el water heater for heat/ figure BTUs
Fonzie
| Posted in Energy, Heating & Insulation on
We have just installed pex in 560 sq ft concrete on 2 inch starfoam. The walls/ceiling are going to be super insulated (R 13 walls + 1 inch R Board) (R 38 ceiling + 1 inch R board). We have the control setup ordered. The heat figured out to be 11000 BTU/hour. I have tried to use the conversion tables and it looks like about 3500 watts approx.
Well, if this is correct I would like to get this in a 20 gal rather than a 30 gal. I haven’t seen it – though I haven’t looked at the cashy ones.
BTW the guy said the pump didn’t move enough to turn on a “demand” el water heater.
Any counsel would be appreciated. This is our first in floor heat.
Replies
With an electric water heater you needn't worry about having a unit that is a bit oveersized -- there will be no significant loss of efficiency so long as you buy a quality unit.
A larger water heater (gallons) will be OK. Sometimes it may circulate w/out actually kicking on the water heater ... so it will cycle somewhat less. If this is new construction, you could combine the doemestic hot water and floor heat into the same water heater ... it is a 'special' water heater ... readily available. Lots of people use them. A great way to combine the two systems (I did it in my new house).
IMO, R-13 w/ foam is NOT 'superinsulated nor is R-38 w/ foam. I put R-21 in my new house and I don't consider it super insulated. R-38 walls is probably considered 'superinsulated' and a minimum R-49 ceiling is probably superinsulated. Your ceiling is about average ... maybe slightly above. Your walls IMO are average maybe below. Especially w/ your cold Illinois winters. Not trying to badmouth, just in case someone told you that you have the baddest insulation that side of the Mississippi.
I made a mistake on that. We're building 6 inch walls and putting R 19 in them with 1 inch R Board on top. The ceiling is R 38 with the R board too. We have had great luck with the R board. In my non engineer trained experience it disperses where the heat and cold meet - unlike visqueen which will give you instant water droplets. We don't use visqueen and have seen problems remodeling where they have. Yeah, Illinois weather - we know a good day when we see one. Actually I like Illinois weather and have become fairly oblivious to it. We are using a closed system with antifreeze. I see your point though - if the customer was using it for both it wouldn't make any difference. This figuring the btu / hour must not be a real straighforward thing. I met with an electrical engineer today and she said she would have to look it up. I have used the conversion tables but I want to get some counsel from experience.
Ok. Yeah, if you can take care of details with respect to the rigid board added to the wall, it's definately a great way to go. For some it's second nature. Just a process of getting used to a different construction detail like anything else.
My radiant floor is closed system w/out antifreeze. I use the water heater that has the HX built into it. Works fine.
Your heat load ... do it piece by piece (walls, windows, ceiling/roof) and you'll be fine. Don't forget about the air leakage ... it is a component just like walls.
Your total floor area is 560 sqft?? that ain't much, but if it's just two people, it can be plenty. If it's right 11,000 btuh sounds +/- reasonable w/ insulation levels you have ... and not knowing whether you have designed a glass box or not (assume it isn't as you do imply at least some wall area :)
Ha, I looked back and I did forget to mention that this is a sharpening shop being built in an existing 2 car garage. Yeah, 560 sq feet, I know you were feeling sorry there. I didn't aim to do that. Actually this is a very specialized type of sharpening - elaborate carbide cutters and draw dies, etc. Not much room is needed. Plus, there will only be one small window. It will be easy to heat. I didn't understand what you meant about having the HX built into the water heater?
a domestic hot water heater that has a heat exchanger built into it for hydronic heating applications. This keeps the potable water separate from the floor heating system, yet they are one in the same heater.
Your 11,000 btuh load may be a bit big for your application. I calc a CONDUCTIVE load at 80 deg delta T of only 3,800 btuh ... plus air leakage, but that should be small, I'm guessing. Even if it was double, we are only talking 7,400 Btuh. That is a ROUGH calc, not knowing any specifics of your application. You keeping the garage door? That alone may increase losses significantly it's not insulated and weatherstripped.
No garage door. There are going to be 2x6 walls on all sides (including the inside of the L which is adjacent to the house) and there will be a 4 inch partially insulated wall outside of that. The only heat loss will be the steel 3ft door and a small slider window. If your figures are right, do you think it could be heated by a 1500wt 120 vt 20 gal mobile home water heater? Or do you have a recommendation on water heaters?
Not sure I understand ... the 'only' heat loss is the door and the window ... you do have exterior walls and a roof, don't you?
Give me a better idea of the geometry/configuration and I can send you a rough cut heat loss. Is a particular space temp critical for your operation?
I assumed a square box w/ 8ft ceilings and a 15 sqft window and a 3x7 door. This was exposed to the weather on the 4 sides. Sounds like your config. is quite a bit different (i.e. some exterior walls are common to the house). 1.5 KW sounds like it could easily be in the ballpark ... and for a majority of the time (except the coldest of weather), will likely be.
If your shop environment is like many, your space temp could easily be set at say 60-65 and be quite comfortable ... but I know zip about the relative level of physical moving around that might justify a lower setpoint. Generally, it doesn't take a lot [of physical exertion] to justify it.
A guy in this area keeps his floor heat set at 61 and it is amazingly comfortable. It is "L" shaped. It would be a 23 x 32 space except the house cuts out a 19 x 9 rectangle out of the corner. We are building 6 inch walls/R board on those two sides too (sound issue as well). There is only one small window - about 16 x 32, and two 3 ft steel doors to the outside. You're right, 8 ft ceilings. With Illinois weather it is important to keep the machines the same temp and control the humidity - not get a dew point going. I'm not sure if I'm taking a risk by trying to use a 20 gal water heater - less residual hot water as well as smaller element. A 30gal isn't much different in price. Plus, some of these are amazingly expensive. Am I missing something about the equivalent of "ball bearings" in water heaters?
The difference between the 20 and 30 gallon heater is likely just the size of the tank. What you really want to check is the wattage of the heating element.A larger tank will provide you with 50% more reserve, but no more "heating" capacity.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
So for that, your CONDUCTIVE load at 70 Fdeg delta T (~60 in and minus 10 outside) is about 2,900 Btuh. add an assumed air leakage rate of say 0.35 ACH and it goes up to 4,900 roughly. That is 1.43 KW. Margin of error ... round to say 1.5 KW ... what are your options rated at? Remember the VAST majority of time equipment rated less than this will do the job ... you size for the peak or worst case that will typically have a good chance of occuring.
water heaters should be oversized by a good 25% or so in most cases, even if dedicated.their heat transfer ability is rated assuming ground temperature incoming water. we have found in the past that they are often not capable of their rated outputs in heating applications.though their efficiency is much better heating than the EF for water heating would indicate as well, as standby loss remains constant and the total load grows.if the water heater does double duty, a fudge factor is good to account for scale buildup as well. constant introduction of fresh water can diminish outputs over time. and of course you need to add the DHW load to the heating load to figure output requirements.I am not sure, however, that the upsized recommendation would hold true for an electric water heater specifically... not sure how hot those elements get, and the difference between radiant return and ground water temperature may not be significant at the dT to the electric element if it's hot enough. However, since it can't be so hot it would flash water to steam, I would assume that it DOES hold up, but I can't swear to it in a court of law.
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-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.comEdited 8/31/2008 11:43 am ET by NRTRob
Edited 8/31/2008 11:43 am ET by NRTRob
> their heat transfer ability is rated assuming ground temperature incoming water.True for a gas water heater, but not for an electric.
In science it often happens that scientists say, "You know that's a really good argument; my position is mistaken," and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion. --Carl Sagan
what's the rating for an electric based on?and, do you know how hot the elements get?-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
The rating for electric is based on watts. Watts convert directly to BTUs. There will generally be a **very slight** rolloff as temps increase due to the increase in resistance of the element (which reduces its actual wattage), but it's pretty negligible.Basically, an electric element will continue to put out its rated watts (or close to the rating) up to the point where it's gets so hot it burns out. This only happens when the unit is run dry.
In science it often happens that scientists say, "You know that's a really good argument; my position is mistaken," and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion. --Carl Sagan
just to restate to make sure I understand then, the element must be hot enough so that the variance in water temperature across the element from ground to heating temperature ranges is a negligable difference, compared to the delta-T from water at either temperature to the element itself.right?if that is true though, I have to wonder how it's not flashing to steam. If it's not flashing to steam, I have to wonder how that is true ;)ah, how I entertain myself. I can't figure out what the element temperature is, but with some digging around, everything hints at "pretty darn hot".-------------------------------------
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Sorry, I couldn't parse that statement.The element puts out X watts of heat. The temperature on the outer shell of the element depends simply on how fast the water can carry away that heat. Presumably the calculations have been done to predict (or experience has shown) that "free" water can carry off the heat without flashing, up to some reasonable upper limit.Understand that essentially 100% of the heat from the element goes into the water -- there's only a little loss through the end of the element where it penetrates the tank.
In science it often happens that scientists say, "You know that's a really good argument; my position is mistaken," and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion. --Carl Sagan
also ... think about installing a 3-way valve to adjust output to the floor based on current average weather conditions. You want hotter water for bigger loads (cold weather) and cooler water for light loads. You'll know if it is right if your pump runs a LOT ... the ideal condition IMO (it keeps the heating nice and even ... no frequent on/off operation).