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Heating suggestions for 40k BTU heating load second floor?

I am planning the second floor of a new build home in climate zone 4 which i am completing in stages due to money constraints. The building envelope is hot roofed with 4" cc spray foam on the roof deck and 5.5" of rock wool beneath that, the walls are 2x4 with cavity insulation and 2" of XPS on the outside of the wall sheathing. The walls are further air sealed with a combination of Knauf eco-seal and cc spray foam and the basement walls have 2" of cc spray foam so the envelope is reasonably tight. Fuel source available is propane which is already piped into the home.

The first floor and basement are heated and cooled with their own dedicated forced air system on 3 zones with a new 3 ton Carrier Infinity Greenspeed Heatpump and electric resistance back up with a Manual J calculated heating load of about 50k BTUs. DHW downstairs is on a dedicated condensing tankless water heater.

My question is, how would you recommend heating the second floor which has a Manual J calculated heating load of about 40k BTUs and DHW requirement for 2 bathrooms? Assume Minisplits and plumbing based hydronic heating are not options for design reasons and i want to minimize the electric load because i backup the house to a whole house 20kw generator which i do not want to overload. Would you recommend a setup similar to the Rinnai hydronic furnace and tankless heater featured in Building Science RR-1206? (link: http://www.buildingscience.com/documents/reports/rr-1206-measure-guideli...)? Or perhaps a Goodman 45k BTU furnace and a/c unit like in your prior blog entry? or a different approach?

thanks,
jbo

Asked by jack ostrick
Posted Fri, 01/04/2013 - 09:40

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15 Answers

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1.
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BSC and Artistic Homes (Palo Duro Homes now) in Albuquerque started using regular HW systems to heat their homes about 10-12 years a go. There are a couple of articles about that at BSC. I used the same system in a couple of homes and folks really like it as well. The tankless option is more expensive and in the end, most people use more water with their unlimited supply.
Also, since you have a conditioned attic, it maybe easy to install all that equipment and ducts up there.

Answered by Armando Cobo
Posted Fri, 01/04/2013 - 10:09
Edited Fri, 01/04/2013 - 11:28.

2.
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I am in NY - does a cold zone (4) negate any advantages to this setup?

Answered by jack ostrick
Posted Fri, 01/04/2013 - 10:50

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If your attic is well sealed and insulated, you should not have any problems; but there are some other possible drawbacks too. If you live in an area of hard water, you must have a rigorous maintenance program with the HW & HVAC system as with time you may get calcification and/or oxidation; and if the installation is in the second floor or attic, good maintenance is most important.
I don't think it matters where your home is located as long as your quality of construction is good.... Albuquerque is in CZ4, Artisia is in CZ3 and the Four Corner area is CZ5.

Answered by Armando Cobo
Posted Fri, 01/04/2013 - 10:59
Edited Fri, 01/04/2013 - 11:07.

4.
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No, we have soft water in the district. Thanks.

Answered by jack ostrick
Posted Fri, 01/04/2013 - 11:11

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Jack,
Your design heating loads seem high to me, considering the fact that your house is relatively tight. How many square feet are we talking?

Answered by Martin Holladay, GBA Advisor
Posted Fri, 01/04/2013 - 11:16

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Martin:
According to the Manual J, 1,826 sq ft, 15,663 volume of conditioned space, .653 CFM / sq ft, 38,225 btuh total heating required including ventilation air, 31,720 btuh cooling load. The sen loss for ductowrk comes in at 14,234 btuh which i thought looked high given it would all be in conditioned space. Assuming that's an error and the correct load is closer to 25,000 - what might your recommendation be?

Answered by jack ostrick
Posted Fri, 01/04/2013 - 11:26

7.
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Jack,
I'm confused. Your original question states that the design heating load for your basement and first floor is 50,000 Btuh, and your design heating load for your second floor is 40,000 Btuh, giving you a total Manual J heating load of 90,000 Btuh. That's a lot.

Now you mention 25,000 Btuh. Is that for one floor or three floors?

What is the rated heat output of your Carrier heat pump?

Answered by Martin Holladay, GBA Advisor
Posted Fri, 01/04/2013 - 11:33

8.
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Martin:
The first floor and basement have a btu requirement of 52k btuh - that is handled separately through existing hvac with a 3 ton carrier heat pump unit (with electric resistance backup) so the heat pump has roughly 30-45k btuh depending on outdoor temperature. The first floor/basement is an additional 4100 sq ft .

The second floor according to my manual j had a 38k btu requirement and i am trying to solve for that heating requirement separate from the existing first floor and basement. the figures above (40k and 25k) are for my second floor only. the confusion came when i noted in the manual j for the second floor that the sen loss for the duct work was 14k btuh which i thought was weird given all of the duct work would be in conditioned space - that's why i subtracted the 14k from 38k to get ~25k. btuh

Answered by jack ostrick
Posted Fri, 01/04/2013 - 11:43

9.
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Those numbers are INSANELY high for a climate zone 4 house that has glass in the windows, and doors that shut. Even with NO INSULATION, but reasonable air tightness and storm-windows over ancient double-hungs you'd still be at or under 50KBTU/hr in a ~2000' house with a 2000' basement at your ~15F outside design temp (NYC & L.I.).

If you have billing information (with meter-reading dates) on what it took to heat the existing space and a zip code (for design temp and heating degree-day history) it's possible to assign a reasonable upper bound on what the actual heat load is.

By way of comparative example, I currently live in zone 5 in a ~2400' 1.5 story 2x4 framed 1923 antique with clear-glass double-hungs + storms, with another ~1500' of semi conditioned (insulated) basement that never drops below 65F, for ~ 3900' total. Based on fuel billing and I=B=R type calculation methods the heat load at my outside design temp of +5F is well under 40K, but over 30K. When I'm done with fixing the known air leaks and insulation gaps it'll be about 30K @+5F (maybe a bit under) even without swapping in better windows.

At +15F my heat load is already under 35K. Yours probably is too, unless your heating it to 90F to run Bikram Yoga classes or something. ;-)

With design temps in the mid-teens heating & cooling with ductless mini-splits would be cheaper than condensing propane, but it's important to get the sizing right. I'm guesstimating that your actual heat load for the second floor would be under 20KBTU, and a 1-2 head ~1.25-1.5 ton ductless solution would handle it just fine all the way down to +5F. It'll be more expensive up front than a condensing propane furnace, but it'll make up the difference in lower operating cost in very short years.

But DO get the heat load calc right. Manual-J methods tend to overshoot measured reality by 15-25%, so the 25KBTU/hr number may still quite a bit to the high side, but not necessarily outlandish, but 38K number is totally bonkers (that's more than the whole load for my not-super-insulated house at +5F!)

Answered by Dana Dorsett
Posted Fri, 01/04/2013 - 12:22

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Here are a couple of well-respected folks who offer Manual J heating/cooling load calculation services:

http://www.energyvanguard.com/hvac-load-calculations/

http://www.optimalbuilding.com

Answered by John Semmelhack
Posted Fri, 01/04/2013 - 12:45

11.
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Assuming everyone is right - i still don't want to lose the forest for the trees here. If ductless mini-splits are not an option b/c of style considerations and the BTUh requirement is anywhere in the 10-25k range, what are the options for heating with propane? thx

Answered by jack ostrick
Posted Fri, 01/04/2013 - 12:54

12.
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Assuming its the ~1800 sq ft with 2 bathrooms that need DHW.

Answered by jack ostrick
Posted Fri, 01/04/2013 - 12:57

13.
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Jack,
My guess is that your existing Carrier heat pump can probably handle the whole house. What you need are more ducts -- not a new heating system.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 01/04/2013 - 13:03

14.
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I I can see where Jack is coming from... most Designers do not take in consideration how an HVAC system should be designed to service three floors. Maybe there is a way to get ventilation from the basement unit, but maybe not, and maybe the HVAC system was an after thought.
My take (only a guess) is that he may need a 3 ton, 2 stages, and 16 SEER/40kbtuh variable speeds AH for the whole house or even a 75gal. WH-hydronic system would work too, but IF all had been done upfront. Now Jack must piece meal a system,
I would think a 2 ton, 2 stage, 16 SEER with a 50gal. WH-hydronic system for down stairs and basement, AND a 2 ton, 2 stage, 16 SEER (slightly oversized) with a 30gal. WH-hydronic system for up-stairs maybe a good compromise.

Answered by Armando Cobo
Posted Fri, 01/04/2013 - 14:11

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Jack - I completely agree with Martin and Dana. It's very unlikely that whoever did your Manual J made only one mistake, they probably made several. To put this in context, having a Manual J done right the first time is about as common as a unicorn sighting. And by this I mean not very common. There should not be a duct load on ducts in conditioned space. You should carefully compare all aspects of your building with the inputs on the Man J. Pay especially close attention to the infiltration #''s, as this is probably the easiest place to hide extra Btu's.

Not sure what your design conditions are, but my quick review of the AHRI directory shows 29 active Carrier heat pumps with a 17F output > 50,000 Btu/hr and an HSPF > 10. Carrier probably makes the best controls for zoning, so IMO you should use one ducted heat pump for the entire house. Also, not that you asked, but Carrier's Tstats have controls for a central fan integrated supply for ventilation air (many Carrier installers don't realize this) so using a CFIS for ventilation air might also make sense.

Answered by Jesse Smith
Posted Sat, 01/05/2013 - 12:25

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