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Jimmery
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| 05/27/2009 6:12 PM |
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First post after visiting often... I've searched the forum and haven't found an answer for what to do for heating with my new construction:
-1 1/2 story, 826 sqft main floor
-25 000 btu/h calculated heat loss (near Thunder Bay, Ontario)
-passive solar considerations with ~7% glass on South wall, main floor acid-stained concrete for solar gain (with 2" foam insulated slab on grade)
-drilled well (neighbors range from 90-130 feet deep) to be done during construction
-wood stove
-no natural gas service at site, only hydro!
So, the following options are considered for heating:
1) Forced-air electric resistance with integrated HRV (Nu-Air ENERBOSS)
-lowest initial cost
-highest heating cost
-can connect to wood stove to distribute wood heat
2) Air-source heat pump (Mitsubishi Zuba-Central: only available in Canada, works down to -30 degrees and afterwards electric resistance would supplement)
-moderate initial cost
-moderate heating cost
-can connect to wood stove
-provides air conditioning (unlike the others)
-can easily connect to HRV to share ducting
3) Geothermal water-to-water heat pump (Waterfurnace EW Series w/ desuperheater)
-water source is drilled well, and would return to same well--called a "standing column well" (this is okay with local building code)
-highest initial cost: radiant floor would be required for this system
-lowest heating cost
-would still need ducting for HRV
-would not connect to stove as there would be no air handler
-also provides water heating, unlike other options
Please advise as I think option 3 is overkill (in systems and in price, for such a small heat requirement and construction cost), but my home designer is concerned with having cold floors. He even recommends installing PEX tubing in the concrete in case I want to install radiant heating later (because of cold floors) if I do option 1 or 2. If that is the case, why not go with option 3 from the start?
Installation costs for options 1 and 2 are estimated at approximately $5000, $12 000, and option 3 has not been priced yet, but I'm guessing it could be $15 000 or more, which seems pretty costly to ensure the floors aren't too cold...
Besides a simple cost-benefit analysis (and payback period), how would you choose one option over the other? Does a cement slab prevent forced air from being a comfortable heating option? |
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jonr
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| 05/27/2009 8:38 PM |
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There are a lot of people living on cements slabs without radiant floor heat (and many without foam either).
I'd do #1 for a cottage and #3 (with careful attention to costs) otherwise.
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Jimmery
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| 05/27/2009 9:54 PM |
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| Thanks Jonr. I wonder if you have also posted on geoexchange.org regarding the geothermal system I would incorporate (standing column well - http://www.geoexchange.org/forums/viewtopic.php?f=2&t=995&start=0#p8079 ). I have not made much progress with this recently (finding an installer who knows/understands this system) and so my option #3 is definitely the more challenging one of the bunch. |
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NRT.Rob
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| 05/28/2009 8:03 AM |
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if you insulate the slab well and don't do aggressive night setbacks, the floor can be more comfortable than you might think without radiant.
that said, slab radiant is cheap, as are electric boilers. if electric is what you are using evaluate straight electric vs geo to see if geo makes sense. I think on this load you will find, especially if electricity is cheap, that the geo doesn't make much sense. |
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Dana1
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| 05/28/2009 8:37 AM |
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To do any sort of reasonable financial analysis, what are your current (and anticipated future) electricity costs? (It's hard to believe that there's any sort of payback for geothermal or even a $12K air source heat pump on a place with a 25K design-day heat load.) If you're heating even part of the season or part of the house with a wood stove the operating costs will probably be reasonable even with resistance electricity, unless Thunder Bay has the most expensive electricity in Canada (I'm guessing it's probably lower than the national average.)
I like Rob's electric boiler + hydronic-floor solution. You get the cush-factor of a radiant floor (not that it's likely to feel very warm to the touch most of the time, but it's still more comfortable than hot air) with the possibility of retrofitting other high-efficiency low-temp options (even active solar) later should the price of electricity skyrocket. Distributing heat with water is more flexible and usually more efficient that air, particularly at the lower water temps needed for slab heating.
I'm not sure how often AC would really be needed in Thunder Bay unless you haven't provided for summer shading of your passive solar. I'd take that aspect of the heat pumps off the table.
If demand-metering is available for residential customers in your area (where the price/kwh rises & drops with grid demand) you can probably make out very well setting the controls to deliver heat to the slab primarily at night when demand is low and the price bottoms out and letting it coast through peak-demand hours. The high thermal mass of the slab and low heat loss lets you do this without discomfort even when it's quite cold out. Your "design day" loads typically occur between 2-7AM, which correlate well with lowest grid demand, and even on design day the afternoon & early evening loads (often the peak grid demand period) are rarely more than 1/4 of the overnight peak load- you could probably coast 4-6 hours even on design day afternoons without an uncomfortable drop in indoor temps. If colder temps correlate with fewer clouds (often does, in the midwest) that's when your passive solar gain would be peaking anyway.
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Jimmery
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| 05/28/2009 8:38 AM |
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Posted By NRT.Rob on 05/28/2009 8:03 AM
if you insulate the slab well and don't do aggressive night setbacks, the floor can be more comfortable than you might think without radiant.
that said, slab radiant is cheap, as are electric boilers. if electric is what you are using evaluate straight electric vs geo to see if geo makes sense. I think on this load you will find, especially if electricity is cheap, that the geo doesn't make much sense.
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Good advise--I will do that.
Regarding "insulating the slab well..." as my drawings are being finished right now, should I ask for more than 2" foam that is already specified? If it makes a difference, there will be no moisture issues as the slab will be built on a raised outcropping of bedrock, with no water sitting under the slab during the winter.
Remarkably, the radiant floor installers that I've talked to in the area said just 1" was enough (either that or the radiant foil, but not both). |
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NRT.Rob
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| 05/28/2009 8:46 AM |
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well if they advocate for radiant foil under a slab, they have some reading to do.
2" is usually adequate, but I'm questioning it a bit. The Passivehouse guys are using something like a foot of insulation. pretty crazy according to the models I use... but the models they are using are significantly more involved.
If it were my house though, I'd use 2" of rigid. |
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Jimmery
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| 05/28/2009 9:45 AM |
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Posted By Dana1 on 05/28/2009 8:37 AM
To do any sort of reasonable financial analysis, what are your current (and anticipated future) electricity costs? (It's hard to believe that there's any sort of payback for geothermal or even a $12K air source heat pump on a place with a 25K design-day heat load.)
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Yes, the area of the cottage will be moving to Time of Use electricity rates sometime in 2010 (currently 4.2 cents at low rate, 7.6 at mid, and 9.1 cents/kWh at high rate). Since "occupancy" will be Spring of 2010, the timing is good. An electric boiler to heat the slab overnight, as you suggest, should work well then.
So, it looks like it's going to be option #1 (forced air electric resistance with integrated HRV) or a new option #4:
-radiant floor in slab, with wall/baseboard hydronic radiators in 2nd floor (loft and bedroom), heated by electric boiler
-separate HRV ducting
-electric domestic hot water: either tankless or tank???
-wood stove for supplemental heating
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Dana1
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| 05/28/2009 10:54 AM |
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Posted By NRT.Rob on 05/28/2009 8:46 AM
well if they advocate for radiant foil under a slab, they have some reading to do.
2" is usually adequate, but I'm questioning it a bit. The Passivehouse guys are using something like a foot of insulation. pretty crazy according to the models I use... but the models they are using are significantly more involved.
If it were my house though, I'd use 2" of rigid.
Yabbut, your house isn't in Thunder Bay, and you're not heating with electricity- I'm thinkin' it's probably cost effective in a 10year NPV calc to go with 3-4" of XPS- a mere 2" would probably be the absolute minimum no-brainer type of insulation level.
The ground temps are probably ~5F or more colder on the north shore of Lake Superior than in your part of Maine, (See http://mb-soft.com/solar/soilmap.gif ) and even if electricity is half the price as in Maine it's still probably more expensive than NG burned at 85% efficiency. (Might be cheaper than propane though.) Assuming a medium-range U value of 1.0 for subsoils you really need substantial insulation against dirt that cold. You'll have slab-to-subsoil delta-Ts greater than 20F for ~6000 hours of heating season there, even in a well-insulated house with relatively low slab temps.
I'd too would be very curious how well the PassiveHouse model simulates different soil types & temps against the thermal mass of the soil & footprint of the house, etc. It's not a simple topic., but a foot of XPS itself is getting to have a relevant thermal mass to characterize (on daily temp change time frames, if not the seasonal time frames as you would want to characterize with soils.) It's not a simple topic- any simple model you come up with has to be quite wrong if it's making generic assumptions about heat transfer characteristics of the soil. Maybe they can get 1 significant digit accuracy out of it, but I'd be surprised if it hit 1.5 digits.
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Dana1
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| 05/28/2009 11:06 AM |
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Posted By Jimmery on 05/28/2009 9:45 AM
Posted By Dana1 on 05/28/2009 8:37 AM
To do any sort of reasonable financial analysis, what are your current (and anticipated future) electricity costs? (It's hard to believe that there's any sort of payback for geothermal or even a $12K air source heat pump on a place with a 25K design-day heat load.)
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Yes, the area of the cottage will be moving to Time of Use electricity rates sometime in 2010 (currently 4.2 cents at low rate, 7.6 at mid, and 9.1 cents/kWh at high rate). Since "occupancy" will be Spring of 2010, the timing is good. An electric boiler to heat the slab overnight, as you suggest, should work well then.
So, it looks like it's going to be option #1 (forced air electric resistance with integrated HRV) or a new option #4:
-radiant floor in slab, with wall/baseboard hydronic radiators in 2nd floor (loft and bedroom), heated by electric boiler
-separate HRV ducting
-electric domestic hot water: either tankless or tank???
-wood stove for supplemental heating
Hot air wouldn't be nearly as effective at time-of-day load-shifting & coasting as a hydronic slab radiant would be. With a hydronic slab the slab stays above the air-temp and has significant thermal mass to coast on. With hot air the slab will be BELOW the average room temp, and be a net draw of heat from the room air until the room cools to the slab temp. All else being equal, option 4 will be more cost effective to run than option 1 (and more comfortable too.)
With time-of-use rates a 2x oversized tank with a peak-hours lockout would probably be cheaper to run than an on-demand. With an on-demand there's never any off-peak pre-loading- it's pay-as-you go, and higher rate times overlap with HW use times by quite a bit. Standby losses on eletric tanks are quite low, so an 80-100gallon tank won't lose much, and you can probably limit tank heat recovery to your lowest-rate periods and still have sufficient daily hot water (unless you have a large whirlpool bath or something.)
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NRT.Rob
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| 05/28/2009 11:15 AM |
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well, I've seen the passivehouse model. the achilles' heel right off the batt is you have to know all kinds of information most people don't have, such as depth of the water table, the speed at which it is moving, conductivity of the soil, and probably a dozen other parameters I didn't commit to memory. but it is crazily detailed, and the goal of course is to get to "no heating system". so if a foot of XPS helps make that happen, it's worth it. Under regular conditions, where you are just reducing load on a heat system, not so much.
The model might be accurate to a wild degree, but the level of detail required to use it is intimidating for sure. I'm probably going to buy it anyway, just as a "heat load educator". I'm sure the modelling is good. Those Germans don't screw around with approximations very much ;)
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Dana1
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| 05/28/2009 12:25 PM |
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It occurs to me that at 4.2cents/kwh, if most of the load can be shifted to off peak it would only take 150 years for a geothermal unit to pay off. :-) Designing in the means to be able to do the shift is still worth some money though.
But if 75% of the electricity use is at 4.2cents/kwh, the relative value of heavier XPS under the slab goes down. 2" inches is still easily cost-effective in a decade time frame, but 4", probably not (unless you assume significant rate inflation). Installing anything beyond 2" would be for reasons beyond the most-simple economics. But if you had PassiveHouse or NetZero type goals, an extra 2-4" may still be cheaper than the PV or active-solar required to drive the extra load. Beyond 6" total might still be tough to rationalize without running the PassiveHouse model though. (I know the depth & temp of the water table at my house, but would have to drill some wells to get a handle on the velocity. That's some model!) |
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NRT.Rob
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| 05/28/2009 12:27 PM |
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| and it's all done in excel! |
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-=Northeast Radiant Technology=-
NRTradiant.com |
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Jimmery
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Posts:25
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| 05/28/2009 2:02 PM |
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Ha! You two are great! 2" foam it will be, then. Unfortunately my "water room" under the stairs (iron remover, water softener, pressure tank, and now the boiler) won't fit an oversized hot water tank, so I think I'll be going with an electric on demand heater, and minimize the heating cost with low flow faucets.
Alternatively, the space where the air handler was going to be could house an oversized tank water heater, but the chance for some extra storage space in this little cottage is pretty nice...
I suppose this is a fairly minor consideration, as a tank heater could be installed later. |
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jonr
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| 05/28/2009 9:58 PM |
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How many winter days/year will it be occupied? Without that input, all the answers are of random value.
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Jimmery
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| 05/28/2009 10:58 PM |
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Posted By jonr on 05/28/2009 9:58 PM
How many winter days/year will it be occupied? Without that input, all the answers are of random value.
True. To give more context, it will be occupied year-round for the first 3-5 years, at which point my job will take me to a new community. So afterwards, I expect it will be used mostly for summer vacations and other random times throughout the year.
I guess with that said, going geothermal is pretty impractical. |
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jonr
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| 05/29/2009 8:46 AM |
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That's an odd case. Since you have to have a well and might as well put tubes in the slab, the only extra expense for geothermal is the 2 ton WW heat pump. I don't know pricing, but isn't a midrange model more like $5K? I suspect it might pay for itself.
Not sure a HRV will pay for itself.
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Jesse Thompson
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| 05/29/2009 8:52 AM |
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HRVs pay for themselves by keeping you from getting sick in your new house...
Exhaust only fans can work as well, but a code constructed building in 2009 HAS to have mechanical ventilation, or bad things will happen to the people living there, as well as to the shell of the building.
My opinion on Under Slab Insulation in Thunder Bay: 4" minimum. 6-8" preferred. The validation work done in Germany has shown that heat loss under slabs has been greatly under-estimated by traditional US heat-loss calculators (DOE-2, etc), especially in low-energy houses. |
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Jesse Thompson
Kaplan Thompson Architects
http://www.kaplanthompson.com/
Portland, ME
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jonr
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| 05/29/2009 9:23 AM |
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I tend to agree although there is a big difference between slab on grade (a short thermal path to outside air) and the middle of a 40' slab at 8' depth (~R28 of soil before you get to outside air).
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Jesse Thompson
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| 05/29/2009 9:35 AM |
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The German math shows how heat transfer shifts modes with air-tight construction. My understanding is that once convective losses no longer dominate, conductive losses then take over. Even if the temp differential isn't great between soil temps and inside temps, there's better conductivity between soil and slab than air and wall.
However, if don't have decent air tightness, it all goes out the window... |
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Jesse Thompson
Kaplan Thompson Architects
http://www.kaplanthompson.com/
Portland, ME
Beautiful, Sustainable, Attainable |
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