I need advice on the design of an energy-efficient heating system
firstcall
| Posted in Mechanicals on
Just completed re-framing a home in NH. Used Zip R on the outside, CC foam inside. I had planned on going with radiant heat but from the articles in GBA, this may be overkill. I have spoke with a few propane companies and 2 plumbing companies and have not seen anything that is right. Most do not seem to be up to date ( old school) Who out there can I contact to design the proper system without trying to sell me on their systems?
I am truly thinking of going with return hot water as this system has served us well in our last home for 25 years.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Joseph,
I'm not sure what you mean by a "return hot water" system, but it sounds as if you are interested in a hydronic heating system. I suggest that you hire a mechanical engineer to help you.
The problem seems to be that he can't find anyone who's up to the job. That in and of itself should be a red flag: do you really want a system that's so complicated that professionals struggle with it? How about high-efficiency wood or pellet stoves instead? You can install them yourself. Then you can supplement this with electric radiant baseboard heaters in the bedrooms for the times when people in there feel chilly.
Nate,
I'm not suggesting that Joseph should install a hydronic heating system, but if he wants one, any mechanical engineer working in New Hampshire can easily design him a system.
When I worked at a plumbing wholesale outfit in Vermont in the 1970s, we provided heating system design as a free service to any plumber who walked in the door and agreed to buy a boiler from us. We measured the house and windows, assessed the insulation levels, performed a room-by-room heat loss calculation, drew up plans showing the piping, specified how many feet of hydronic baseboard would be installed in each room, and indicated where it should be located -- and if a kickspace heater was required for the kitchen or bathroom, we showed where that would be installed, too.
We also prepared a takeoff sheet listing all the parts, and we priced out the whole system.
These design services aren't rocket science. Hydronic systems are common in New Hampshire and Vermont.
Before you even talk to contractors, run your own room by room I=B=R heat load calculations at the local (or nearby) 99% outside design temp, using 68F as the interior design temp (even if you like to keep it warmer than that at night.) Don't exaggerate the infiltration rates (as many calculation tool defaults will do by an order of magnitude or more.) For design temps, see: http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf
Have that in your pocket when talking to them, and don't be shy about sharing that information.
When you have the room numbers, subtotal up the rooms that you want zoned together, and the whole-house number. If you can share the zone & whole-house numbers there are readers & contributors here capable of making reasonable recommendations, and warn you away from problematic proposals.
If the numbers are low enough there may be reasonable modulating air-source heat pump solutions that would have half the operating cost of condensing propane, but without the heat load numbers (and the 99% outside design temp) it's not possible to make that call at this point.
It's too bad the framing & insulation project is already complete. Using cc foam between studs improves air tightness, but the very high center-cavity R value is cut off at the knees by the thermally bridging framing, adding only about R2 to the average or "whole-wall" performance of a 2x6 framed house, even less on a 2x4 framed house. Saving the foam budget for continuous exterior insulating sheathing (where it isn't thermally bridged by wood framing) is almost always significantly better bang/buck. But unless the siding isn't up yet it's water under the bridge.
Interested in what is meant by return hot water system . Could you be more specific ?
Return hotwater.. Water heated at a boiler sent through a zone to heat room and returned to the boiler for re heating????
Dana, could you explain the last paragraph of your answer a little better. I do not understand it. cc foam and Zip R. can it be any better than that?
Joseph,
The system you describe -- "Water heated at a boiler sent through a zone to heat room and returned to the boiler for reheating" -- is usually called a hydonic system, not a "return hot water" system.
As I wrote earlier, any mechanical engineer in the state of New Hampshire who works on residential projects should be able to design a hydronic system for you.
You asked, "closed-cell foam and Zip R [sheathing]. Can it be any better than that?"
Well, yes, it can. The problem with putting your insulation between your studs is that this method of insulating doesn't stop thermal bridging through the studs. There are several ways to address the problem of thermal bridging; the two most common ways are to install rigid foam insulation on the exterior side of the wall sheathing, or to build a double-stud wall.
Joseph,
There are folks who will speak with you , conduct an in depth interview of what you want and make recommendations for whole systems based on what you desire and what will work . Don't confuse someone telling you what will work with them trying to sell you their system . Many times what someone wants is not a good fit for their particular building .
I perform these types of services and am able to design utilizing any number of technologies as long as there is a wet or hydronic component . A designer , consultant , installer worth his salt will not hesitate to tell you what will work well and what will suffer in certain situations .
There are very few websites worth looking at to find these people , Caution . 1 that offers best in class advice by those at the top of the industry is heatinghelp.com . There is also a find a contractor feature there that has been more than helpful to many . GBA is a great source for building issues , the advice given by the regulars is top notch . However if having a well done hydronic system is your goal you may be better served elsewhere .
I do have thermal bridging. Zip R has 1" rigid foam on its back that is nailed over the studs.
I want to thank everyone for the rapid response to my question. Great response and I am now searching for a mechanical engineer. Return hot water is the term most home owners use in New England.
Joseph,
Thanks for the clarification. Even though I copied what you wrote, I missed the fact that you are talking about Zip-R sheathing -- I misread your words, and thought that you were talking about ordinary Zip sheathing.
One-inch Zip-R has an R-value of R-3.6 -- which is OK, but not much for New Hampshire. However, you are correct that the R-3.6 foam will reduce thermal bridging through the studs.
"Return hot water is the term most home owners use in New England."
I've lived & worked in New England for decades and have never encountered that term . I've often heard hydronic systems referred to as "forced hot water", and "pumped hot water", but never "return hot water".
" I do not understand it. cc foam and Zip R. can it be any better than that?"
Yes.
Closed cell foam costs about a buck a board foot, installed for R6-ish, or 17 cents per R-ft^2. The installed cost of rigid EPS foam costs about 40 cents/board foot for R4-ish, or 10 cents per R-ft^2. For the same cash investment in foam you get much better return out of EPS. But it's even worse than that...
At 16" o.c. stud spacing about 25% of the wall area (not counting windows & doors) is thermally bridged by framing, which has an R value of about R1.2/inch. When you fill the other 75% of the wall area with R6/inch foam it does nothing to reduce the heat transfer through the 25% that is bridged by framing. In fact, the framing fraction is conducting well over half the total heat through the the wall in that scenario. If the cavities were filled with much cheaper R3.7-R4.2/inch stuff (like cellulose, rock wool batting, open cell foam) the framing fraction would still be conducting about half the total heat. But if you then spent the foam budget on continuous foam exterior to the sheathing you end up with a much higher performance wall for the money.
The total R of 1" ZIP-R may be R3.6, but it's only adding R3 beyond standard sheathing. That doesn't even double the R-value of 2x4 framing, and only improves 2x6 framing by 45%. Worse still, ZIP-R is polyiso, which takes a severe performance cut (by about half!) when the average temp through the foam is below freezing, which it will be MOST of the winter in NH when it's on the exterior of R20+ closed cell foam. So in practical terms using 1" ZIP R is adding something like R1.5-R2 to the whole assembly, which could have been achieved with 3/8" fan-fold XPS siding underlayment at much lower cost. (Polystyrene foams like EPS and XPS gain performance at much lower temps, unlike polyisocyanurate.)
If the siding isn't up yet it's possible to add significant performance & resilience by installing an inch or two of rigid rock wool, which will improve the average performance of the ZIP-R (by keeping it a bit warmer), and putting a more substantial thermal break over the framing fraction. With ZIP-R you can't add exterior foam, since that becomes too much of a moisture trap for the sheathing, but rigid rock wool is as vapor permeable as housewrap, and would not be a problem.
For comparison purposes, assuming a 25% framing fraction fiber cement siding, and half-inch wallboard the "whole-wall R" performance is roughly...
2x4 construction:
non-insulating sheathing w/R13 open cell foam: R10.1
non-insulating sheathing w/R13 open cell foam, with 1" exterior EPS: R13.7
non-insulating sheathing w/R13 open cell foam, with 2" exterior EPS: R18.8
...as compared to...
non-insulating sheathing w/R20 closed cell foam: R11.3
That's an R1.2 improvement over the cheap stuff, barely more than 10% more performance at several times the cost.
Then:
1" ZIP-R w/R20 closed cell foam: R14.3 (fully rated) or R12.8 (derated for climate.)
That's an total improvement of only R2.7 over the bare bones studwall, and doesn't even perform at current code min.
If you added 1" of rigid rock wool to that you'll be at ~R16.8, and still be under IRC 2012 code-min performance, but if you added 2" you'd be there.
If it's 2x6 framing we can run the numbers there too, if you like. But with 5.25" of R6/inch closed cell foam and 1" ZIP-R it would still be barely code-min even if fully-rating the ZIP-R.
For reference, NH is mostly climate zone 6:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm
Thank You extremely informative
If you tell me the cc foam thickness, the siding type, and whether you'll be adding exterior insulation (and of so, how much) I can cook up reasonable U-factors to use when calculating the heat loads.