentially an energy balance for the entire 24 hour period.
Pacific Gas and Electric is developing a laboratory in San Ramon, CA which will do side-by-side testing of various water heaters with more realistic use patterns than are used in the DOE test. No test results are yet available and I am anxious to hear about them. My experience would tell me that if the usage is lower than the DOE test, the tankless will be favored but if the hot water usage is higher than the DOE test, then the efficiency advantage of the tankless will be reduced.
Since the EF test is a simulation, the DOE then uses the EF to calculate an Estimated Annual Cost of Operation. Those calculations are the ones used to estimate that a typical tankless would save 75 Therms per year over a run-of-the-mill storage unit. If a high performance storage water heater is used such as a power vent, the estimated savings are smaller.
Bill
Bill,Thanks so much for that concise explanation. I've saved it to my records for reference.Steve
My pleasure to try to assist in logical, informed decisions. If you have further questions, you may contact me a [email protected].
Bill
Thanks Bill -- good information."... If a high performance storage water heater is used such as a power vent, the estimated savings are smaller."I've not seen a power vent water heaters with EFs much better than regular gas heaters -- am I missing some?Gary
Another point that hasn't been brought out is that side vent or power vent tank hwhs are in the same price range as tankless hwhs.
That is ultimately why I decided on a tankless - I wanted a through-the-wall vent, due to issues with the chimney, and the tank type heaters that could vent through the wall were in the $600 to $900 range. At those prices, the $500 for the tankless and $150 for a vent kit made sense - especially with the government covering $300 of that.
So for the cost of a conventional vent hwh, I got a tankless - with the benefits and energy savings for free.
Others have mentioned in other threads that if you use a lot of hot water on an ongoing basis - so that a tank type hwh won't be turning on to reheat water that is just being stored - you likely would not see much difference in energy usage between tank and tankless.
If you use hot water infrequently (e.g. two adults, two showers a day, maybe a couple loads of laundry), the energy savings with a tankless can be huge.
I have an older, poorly insulated home. My gas consumption dropped by about a third when I installed the tankless. Now, the old hwh was an older model, not as energy efficient as the latest tank hwh, but still, that was a lot more than I expected. I'll recover my cost for heater in about 14 months.
You claim a huge drop in your energy consumption when you put in a tankless water heater. Just how much to you consider huge? The average gas customer in the US uses about $300 worth of natural gas to hear their hot water per year. Even if the savings is 50% which far exceeds what the DOE predicts, the savings is still only about $13/month.
You quoted the cost of your tankless water heater, I did not see that you gave us the cost of installation...electric power, venting, plumbing, increasing gas line size and increased make-up air. Are you willing to share those numbers with us??
You claim a huge drop in your energy consumption when you put in a tankless water heater. Just how much to you consider huge?
In my post, I said 30% - that's huge, IMHO, for hot water heating, and certainly more than I expected.
The average gas customer in the US uses about $300 worth of natural gas to hear their hot water per year.
Guess that's an average - which means there could be a LOT of areas where the cost is substantially higher. My best estimate, based on my experience, was that I was spending around $650 a year for hot water heating, which has been reduced to $400 - I'm saving around $20 a month. Now, that's HIGHLY DEPENDENT on usage patterns - but that's what I am seeing.
You quoted the cost of your tankless water heater, I did not see that you gave us the cost of installation...electric power, venting, plumbing, increasing gas line size and increased make-up air.
Sure. I did a lot of the work myself, though, so my costs would be lower than someone who had to hire a contractor. Electric was $20 in parts, venting was $150 in parts, plumbing was $30 in parts, gas line size did not have to be increased, and there was an existing tap in the area where I installed the tankless hwh. Probably another $50 for flexible couplings, unions, etc.
Now, keep in mind that most of those costs would have applied to a direct vent tank hwh, too.
One other point to mention - finding a code compliant place to vent a direct vent appliance can be tough. The requirement to be 8' from an opening (including doors and windows) is tough in basements. I could not (legally) install the tankless in the location of the old tank hwh.
That's the highest annaul hot water heating cost I have come across in a long time.
That does not sound like the one that I was thinking of, but it lead to someothers.I found this.Comments on the Energy Star Residential Water Heaters Draft ...
http://www.energystar.gov/ia/partners/prod_development/new_specs/downloads/water_heaters/LBNL_052907.pdfWhich I had found ealier. It was the only place that showed found the basics of the test proceedure.And it referenced this article."Davis Energy Group, Inc., ôField and Laboratory Testing of Tankless Gas Water Heater Performanceö,
sponsored by the California Energy Commission, April 14, 2006."Which is probably the one that you are thinking about.But I could not find it, but I did find this one.http://www.energy.ca.gov/title24/2008standards/prerulemaking/documents/2006-05-18_workshop/2006-05-11_GAS_WATER.PDF"Tanklless Gas Watter Heatters"It appears to be a following to see why the large difference
in EF."Figure 2 compares monitoring data for both the storage gas water heater (pre-retrofit) and
tankless unit (post-retrofit) at the field site. The storage water heater data shows
efficiency approaching zero at very low daily hot water usage rates. As loads increase
(the Qout term in Equation 1), standby loss term becomes a smaller fraction of the overall
energy consumption. The trend of the storage water heater efficiency curve fit suggests
that as water loads approach the 64.3 gallons used in the Energy Factor test, the
monitored efficiency converges to a value close to the rated Energy Factor. The
instantaneous water heater demonstrates a very different relationship. As hot water
consumption decreases, the daily efficiency falls slightly. What is more significant is the
large variation that exists for days with similar hot water usage. For example, the three
data points around 25 gallons/day demonstrate efficiencies ranging from about 55% to
82%. This large variation suggests that differences in hot water use characteristics affects
the calculated daily efficiency.Testing was performed at the Davis Energy Group shop to better understand the impacts
of draw volume size, flow rate, and time intervals between draws on overall efficiency.
Figure 3 plots data from a series of tests with varying flow rates (1.2 to 2.3 gpm) and
varying time intervals between hot water draws (5 and 45 minutes2). The data clearly
depicts the impact of ôcool downö time on system efficiency. The ô5 minutes between
drawö tests show a 10-15 percentage point drop in efficiency at draw volumes of 1 gallon
(relative to larger 10 or 15 gallon draws), while the ô45 minutes between drawsö show a
much more significant drop. This efficiency disparity is largest at small volumes and
approaches zero at about 4 gallon draw volumes. The impact of flow rate appears to be
negligible for the ô5 minuteö data, although the ô45 minuteö interval data does
demonstrate some variation due to flow rate. This may be due to the effect of the lower
flow rate allowing the heat exchanger more time to fully come to temperature than at a
higher flow rate."Looking at the curves you can see where some of the wild differences
in reported savings come from. Large variations depending on the
amount of water used and the water usage paterns.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
BTW do you have any suggestion on the type of WH for my application.I currrently have 2 furnaces and WH in a closet with external air vents.I want to replce them with a HE furnace and WH with external combustion air and seal off the external vents.I think that a basic direct vent WH would work. But the last time that I looked all of those had relatively low EF's.It looks like my only other options are the tankless or the Vertex/Polaris styles which are even more expensive than the tankless..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
when it breaks the average plumber may not be able to fix it, plus Ace hardware will not have the parts- they will be from the manufacturer only.
What does a flow sensor cost.
what does a fan cost, plus labor- 2 trips since they will not stock your parts I bet.
Bill,
I completely agree that if you are moving to a HE condensing furnace, a direct vented water heater is completely logical.
A comment about the price of the Vertex. You need to consider the total installed cost of your options. We believe that in most retrofit applications, the installed cost of the Vertex will be lower than the installed cost of a whole house tankless...no increase in gas line size, no stainless steel venting (just pvc), no increased makeup air.
Have you considered a high efficiency power vent model. I know ones with an EF as high as 0.67 are available at a cost which should be a little less than a tankless. These could be a better alternative for you.
The real question is cost effectiveness of all your options. I have a spreadsheet on my work computer (I am at home), which calculates estimated cost of operation and paybacks based on the EF numbers and the installed costs. If you send me what you think your options are, I will be glad to run the numbers for you.
Bill
I am sinlge and HW usuage is low.Gas usage charges during the summer months is only about $15 (plus a $25 fixed fee). That that is for 2 furnace pilots and the gas grill besides the WH.So I know that I can't expect to save enough in gas to pay for a change.Space and/or venting location might be the determination.I really won't know the limits until the furnace is selected and what is needed in the way of ductwork changes. I am want to go from 2 furnaces to one with zoning.An electric tank might be the way for me. We don't have the cheapest rates, but it should be reasonable. I believe that about 50% would be at 5 cents, 25% at 9 and 25% at 10..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
You can save a bit on the electric by having a "Saver Switch" installed. It requires wiring changes, but you'd need new wiring anyway.
If your view never changes you're following the wrong leader
Depending on your usage, you might consider an electric tank. The Marathon unit has essentially zero standby loss and should last 20 years in most applications.
If your view never changes you're following the wrong leader
Tankless water heaters are considered by the Department of Energy to be somewhat more efficient than storage water heaters. If you compare a typical new storage water heater to a typical tankless water heater, you will see that the storage water heater will have an Energy Factor of 0.58 while a typical tankless will have an Energy Factor of 0.82.
In my experience, storage water heaters are more in th 40% range while tankless are more in the 90% range.
FWIW, I installed a tankless in my own home just before the energy credit expired (:-)). It does not have the delay you speak of - it's nearly instant.
Woodturner9,
I don't know where you get your experience, I have only been quoting data from the DOE. If you have not conducted laboratory testing under controlled conditions, your observations are not necessarily accurate or broadly applicable.
As far as the delay goes, it is interesting that your tankless defies the laws of physics and the ANSI code requirements. Again, unless you have data, I guess all I can say is that I am happy you like your tankless. Laboratory data and logic says there is the delay I refered to and it is unavoidable.
Bill
I don't know where you get your experience
As I said in my post, it's personal experience with a unit I installed in my own home.
I have only been quoting data from the DOE.
Please post a link to that data. Perhaps you are misinterpreting the data?
If you have not conducted laboratory testing under controlled conditions, your observations are not necessarily accurate or broadly applicable.
They are accurate, for my situation, but of course one example does not mean they would necessarily be broadly applicable.
As far as the delay goes, it is interesting that your tankless defies the laws of physics and the ANSI code requirements. Again, unless you have data, I guess all I can say is that I am happy you like your tankless. Laboratory data and logic says there is the delay I refered to and it is unavoidable.
Not sure where you are getting that information, but there is no "defiance of the laws of physics" or any ANSI or other code violations required. I wonder if you may be misunderstanding the code requirements, or "reading into" them unintended restrictions.
Any product is designed with tradeoffs, which result in different operation of the product. Some manufacturer's emphasize fast hot water delivery, others emphasize efficiency more, etc. There are many different ways to achieve the efficiency goals - using a fan to blow out the exhaust is not the only way (nor the most effcient) way to vent a tankless hwh.
Woodturner9,
With all due respect, I am glad you have had a positive experience but to state your observations about efficiency based on what happend in your house is stretching it a bit. In order to compare efficiencies between two water heaters you need to have exactly the same use pattern, the same incoming water temperature, the same outgoing water temperature and the same ambient temperature. Unless you have controlled these variables, it is not reasonable to quote efficiency numbers.
The data I have quoted comes from the GAMA (Gas Appliance Manufacturer's Assoc.) which is the offical keeper of gas appliance certification data. This guide also provides the equations for calculating Estimated annual cost of operation numbers. All gas water heaters sold in the US are supposed to be listed in the GAMA guide.
As far as the ignition delay, upon opening a hot water fixture, all tankless water heaters must sense flow, start their fan, purge two air exchanges out of the combustion chamber (ANSI requirement), ignite the burner and get the heat exchanger hot. I believe physics says that all of these steps take a finite time and during that time delay, additional cold water is flowing into your hot water line adding to the wait time and the water waste. Our lab tests indicate that many of the tankless heaters have delay times in the 10-15 second range (one manufacturer is currently claiming only 3 sec...no data yet).
In any case, there is no delay time for hot water to leave a storage water heater and there is a very real and necessary delay for hot water to leave a tankless water heater. While manufacturers have design options, tankless manufacturers can not avoid a measurable hot water delivery delay to allow time for the necessary ignition sequence.
In any case, there is no delay time for hot water to leave a storage water heater and there is a very real and necessary delay for hot water to leave a tankless water heater.
The efficiency numbers I referenced are the efficiencies the manufacturer claims for their product.
When I said that the tankless was "nearly instant", what I meant was that I see no difference between the tankless and tank water heater in this regard. In other words, it takes 3 to 5 seconds (rough estimate) for my tank hwh to produce full temperature hot water in my shower, and the tankless produces hot water in my shower in about the same time. So there is a delay, it's just not significantly different than a tank hot water heater, in my application.
We replaced our old cast iron boiler and 50-gal WH (both gas fired) with a tankless combination unit. Not exactly your situation, but we found our gas bills for baseboard heating and hot water decreased 15%. That really isn't enough difference to justify the extra cost of the tankless heater, but we needed a direct vent appliance to meet the new building code and the higher efficiency was a bonus.I don't know about the delay from the Bosh, but with our unit the hot water is available within a few seconds after the flow switch is tripped (1/2 GPM). Faucets far from the heater have a slug of cool water in the pipes and that has to run out just like a tank unit before the water gets hot.In our set up the DHW goes through a regular, 30-gal electric water heater used as a buffer and storage tank. Normally we leave the electric elements off. If we have guests and need to run two showers at once, or if the combi unit is ever down for repairs, I can flip a circuit breaker and use it as an electric water heater.
A 15% decrease in your heating bill sounds pretty reasonable considering you were replacing a old boiler (assumed to be inefficient) and a gas water heater.
The delay I spoke of is the delay from the beginning of flow until hot water exits the tankless heater. With a storage water heater, hot water immediately leaves the water heater upon the beginning of flow. It is true there is still a wait until the hot water clears out the slug of cold water in your hot water pipe but with a tankless, that wait is always longer wheither you notice it or not.
You are very wise to leave the electric buffer tank unplugged, otherwise your electric bill will be enhanced. The purpose of the buffer tank was probably to eliminate the "cold water sandwich" but if you don't have that problem with your set up, that is great.
One other benefit of tankless hw heaters is that they don't ever run out of hot water.
One of the complaints I had with the tank hwh is that I often got lukewarm showers. I would be out all day, and to conserve energy, set the temperature on the hwh so that on a cold winter day the temperature with just hot water was comfortable. However, the water would not be hot after sitting all day, just warm, depending on when the heater had last cycled. So I had to run enough hot water to make the heater turn on, then wait 15 minutes, or I got a warm but not hot shower.
Your warm showers were a problem with your water heater thermostat and not the water heater itself. A few dollar part would have fixed the problem.
Tell me, if you never run out of hot water with your tankless, do you use more hot water and hence more energy?
Your warm showers were a problem with your water heater thermostat and not the water heater itself. A few dollar part would have fixed the problem.
Maybe, but all of the 12 or so hot water heaters I have owned in my life exhibited this characteristic, even brand new ones. Thermostats have a certian amount of hysteresis - they have to or they will not work properly. Many people set the thermostat to a higher temperature to overcome this issue, but that increases energy usage.
Tell me, if you never run out of hot water with your tankless, do you use more hot water and hence more energy?
It's tempting at times, definitely some temptation to take longer showers, but I haven't really noticed that.
FWIW, it was fairly unusual to run out of hot water with the tank heater, too, but from time to time we did, usually from doing laundry and then taking showers without waiting long enough for the heater to recover.
Edited 3/20/2008 12:01 pm ET by woodturner9
we know what tank parts like elements cost. get em at hardware store.I still want to know what the fans, controllers, ignitors, flow sensors and logic board costs in a tankless. why cant someone post that?
"I did my monthly test and it kept dribbling..."Thats why it leaked I bet- grit in the seat caused by the test
Thats why it leaked I bet- grit in the seat caused by the test
We have hard water, and the calcium buildup on the valve shaft appears to have been the cause of the leak. I did actuate it several times to clean off the shaft, and it did help, but I didn't trust it.
What kind of engeneer are you?
we know what tank parts like elements cost. get em at hardware store.
I still want to know what the fans, controllers, ignitors, flow sensors and logic board costs in a tankless. why cant someone post that?
I haven't actually priced them, but I would assume they would be more expensive.
Perhaps the greater question would be whether or not they are likely to fail in the usable lifetime of the heater and outside the warranty period.
The information I found suggests that they are no more likely to fail than similar parts on gas furnaces. Like gas furnaces, therefore, it is likely that 5% or less of the tankless hwh's will need repaired in their lifetime.
And lets not forget that parts for newer gas tank type hw heaters are not so readily available - you are unlikely to find many of them at the local hardware, box, or plumbing supply store.
Wouldn't running your HW through a storage tank mean you have a 30 gallon slug of cold water in your pipes if you had no HW usage for several hours?
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Yep.
If your view never changes you're following the wrong leader
"Wouldn't running your HW through a storage tank mean you have a 30 gallon slug of cold water in your pipes if you had no HW usage for several hours?"That's a good question and it might apply to older heaters with inefficient insulation. Modern heaters are well insulated, and since we used an electric tank, there is even less heat loss because it has no flue pipe through the middle. I'd guess that water in the tank loses 1/2 to 1 degree (F) per hour at most. In our daily routine we haven't noticed any problem with lack of hot water.
Edited 3/21/2008 1:41 pm ET by TJK
So basically you have the standby losses associated with a tank.
If your view never changes you're following the wrong leader
"So basically you have the standby losses associated with a tank."Yeah, about 250 BTU/hr, or 0.2% of the capacity of the tankless heater. IOW the loss in efficiency due to the buffer tank is not meaningful.
Which is to say that standby losses from an electric tank are in general not meaningful.
If your view never changes you're following the wrong leader
Which is to say that standby losses from an electric tank are in general not meaningful.
Unfortunately, that is not true in general - that assumes a perfectly insulated tank, an impossiblity.
Whether a tank is gas or electric heated is essentially immaterial - it's the insulation on the tank and the usage of hot water that determines the standby loss. If you use hot water infrequently, more (wasted) energy will be consumed to maintain the temperature of the water. If you use hot water constantly, there will essentially be no standby time and thus no standby loss.
"Which is to say that standby losses from an electric tank are in general not meaningful."Not exactly. I said the loss of _efficiency_ due to the buffer tank is not meaningful. Compared to 120K BTU, 250 BTU is small by comparison. Now if we only pulled water out of the storage tank once a week then yes, the 250 BTU/hr lost through the tank's insulation would be a serious problem and boy would I hear about it. I know when we're gone for a day or more, we need to run a hot water tap to clear the cool water out of the storage tank - no big deal.In our household we run hot water at least once every eight hours, so the 4F to 8F temperature drop of water in the tank is insignificant. Initially I designed-in valves and a pump to recirculate the water in the storage tank through the heater to maintain a set temperature, and I even installed the $$ parts for it. After using the system for a month we found the recirculation wasn't needed in our daily routine - in fact the pump was quite noisy when it kicked in. So I stripped out the extra plumbing and stored it away. Someday I might use the 3-way valves and pump for a rooftop solar water heater that ties into the existing system.
But if you went ahead and turned the tank heaters on you'd pay very little extra to run them, especially if you set them 5 degrees lower than the gas heater. They'd only kick on when that "not meaningful" standby loss added up to enough to drop the temperature 5 degrees. No electricity would be spent heating the water, just keeping it hot.
If your view never changes you're following the wrong leader
"No electricity would be spent heating the water, just keeping it hot."???Electric heat is electric heat. In our market it costs 3X more to heat a gallon of water with electricity versus natural gas. You are right though, I could flip the breaker on and use the built-in elements to keep the storage tank contents hot-er. But why spend the extra money?
Just making the point that the additional expense would be minimal. And eliminating any misconception on the part of other readers that somehow by turning the tank heaters off you're significantly reducing standby losses.
If your view never changes you're following the wrong leader
Just making the point that the additional expense would be minimal. And eliminating any misconception on the part of other readers that somehow by turning the tank heaters off you're significantly reducing standby losses.
But that IS the point - having the tank heaters off DOES significantly reduce the standby losses, in many cases.
The usage patterns determine the actual results, however. If you use a lot of hotwater, so that the tank is constantly running to heat up "fresh" water, there will be little standby loss because there is no standby time.
If you use hot water rarely, however, the standby losses will be large.
Like DanH said, why not just get a 60 gallon electric and wrap the tank with an insulating blanket. You would save the expense of the tankless with seemingly little extra heat loss.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
"Like DanH said, why not just get a 60 gallon electric and wrap the tank with an insulating blanket. You would save the expense of the tankless with seemingly little extra heat loss."1) We didn't install it to save money, but it turns out our gas bills are 15% lower. 2) A 60 gallon WH is only 60 gallons, the tankless can run all day at 240 GPH if we want.3) Where I live the cost for electric heating is 3X the cost of NG.
Wouldn't running your HW through a storage tank mean you have a 30 gallon slug of cold water in your pipes if you had no HW usage for several hours?
Depends on whether the tank is before or after the hwh.
Charles Wing, in his books on saving energy, talks about using a hwh as a storage tank on the INPUT side of the active hot water heater. This would work well with a tankless as well. The idea is that the water is in the tank, inside the envelope of the house, and uses the "waste heat" from the room to warm the water in the tank.
The net result is that the demand hwh has higher temperature input water, so it consumes less energy to heat the water to the setpoint. Of course, if you run all the water out of the storage tank, the benefit is lost.
Aren't you essentially heating the water with your HVAC system rather than the WH? Especially so in a climate that is primarily heating.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Aren't you essentially heating the water with your HVAC system rather than the WH? Especially so in a climate that is primarily heating.
Yes, that's correct. But the argument is that it is essentially waste heat that would be used whether or not the water heater were there. That's a good assumptiong, if the hwh is in an "unheated" basement. If it's in the living space though, it may not be as good as assumption.
I had a tankless water heater installed in my shop and home office because it took up considerably less space and because I don't have to worry about keeping water hot for long stretches when I'm not there.
When the water heater has to be replaced for the second floor of my house, I will change that to tankless as well. With the kids now gone, the second floor is mainly used for guests.
But I'll probably always keep the standard water heater for our first floor, with the master bedroom, master bathroom, and kitchen.
Any tangible idea of savings?
Have a good day
Cliffy
No, it was more peace of mind type of thing. Not having to have the gas running all the time to keep the water hot when no one was there.