Relocating thermostat to allow heating with wood stove?

Users who are viewing this thread

MikeKenmore

Member
Messages
47
Reaction score
2
Points
8
Location
Kenmore, Washington
so, here's the background:
2 story house (basically a ranch with a basement and garage on the bottom level).
basement has rec room, bedroom, bath and washer/dryer.
upstairs is kitchen, living room with fireplace, 3 bedrooms, 2 baths.

the thermostat is currently in the hallway that accesses the bedrooms on the main floor - about 25ft from the fireplace.

we find that when the thermostat reaches 70, we're perfectly comfortable in the living area, which is where the fireplace is located. however, if you go into the basement, since the electric furnace (with heat pump) is never on, the basement is probably 58-60 degrees.

what i'm thinking of doing is relocating the thermostat to the basement so that i can turn AUTO-HEAT back on and maybe set the temp to 67 degrees. i know when we've test-run the furnace, there's a strong rush of air circulating down the stairs (2 of the 3 cold air returns are near the bottom of the stairs), so i'm confident the warm air will be able to circulate down there.

i think i have relatively easy access to the thermostat wiring, so getting the thermostat hardwired into the basement shouldn't be a big deal. we've heated 100% this winter with wood. the basement is under construction right now after a complete gut and remodel.

any issues with this approach? drawbacks? etc?

thanks
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
Warm air circulating randomly it's the same as ducted hot air, and has fairly low heating efficiency in terms of how much heat you are delivering with the power used in the air handler. The air coming out of the duct is typically 110F, whereas the draft going down the stairs would be MAYBE 70F(?), and it would take an order of magnitude more flow to deliver the heat. It can end up taking more air-handler power than delivering the heat directly to the rooms with electric baseboards, and will always be more power than delivering the heat with a point terminal heat pump (PTHP).

Since you're doing a gut rehab of the basement, it's an opportunity moment to insulate and air seal it properly (and up to at least IRC 2015 code minimum) in such a manner as to avoid creating mold-farms. I've covered that topic in detail many times on the remodel forum of this site, if you care to search it out. In your location (US marine zone 4C) the IRC calls out performance equivalent to R15 continuous insulation (no thermally bridging studs) , or R19 if thermally bridged by studs, but the 2x6/R19 solution would have high risk of mold problems. An inch of rigid foil faced polyiso trapped to the foundation with a non structural 2x4 wall with unfaced or kraft-faced R13-R15 batts would get you there (unfaced is better, but sometimes kraft faced is cheaper.) An inch of EPS (not polyiso) under the bottom plate of the studwall is advisable if there is even a remote question of ground moisture, and you then wouldn't have to use pressure-treated.

The heat loads of typically sized reasonably tight code-min insulated basements at Kenmore's 20-25F outside design temp are well within the output of a half-ton PTHP, and walk-out basements would usually be within range of a 1-ton. If you ever wanted the basement to be on it's own zone, this is the time to think about doing it right.

Run a room-by-room heat load calculation of the "after rehab & insulation" picture, and report back.
 

MikeKenmore

Member
Messages
47
Reaction score
2
Points
8
Location
Kenmore, Washington
dana,

thanks for the feedback. alas, i haven't seen all your posts regarding insulating and preventing mold farms. here's what i did:

- gutted down to the studs.
- performed seismic retrofit (anchor bolts, plywood on inside face of walls, etc)
- installed R11 behind plywood in the exterior wall bays
- covered plywood and concrete foundation walls with 3" XPS foam
- installed 1.5" XPS in rim joist bays
- installed 1.5" XPS over concrete floor slab and placed 3/4" tongue and groove advantech OSB
- taped all joints where foam meets wood to airseal if not already spray-foamed tight

therefore, i think it have it airsealed quite well. R15 on the foundation walls; R26 on the exterior walls.

i had an HVAC tech come out to look at the air handler and want to say he measured the output air at 100*F.

my thought with the draft going down the stairs isn't so much to get air to flow that way, but rather to indicate that the system is recycling the air. i'm an HVAC novice, of course, so i might be way off.

would installing electric wall heaters (hardwired, in the wall) be suitable for this kind of situation (assuming the current electric furnace and heat pump can't keep up)?

ultimately, i think my biggest issue is that we do plan to heat primarily with the fireplace (EPA woodstove in disguise) and would like to balance out the air temps a bit more.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
Did you put studs & R11s between the 3" XPS and concrete, or are the R11s only in pony wall stud bays above the foundation sill? The 3" of XPS has a vapor permeance of less than 0.5 perms, so anything on the exterior side of the foam needs to dry toward the outdoors, anything on the interior side of the foam needs to dry toward the interior. If there is any wood trapped between the foundation and foam it has a high risk of developing rot from groundwater moisture drives. Even with exterior side waterproofing on the concrete, ground moisture wicks up the foundation walls from the footing. With reasonably vapor-open walls it will usually dry toward the interior without creating a problem, but not through < 0.5 perm foam.

With continuous R15 on all exterior walls, and another R11 studwall (about R8 after factoring in the thermal bridging of the framing), the U-factor of the above-grade exposed concrete is about 0.06 BTU/hr per square foot per degree difference, and the U-factor with the R11 studwall behind the R15 foam is about 0.045 BTU/hr per square foot per degree. If you use an interior design temp of +68F and an outdoor design temp of +23F (your real 99th percentile might be warmer than that by a degree or two, but whatever...), which is a delta-T of (68F-23F = )45F.

Measure up or estimate the square footage of above grade basement wall by wall type (concrete separate from studwall) and run the numbers. Say it comes to 100 square feet of each, excluding window & door area. The wall losses are:

concrete:

100' x U0.06 x 45F= 270 BTU/hr

studwall:

100' x U0.045 x 45F= 203 BTU/hr.

Say you have 50 square feet of clear glass (not low-E) window down there, that runs about U0.5. The window losses are:

50' x U0.5 x 45F= 1125 BTU/hr

Assuming you have 10 cfm (= 600 cubic feet per hour) of ventilation or infiltration, which at 0.018 BTU per degree per cubic foot the ventilation losses come to;

600 cfh x 0.018 x 45F= 486 BTU/hr

The below-grade losses are next to nil, add maybe 1000 BTU/hr for losses out the slab (if it's uninsulated, when you add it all up you're looking at about 3000 BTU/hr.

From that subtract at least one conscious upright human adult at 300 BTU/hr, and you're at 2700 BTU/hr. If you have a hot water heater or some other appliances that run 24/7 down there you'll have to subtract the standby losses there too.

To convert that to space-heater watts, divide by 3.412 watt-hours per BTU to come up with watts, eg. 2700 BTU/hr/3.412= 791 watts. That would be enough baseboard or other heat to keep the place warm.

A half-ton PTHP can deliver about 6000 BTU/hr, but would use about 1/3-1/2 the power. If it's a fairly open space, not all doored-off that might be a good option. If it IS all doored off, run the heat load numbers on each room individually (subtracting off the room's plug load/standby and one 300 BTU/hr human for that room), and install the smallest amount of electric resistance heater that still fills the bill. You'll find that most individual rooms probably won't have more than 100 watts of heater required, which isn't very much at all.

So, measure it all up, if you have better windows than that, look up the published U-factors (if you can), or describe them and we'll come up with a reasonable guesstimate for a U-factor.
 

MikeKenmore

Member
Messages
47
Reaction score
2
Points
8
Location
Kenmore, Washington
dang, that is a seriously thorough and helpful reply.

i will double check numbers sometime this week. i want to say the overall floor area is about 900 SF. pony walls, on average, are 4ft tall.

windows are likely U=0.31. there's about 54SF of them. 2 new doors - 1 exterior, 1 fire rated garage entry door.

for the insulation, yes, i placed the r11 between the pony wall studs. then placed plywood over those studs (as part of a seismic retrofit). then placed the 3" foam over the plywood and the concrete foundation. for my interior walls, i offset them an inch or so from the foam. they will likely not have any insulation in them. my concern about creating a moisture sandwich is with the plywood layer. the r11 i put back in there had craft paper on it (facing the plywood, not the exterior). hopefully that doesn't create an issue...
 

MikeKenmore

Member
Messages
47
Reaction score
2
Points
8
Location
Kenmore, Washington
IMG_0021.JPG
IMG_0024.JPG
IMG_0025.JPG
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
Asphalted kraft facers are not vapor barriers, but rather "smart" vapor retarders, as is half-inch CDX plywood. When dry kraft facers they are about 0.4 perms, but when there is enough moisture in them to support mold growth they are over 10 perms (far more vapor-open than interior latex paint on wallboard, which runs 3-5 perms). When half inch CDX is dry it's somewhere between 0.5-1 perm, but at 20% or higher moisture content it's over 5 perms. The only highly vapor retardent layer in the stackup is the 3" XPS, which will always be under 0.5 perms no matter what the humidity is like on either side.

There may be code issues with the closed-in cavities with no vertical firestops. In the event of an electrical fire in your studwall lighting off the polystyrene insulation the smoke and flame spread is a problem. Putting the studs tight to the foam and installing firestop blocking limits how far and how fast it moves.

A cheaper way to have insulated the foundation at a comparable performance level is an inch of polyiso trapped to the wall with 2x4 studwall, with R13s in the cavities.

Slab losses are complex to calculate correctly, but a really crude method for estimating the loss of a slab that is below the frost line is to treat it as a constant, unvarying heat loss using a U-factor of 0.04 against the difference between the average room temperature and the deep subsoil temperature. In your case your deep subsoil temp is about 53F, so assuming a 68F room temperature that's a 15F difference in which case the whole 900' basement you're looking at 900' x U0.04 x 15F= 540 BTU/hr of background heat load, if you care to factor it in. It's a very squishy number- a 900' slab poured directly on wet clay soil it could easily see losses over 2000 BTU/hr, but if poured over a couple feet of compacted dry pumice it could be under 100 BTU/hr- expect a bit of error.
 

MikeKenmore

Member
Messages
47
Reaction score
2
Points
8
Location
Kenmore, Washington
my understanding of polyiso is that it loses performance over time. plus, when actually cold out, it performs poorly. the price i got on the xps was $0.32/SF per inch thickness, so it seemed reasonable to me.

i hear you on the fireblocking. i struggled with finding a way to install it without creating a thermal bridge. before the drywall goes up, i will install some drywall in the openings and put some mineral wool (roxul) in the bay above it. not perfect i admit.
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
Polyiso loses performance at very low temperatures, when the average temperature through the foam layer is below freezing, but at Puget Sound area regional winter average temperatures it will exceed it's labeled performance. It varies a bit by vendor. The following curves are multiple 1" samples of aged polyiso tested at a 50F temperature difference from one side to the other, which exaggerates the effect a bit.

See: http://www.greenbuildingadvisor.com/articles/dept/musings/cold-weather-performance-polyisocyanurate

With R6 polyiso stacked on the exterior of R13 cavity fill, the temperature on the warm side of the polyiso is about 1/3 of the difference between the interior and exterior temps. At 23F (roughly your 99% outside design temp) and a 68F room, the warm side of the polyiso will be about 38F, and the average temp through the foam layer would be about 30F. But 99% of the time it'll be warmer than that. The binned hourly mean temperature in Kenmore for the month of January is about 40F, which would make the warm side temp about 49F, for a mean temp through the foam of about 45F, a temperature at which all but one vendor's sample test above R5, and some over R6.

This is the temperature norms for Seattle:

temperature_temperature_f.png

See: https://weatherspark.com/history/29735/2015/Seattle-Washington-United-States

The labeled R is tested for labeling purposes is at a mid-foam temperature of 75F, and yes some fail to meet R6, but all are above R5 at that temp. But at your full seasonal average temperatures all samples will have average performance north of R5/inch.

XPS is blown with HFCs, that bleed out over time. The labeled R5/inch is the average performance over it's first decade or so (I forget the FTC's required ageing interval over which to average for lableling purposes). It will eventually fall to R4.2/inch @ 75F, but will outperform that at cooler temps. At ~40F mean temp through the foam, blowing-agent-depleted 1.5lb & 2lb density polystyrene will deliver about R4.5/inch.

The other advantage to polyiso is a slightly higher kindling temperature, but better still, even when fully burning it chars in place rather than melting, and spreading as a burning liquid, which is what happens with polystyrene (either EPS or XPS.)

Both polyiso and EPS are blown with pentane, which is fairly innocuous stuff. Pentane has a 100 year global warming potential of about 7x that of CO2, but most of it escapes during manufacturing, often recaptured and burned for process heat. The primary HFC used for XPS in the US is HFC134a (automotive AC refrigerant), which has a 100 year global warming potential of about 1400x CO2. There is discussion within the EPA about phasing it out over time to be replaced with low-impact HFO1234 variants, and HFO1234yf has been shown to be an effective blowing agent for both polyurethane and polystyrene- they'll eventually fix that bit of damage, but it will still be lost over time. HFO blown XPS will still have a decades long decay down to ~R4.2/inch.

[edited to add]

If you're interested, today's GBA blog has an update of where they're going with polyiso blowing agents, not that the cold temperature derating of pentane-blown goods would be an issue in your climate, the way it would be in North Dakota or Manitoba. (You'll have to sign up for a free trial membership to read it.)
 
Last edited by a moderator:

MikeKenmore

Member
Messages
47
Reaction score
2
Points
8
Location
Kenmore, Washington
damn, you know your stuff!!!

maybe on the next basement remodel i do (which is never, i hope), i will use polyiso. it's tough to know what you don't know until you know it :)
 

Dana

In the trades
Messages
7,889
Reaction score
509
Points
113
Location
01609
damn, you know your stuff!!!

maybe on the next basement remodel i do (which is never, i hope), i will use polyiso. it's tough to know what you don't know until you know it :)

Naw, I just make it up as I go along! :)

In many places it's possible to get used foam from reclaimer/demolition companies at dirt-cheap prices, which is where I tend to look first. Fiber faced roofing polyiso is typically 2lbs density, and R5.5/inch. If it's old stuff blown with CFCs or HCFCs it's performance actually increases with falling temperatures (like EPS & XPS) , and it doesn't lose the blowing agents anywhere near as fast as the HFC blown foams (big fat molecules can't get out.). When I did my basement I found some 3" roofing iso (R17 or better) at $20 for a 4' x 8' sheet coming off an old factory building a few towns over. At 4 cents per R-foot^2 it beats the pricing of any virgin-stock insulation, including fiberglass batts. One can often find something suitable in Craiglist (or similar) listings.

When using polyiso on basement walls it's important to keep the bottom edge of the foam off the slab, since it is slightly hygroscopic and can wick groundwater moisture over time.
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks