Best radiant design for new build

Data

I’m looking for experienced design opinions... I'm building a house (slowly) for retirement in south-west BC (north of Seattle). The site has no access to natural gas, so heat is going to be electric. Probably also have a wood fire (hopefully centrally located masonary stove). Of course we are looking for cost-efficiency in both design and on-going cost. We are building in two stages and I’m doing what work I can myself:

Stage 1: A two-storey (2x600 sq/ft) box is built to lockup. This is “the cottage” until we actually retire. The lower floor is the workshop. We’ll spend a lot of time there and we plan on hydronic heated slab. Upper floor is an apartment for now and will probably just use supplementary electric heaters since it won’t be used often. Plan on a tankless boiler to heat the slab. Max heat loss expected just over 14000 BTUH.

Stage 2: A 2000 sq/ft open-plan addition to be added later. Most efficient option is probably thin hydronic slab over all floors, but the good wife does not want “hard” floors. I don’t want staple-up hydronic - too many problems, and/or too much experience and $$ to do it right. Whole house max heat loss is expected to be 40,000.

I love the idea of geothermal, but we’re on solid rock so it would be pricey (although efficient). An air-source heat pump works well in our area. So final design could be:

1) Find a way to keep wife happy with fully hydronic radiant (maybe radiators, or bite the bullet and go warmboard). Use air-to-water heat pump supplemented by the electric boiler when required. Still need ducting for HRV.

2) Go forced air, but still use air-to-water heat pump supplemented by the electric boiler.

3) Forced air, but use a completely separate air-to-air heat pump with electric or propane furnace for backup in the addition and stick with small boiler for the slab in the initial house.

Biggest urgent question is should I spring for the big electric boiler now because it will make sense to use it to supplement the heat pump later.

SWEI

options

No need for a thin slab with all the dry floor systems available in today's market.  Staple-up is for retrofits.  If you can't live without wall to wall carpet and thick padding, consider putting heating the lower portion of your walls.



How much wood do you have available to burn?  You might want to look at a wood gasification boiler and some thermal storage.



The term 'tankless boiler' is a bit vague, though wall-hung modulating electric boilers are  quite affordable.  Adding a heat pump later is easy if you plan for it.  Most electric boilers have both low mass and low friction head so you can usually just pipe them in series with another source (like solar or a GSHP.)

Data

Thoughts on options

Really appreciate the input. Yes perhaps a dry floor system will be worth considering in areas without carpet, although thin slab still sounds like it would offer best cost/efficiency. Hadn't considered heat in the lower part of the walls. I assume you mean baseboard radiators, or would you actually run the pex through the studs?

Wood-fueled boiler sounds like more work cutting/stacking/feeding than I'll have the stamina for later in life so I think I'll stick with the masonary stove idea. Easier to put down the book and drag yourself out of the chair to throw in another log.

I guess "wall-hung modulating electric boiler" is what I meant by tankless boiler - the local supplier suggested Argo AT series. I'm happy to hear that plumbing it in series with a heat pump is a sound idea. So I should plan for a large boiler (and required electrical supply)... probably should get some professional design assistance. Thanks SWEI.

SWEI

electric boilers

Over the past decade or so, many designs have moved to low mass floors and higher mass walls (or other building elements.)  By separating the thermal mass from the emitter, the system becomes far more responsive to control input and generally works better with variable winter sunshine events.



With regard to walls, I was referring to PEX tubing and aluminum plates (the working parts of a dry floor system) mounted in the lower 4 feet or so of a wall.  Both vertical and horizontal layouts are workable depending on the type of construction and the design.



One really nice thing about electric boilers is that they come in sizes far smaller than any mod/con gas unit I have seen, allowing them to be sized properly for small, tight spaces.  The gas manufacturers are just not there yet.  No experience with the Argo, but it appears from the brochure as though it has a manually adjustable temperature control.  Outdoor reset will give you far better control and comfort plus save a bit of energy.  Thermolec boilers (made in Canada) have built-in ORC and are priced quite reasonably.  I'm sure there are others, but with our energy rates in the southwest (relatively expensive electricity and cheap NG) we don't see too many electric boiler installs.

Zman

Fuels

What are are your local electricity costs? Also what else is available for fuel as well as costs? Wood, pellets, propane? How will you heat your domestic hot water?

NRT_Rob

lots of options

for air source you want low water temps as best you can. Depending on your water temps, outdoor design temp and final load you may or may not even need much external backup to the ASHP...



for low cost deployment in flat ceiling situations, a wider on center radiant ceiling can give good performance at low temps.



Staple up is NOT just for retrofits... it also is much less prone to puncture issues. however to get low temp operation you either need a very low per square foot heat load or more expensive, heavy plates.



You can do better with overfloor methods. Warmboard is the best, no doubt. There are DIY variants which save money and sacrifice some performance and add a lot of labor.



Panel radiators are also often cost effective. You can do those WITH suspended tube joists pretty cheaply and get floor warming and low temp operation as well. but you have visible emitters. can be a nice way to hit a great room in a house that did radiant ceiling elsewhere though.

Data

Wall radiant

Interesting idea - the pex and aluminum plates in the walls. That would be behind the drywall, or would you typically use some other covering?

Need to look into this outdoor reset thing...

Data

Fuel

Electric is averages out to around $0.10 KwH. Propane and wood pellets are an option, but we're on an island so everything is more $ for shipping. That's why the plan is electric, with as much help from passive solar and masonary stove as we can muster. The masonary stove will be large and central - even wondered about running the pex through it with the thought that it would pick up heat from the fire when burning, or store it in the thermal mass when not.

Domestic water I figured electric - although solar in the summer sounds like a good use. Probably relatively easy to do too? Our current house has a pre-heat tank that is heated by the wood stove, then the EHWT is supplied from that.

SWEI

hydronic walls

Drywall is good up to 120F -- no problem in a modern well-insulated building.  You get less limitation on furniture layout than panel radiators and the comfort level feels a bit better to me than ceiling radiant.



I'm doing a commercial remodel with 160F water (design temp) and magnesium oxide cement board on the walls.  Weighs over 80 lbs per sheet but practically indestructible.  Walls are a good option for retrofitting existing buildings -- especially multistory, where raising a floor an inch is simply not practical due to elevation issues with stairs and thresholds.

Data

Options

Yes, I think the ASHP will not often need help, that's why I'm thinking the most cost-effective way to do it would be just put in a larger boiler than we initially need - likely not much extra $ to do that if it's a sound idea.

The great room will be vaulted ceiling and likely carpet, so I suspect radiators might be the way to go - or SWEI's idea of radiant IN the walls... unless we just go forced air.

I'm anxious to find out what the temp difference is going to be between the workshop with heater slab, and the living space above it with no extra heat source. If the temp dif is not great, perhaps the great room could have a heated slab in the crawl space UNDER it...

Data

Wall Board

That brings up another thought. We went with Hardie board (stucco finish) on the outside and I had wondered whether anyone has tried it on the inside. Would provide some thermal mass and harder surface than drywall.

NRT_Rob

it is almost guaranteed

you will not enjoy those results. max transfer in that case is probably 10 BTU/sq ft or less and that only if the crawlspace under is very hot.



Radiators sound like the winner there, perhaps with some joist assist.



Radiant wall works but I'm always concerned about punctures in wall apps.

bld999

Hardie inside

"Wall Board

That brings up another thought. We went with Hardie board (stucco

finish) on the outside and I had wondered whether anyone has tried it on

the inside. Would provide some thermal mass and harder surface than

drywall."



I would guess hardie has more thermal mass per cubic foot, but it's half the thickness of gyp, so it may be a wash. Drywall is easy to repair, hardie less so, usu. you wold be replacing a whole panel instead of patching. It is certainly more acoustically reflective than gyp. Might look a bit industrial, too. Any exposed edges receiving knocks or abraision may delaminate and "broom."