Tap water flat plate heat exchanger
Riversong
| Posted in Energy, Heating & Insulation on
Sounds like you’re talking about another form of instantaneous heater.
It depends on the efficiency of the heat exchanger and the flow rate demanded by multiple fixture use. How much temperature rise at desired flows? Can it meet your hot water needs? How many bathrooms? How many occupants competing for hot water at one time?
Today’s indirect tanks are well insulated and the standby losses are less significant, particularly in winter when it becomes space heating. A tank generally offers much better capacity when multiple fixtures are demanding hot water.
Riversong HouseWright
Design * * Build * * Renovate * * Consult
Solar & Super-Insulated Healthy Homes
Solar & Super-Insulated Healthy Homes
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Riversong,
Not quite on topic here but seeing your post and tags, you might have some advice for me.
I was looking into radiantec's solar water heating. I don't know how well it would work here in Portland, OR, but as I measured my incoming tap water from my well the other day at 44.5 degrees F., it occurred to me that even without solar, but just a passive 15 gallon tank upstream of the 80 gal. electric water heater might use the conditioned house heat to save some 20 degrees of heating thereby saving a bunch...No?
Does this make sense to you and do you know of any large surface area type tanks (tall and thin I'm thinking) that would work?
Any ideas, thoughts appreciated.
It's called a tempering tank. Folks sometimes make them by stripping the insulation off an old water heater. Of course, in the winter you still pay for the heat, but they're a win-win in the summer.
If your view never changes you're following the wrong leader
Hmmm... Take such a tank, and a circulating pump, and run a loop of copper through the furnace plenum.
If your view never changes you're following the wrong leader
There's no free lunch.
The 15 gallons gets heated by whatever heats your house = zero gain.
Joe H
Except that he has an electric water heater and likely a gas furnace.
If your view never changes you're following the wrong leader
I understand there is no "free" energy, there is just some you don't have to pay for. I pay for all the electricity to heat the water, where as the basement where the water heater is probably wouldn't get much below 55 degrees in our Oregon climate with no heat added but is generally a constant 65 d. I figure that's 20 degrees of "free" heat. Furthermore I use wood from the property (free with all the usual caveats in "free" firewood) supplemented by an oil furnace. Thus, I'm really interested in cheaper heat.Does anyone have experience with these tempering tanks and how many gallons they can "heat" in say 8 hours.I'm thinking if most of the morning showers useage could be brought up 20 degrees during the night that ought to save a considerable amount of electricity.Does anyone know the approximate formulas/calculations on what such saving would be in watts, $, etc.?Thanks
JoeH said: "There's no free lunch."
While that's true thermodynamically, it's not true financially. For instance, pumped storage reservoirs (on mountaintops) are used here in New England for "extra" generating capacity even though it requires more energy to pump the water up than can be reclaimed when the water comes back down because of friction losses. But utilities pump the water up at night when there is little demand for baseload power and let it come back down during the day when they can charge more per kWH.
As some have pointed out, some purchased energy costs more per BTU than others, though in the Pacific NW you have some of the cheapest electricity in the country.
Does this make sense to you and do you know of any large surface area type tanks (tall and thin I'm thinking) that would work?
You're flailing in the right direction, in that it would require a tank with a high surface area to volume ratio to get good heat transfer. But, if you're looking at an 8 hour pre-heat time...
Since water requires 1 BTU/lb to raise it 1°F, and a 15 g tank would contain 125 lbs of water, it would require 2500 BTUs to raise that volume 20°. For the tank to warm 20° in 8 hours would require a transfer rate of 312.5 BTU/hr. At a 20° deltaT, assuming the tank wall had an R-value of 0.5, you would need 7.8 SF of surface area. A 15 g (2 cf) cylindrical tank that was 16" in diameter and 17" tall would have about 7.24 sf of surface area counting the sides and top. So this would be close.
However, a standard shower head uses at least 5 gal/min, so that 15 g of pre-heated water would last 3 minutes before cold water were being drawn into the water heater. And, even with a low-flow 2.2 gal/min shower head (as required since 1992) it wouldn't give you quite 7 minutes of shower.
Maybe with a much bigger tempering tank... but then you would definitely need a tall narrow tank. And this is why solar pre-heating systems generally require a very large storage tank to have any usefull effect and very thin collectors to have a good collection rate.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 1/30/2008 7:51 pm ET by Riversong
Riversong,Looks like I can get a bigger tank and need one.Thanks for the input. I suppose I should see what kind/size tank I can find for cheap first and then see if the gallons and surface area work out. I can see how you came up with a transfer rate of 312.5 btu/hour which I'm not sure if thats a reasonable rate to expect or not.I'm not sure what the equation you used with delta T to get Sq. Ft.
I'm thinking that the transfer would slow down as delta T gets smaller(that is, as the tank warms up) but maybe you already accounted for this or that it is negligible.I came across the following numbers in a popular mechanics article. It takes about 100 watts to raise 40 gallons 1 degree. Sometimes these references can be pretty old not reflecting current electric water heaters or just missing info. Does that sounds about right to you and /or is there a number to verify this on electric water heaters?If so, and I can use a 40 gal. tempering tank to raise my well water from 45 to 65 before flowing into the water heater, would you figure a savings of .20 cents per fill up at a rate of .10/Kw?Thanks in advance for any more info
I can see how you came up with a transfer rate of 312.5 btu/hour...I'm not sure what the equation you used with delta T to get Sq. Ft.
If R=0.5 (assumed), then U (conductance) = 1/R = 2
U = BTU/(HR*SF*DeltaT )
2 = (312.5 BTU/HR)/(SF*DeltaT)
SF = 312.5/(2*20)
SF = 7.8125
I'm thinking that the transfer would slow down as delta T gets smaller(that is, as the tank warms up) but maybe you already accounted for this...
You're right, and I realized that after I posted but couldn't remember what thread this was in to make a correction. Since the DeltaT would be changing from 20° to 0°, the average DeltaT would be 10° and the surface area would have to be double what I had calculated, or 15.625 SF, or a cylinder 16" in diameter and 41" tall. A tank of that size would hold 36 gallons of water.
I came across the following numbers in a popular mechanics article. It takes about 100 watts to raise 40 gallons 1 degree.
A BTU is defined as the amount of heat required to raise one lb of water 1°F.
40 gallons of water weighs 333.6 lbs (8.34 lbs/gal), so it would require 333.6 BTUs to raise it 1°F.
1 watt = 3.4 BTU, so 100 watts = 340 BTUs.
If so, and I can use a 40 gal. tempering tank to raise my well water from 45 to 65 before flowing into the water heater, would you figure a savings of .20 cents per fill up at a rate of .10/Kw?
Your electric rate is $0.10/kilowatt-hour, or 10 cents for every hour that you're consuming a kilowatt. If installing a 40 gallon tempering tank would save you $0.20 each morning, is it really worth the effort? Wouldn't it be easier and more cost-effective to improve the insulation on your water heater, which is the first thing that all Home Weatherization programs recommends?Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Didn't mean to ignore your comment on the water heater insulation.I was assuming... that since it is a new (3 years) water heater that adding an additional layer of insulation may not help that much...that may not be correct. Any figures on the water heater that can help determine this?I know adding a tempering tank would help heat up the water. Also do you know on the connections for the water heater, if you can just use a long enough flex hose to put a loop in creating a "heat trap"? Sorry if this has all been covered before by others.
Riversong,While I'm asking and saw some of your other posts, I'm looking at "repairing" my barn which saw a few too many years without maintenance before I got there along with way too much snow.The shed addition on the lower side is the only part that is still good. I'm hoping to add new supports and slice it away from the existing structure so that I can take down the firstand preserve the good part.I'm want to put in a slab floor with radiant tubing to be used only with a solar or a wood heat exchanger.I read your comments on insulation underneath on the other topic. For a slab with radiant in our climate, would you suggest 2" blue(DOW) board underneath? Pex tubing?How far above the insulation do you typically suspend the tubing and do you hold it up with the wire mesh?Being on top of the foam, do you need to pour a thicker slab say 6" instead of 4" or what would you recommend.Any help appreciated. I'll attach a few pictures.
I should say that my goal is ultimately a woodshop.
I read your comments on insulation underneath on the other topic. For a slab with radiant in our climate, would you suggest 2" blue(DOW) board underneath? Pex tubing?
What DD climate are you in? In a cold climate, I would use 2" of XPS under a radiant slab, but it's even more important to have good slab edge insulation as that's where most heat is lost. Be sure to place a vapor barrier underneath as well (without any sand above it).
How far above the insulation do you typically suspend the tubing and do you hold it up with the wire mesh?
I lay the 6" mesh on the foam board and tie the PEX to that with zip ties. Whoever places the slab should lift the mesh an inch up into the concrete. If it stays on the bottom, you don't get much tensile advantage from the mesh but raising it up too much and you risk nailing into the PEX if partitions are installed.
Being on top of the foam, do you need to pour a thicker slab say 6" instead of 4" or what would you recommend.
Anything thicker than 4" and the heat response time will be too slow. Ideally, the PEX is in the center of the slab, but that places it at risk for puncture, so I keep it in the bottom 1".
I assume the barn remodeling has been finished about a year ago. Can you post new pictures?
How did the project go?
Flat plate heat exchangers
Pex Tubing
"The boiler is always hot so why store heated water?"
Is IS??????
What about in July?
.
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 indicated, I have a Bock indirect water heater so the boiler runs all year round.
My point is why heat the water in a 35 gallon tank when a flat plate heat exchanger would heat it on demand?