Can anyone give me some guideance on what kind of ratio you might design a ASHP & BBE system to? I am thinking it might be a bit foolish to spec a heat pump for those 3 really cold "design temp" days - better to design it for the majority of the heating days and rely on BBE for those exceptionally cold ones.

Is it common to design this way?

Would 80/20 seem reasonable?

(I am in the early stages and may end up on the tipping point of a much larger system etc. Also, I don't mind the idea of the BBE as backup, even if its not going to keep it fully heated.)

thanks

John

First of all , what climate region do you reside. ASHP can be quite effective as primary source for heating and cooling. Electric heating can be quite expensive depending on utility rates for your region. Post some additional info .

Yes - best is to get some software that will estimate total heating costs and play around with different mixtures.

Sorry I was a bit tired after work when I posted that.

I am in Vancouver which according to our building code has a design temperature of 18F and heating degree days(F) of 5310. This is pretty mild for Canada and an ASHP will function very efficiently.

House design is 1000sq ft basement, 1100sq ft main, and 800sq ft second floor. I am aiming for a "code plus 50%" level of insulation. (For this calculation I am actually comparing the clear-wall values since the anti-thermal bridging benefit of the continuous exterior XPS is lost comparing the code "nominal" R values if you know what I mean.) Insulation to be dense pack cellulose with 2" XPS continuous exterior insulation. Serious air sealing and blower door testing. Although there is not much potential for solar heating in the winter here, roughly half the windows are on the south exposure, whereas north facing windows are limited to just two 24" square units. Windows double glazed, low-e, argon fill (actually nothing special although leakage minimized by installing operable windows only where they are actually needed (fixed glazing otherwise) and those windows are better sealing casements). I think that's about all I can say about the envelope.

I'll be running HOT2000 heat loss software to get a good feel for the demand. I expect my radiant heating designer/supplier to run their own software so I'll have a clue if its really out to lunch.



Still I am just looking for some advice on the typical or appropriate modal split between ASHP and BBE. As mentioned it seems foolish to size the ASHP for rare maximum demand events when the temperature dips to the design temperature (besides the envelope is going to retain the heat pretty good too). BBE is so cheap to install (ignoring operating costs) so there must be some sweet spot like 80/20 or something.

Thanks for any input.

John

I don' t know what model you're talking about, but the altherma has an electric backup element built in, and plenty of output available at 18F. Seems like for 2900 sq ft of well insulated house you should be able to hit full capacity on the heat pump. Maybe you'd choose to undersize and save some bucks but when you add in the EBB I'm not sure what you'd really find the difference to be.

80/20 isn't a ridiculous rule of thumb, but even better is to simply see what the damage is for designing to capacity.

I also wouldn't mind the redundancy of BBE should the ASHP circulation pump or something fail and I have no heat. It could at least keep the pipes from freezing!

John

If your design temp is 18 deg F I don't think freezing pipes is likely to be much of an issue in the envelope you describe.

Posted By NRT.Rob on 27 May 2011 09:33 AM
If your design temp is 18 deg F I don't think freezing pipes is likely to be much of an issue in the envelope you describe.

Rob is right- in a air-tight  house with ~R25 whole wall values (assuming dense-packed cellulose in a 2x6 studwall + R10 XPS sheathing + rainscreened  wood siding) freezing pipes are definitely not in the cards unless you're out of power for several days during the coldest week of the century.

A +18F design temp is about the 97.5th or 98th percentile hourly weather for Vancouver airport, and at those temps a house like that is probably going to have a total heat load around 20,000BTU/hr or under, unless you have a large glazing fraction.  You could heat the whole place with a 6kw hot water heater.  COPs on relatively inexpensive R410 mini-splits are in the 2.7-2.8 range at that temp, and I'd assume the Daikin Altherma hydronic  would be about that or slightly better and has more than 20K of output @ 18F output even on the smallest versions.  At the 99.9% condition the heat strips would keep up, or you could choose to let some zones run cooler, but baseboards are probably never going to be necessary.

If you're thinking of baseboards simply as back-up should the heat-pump fail, consider that you can probably heat the entire place at 18F with four 1500W portable space heaters (the little oil filled radiator type rather than a glowing orange radiant type, for better fire safety) for an up-front hardware cost of under $200.

But run the heat loss calc, see what you come up with.

BTW: What's your insulation & air sealing strategy on the basment? (IIRC Hot 2000 tends to be a bit off on that, but an inch of XPS against the foundation with an interior unfaced batt insulated 2x4 studwall w/no interior poly wouldn't be unreasonable in Vancouver. You might even go with 2-2.5" of unfaced EPS rather than 1" XPS, and put 2" of EPS under the slab. See:

http://www.buildingscience.com/documents/reports/rr-1003-building-america-high-r-foundations-case-study-analysis

Note, Vancouver is much warmer than Minneapolis, and you can get by with a lower foam/fiber R-ratio., But with R25 whole wall R above grade, R15+ makes sense for below grade walls. A 2x4 studwall w/R13 Roxul runs ~ R10, add an inch of XPS and it becomes R15. With ~1-2perm foam on the foundation and no interior vapor retarder both the concrete & studs will stay dry, protecting the foundation sill from groundwater wicking up the concrete, yet the cold edge of the stud stays above the dew point of the interior edge. Interior poly or kraft facers would trap ground moisture in the studwall, so skipping them would be the right thing to do, but you may have to argue with the inspector that the foam is the vapor barrier, or show them the math on the dew-point calc or something.

In all seriousness John, why not stay with your original premise of HP for the 'normal' days, and do what DW and I did once, as advised by the motel owner/clerk, when we stayed in Vancouver once and it got down to -10C.  Unit had one of them old noisy wall unit heat pumps, and a small kitchen.

"leave the heat pump on, turn on the oven and leave the oven door open"


For those few days every few year when you need supplemental heat, you cold just put on an extra sweater.   


Got off a ship at  Adak, AK base one time at 1 AM and the oil heater and water were off at the unit assigned to sleep in that night.   Close to -15C inside, to tired to find and start the oil heater, so just piled on the blankets wearing all clothes and parka and fur hat, slept like a baby......

What are your fuel choices and their relative cost?

with the mix of full height concrete wall with interior stud wall, and partial height concrete wall with pony and interior stud wall, the insulation levels are likely higher in the basement than the main floor.

i am getting input from experts here that suggest interior XPS applied to the concrete wall and then an interior stud wall with cellulose and no vapour barrier (air tight drywall). the upper portion where there is an additional frame pony wall atop the partial height concrete wall the amount of cellulose would be higher (perhaps ridiculously high).

shouldnt have a problem with the local BI.

john

Posted By junkhound on 30 May 2011 09:59 AM

In all seriousness John, why not stay with your original premise of HP for the 'normal' days, and do what DW and I did once, as advised by the motel owner/clerk, when we stayed in Vancouver once and it got down to -10C.  Unit had one of them old noisy wall unit heat pumps, and a small kitchen.

"leave the heat pump on, turn on the oven and leave the oven door open"


For those few days every few year when you need supplemental heat, you cold just put on an extra sweater.   


Got off a ship at  Adak, AK base one time at 1 AM and the oil heater and water were off at the unit assigned to sleep in that night.   Close to -15C inside, to tired to find and start the oil heater, so just piled on the blankets wearing all clothes and parka and fur hat, slept like a baby......

That would certainly work but that solution shouldn't be my goal. Besides I am sure I won't get a "heating permit" with the oven as part of the HVAC system!

John

Posted By BadgerBoilerMN on 30 May 2011 04:56 PM
What are your fuel choices and their relative cost?

I wish it was that easy here. In the past the gas company and the electric company were on the same bill! Our electric rates are quite reasonable but I was just reading that the provider is applying for a 30-50% rate increase over the next few years. The provincial government also has a policy lately of encouraging electric rather than gas use.

Then there is always Peace River Site C....

In lieu of a clear direction on rates I am focusing on insulation and reducing demand. I can't worry about world markets and government manipulation.

Thanks

John

Hydronics.

Panel radiators driven by a dual function condensing natural gas water heaters. Mini-split for the AC and don't forget the ERV.

Natural gas is abundant (made in Canada) and does not have to be converted like electricity. With hydronics you have the option to change your mind.

Posted By John7 on 30 May 2011 11:14 PM
with the mix of full height concrete wall with interior stud wall, and partial height concrete wall with pony and interior stud wall, the insulation levels are likely higher in the basement than the main floor.

i am getting input from experts here that suggest interior XPS applied to the concrete wall and then an interior stud wall with cellulose and no vapour barrier (air tight drywall). the upper portion where there is an additional frame pony wall atop the partial height concrete wall the amount of cellulose would be higher (perhaps ridiculously high).

shouldnt have a problem with the local BI.

john

The wall insulation sounds great, but if there's any likelihood of a bulk water incursion rock wool might be a better choice for the below-grade bits. As much of a fan I am of cellulose, it doesn't dry quickly once saturated, so caution is advised in basement apps.  But if you're confident, just be sure to have a capillary break between the studwall plate and the ground moisture in some part of the floor stackup.  Such as...

Is there any foam going under the slab? With an R15-R20 wall, with your subsoil temps R8-R10 of EPS under the slab starts to make some sense.  It's comparatively cheap (compared to XPS) stuff per unit-R (~$1/m² per R, or ~$1/ft² for R10, if you prefer to think in feet.) It won't run into insect problems in your climate the way it can in much warmer regions.  Even Type-I lo-density EPS can handle the loading under a residential slab, but mid-density Type-II is still cheap.

Using table 0.2, p 10 as a rough guide for what's cost-effective in the longer term for whole- assembly R values, you'd be equivalent to a US-Zone 4 climate. They suggest R15 foundation wall, R7.5 sub-slab, or a 2:1 relationship between wall:slab.

Dana

Interesting point about the cellulose drying very slowly. I think that is why they stopped doing the wet-blown stuff around here (if installed in the fall it wouldn't dry until June!). I'll consider rock wool.

Our code requires R10 under slabs, and R12 under radiant slabs. Also interesting that you deduce from that table a 2:1 wall-to-slab insulation level. (Full code spec: Attic R40, Cathedral ceiling R28, Walls R20, Suspended floors R28, Foundation walls R12, Slabs R10, Radiant slabs R12 - yes the foundation wall value seems low.)

Do you have costs for EPS type 1, EPS type 2, and XPS? I can think any units!

Thanks

John

Posted By BadgerBoilerMN on 31 May 2011 06:49 AM
Hydronics.

Panel radiators driven by a dual function condensing natural gas water heaters. Mini-split for the AC and don't forget the ERV.

Natural gas is abundant (made in Canada) and does not have to be converted like electricity. With hydronics you have the option to change your mind.

By Panel Radiator are you referring to something other than typical radiant floor?

What's AC? (just kidding, it only gets that hot for about a week to ten days here so its rare to have AC in Vancouver. Our summers are low humidity too.)

John

Type 1 & 2 EPS are pretty similar in price and will run 7-9 cents/ft/R f.o.b. the distributor, ~10 cents/ft/R, installed. XPS is ~10cents/ft/R f.o.b. the distributor's yard.

I've seen up to 2x $/R from XPS vs EPS. And as little as the same price.