Help w/ Sizing Dual Fuel Heat Pump

13 years ago

I could use some help with properly sizing a hybrid heat pump/gas furnace system for my home. Thinking about getting a 16 SEER 95%, AFUE system.

I've had 3 contractors work on proposals for me and all are telling me that performing a manual J calculation is kind of pointless because the size of the house means that even the biggest residential systems (5 tons?) would still be undersized. Is it really possible to ballpark it given the size of the house? Should I force the issue and get a manual J (have to pay extra $ for it).

16 year old wood frame single family house in Mid-Atlantic (NJ)

2 story colonial w/ un-conditioned full basement

5 bedrooms

4,065 sq ft

37,941 cu ft volume

single zone forced air heat/cooling

They all say this house should have been built with 2 zones (I didn't build it) and recommend converting to 2 zones or installing in-duct zoning. Economically at this point I'm thinking to replace the current system but keep it a single zone, then in the future install a 2nd floor zone if necessary.

Any help would be appreciated.

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Comments (5)

tigerdunes
13 years ago
smackboy

if best comfort and control is important to you, then go with two systems. if not two systems, then one system with electronic zoning controls. if not zoning controls at least have adequate damper control to help balance supply air and temp between the zones.ductwork both supply and return should be inspected. any hot/cold spots should be addressed. if five ton of cooling is not adequate based on load, then you will be very disappointed with just one system.

and do an analysis of cost of electric vs nat gas. I had always heard that NJ had high electric rates and HPs were not recommended.

what size AC and furnace are you replacing? find a good dealer willing to put in the time to provide a professionally performed load calc for correct sizing.

IMO
david_cary
13 years ago
I'd get a manual J just to see what you are dealing with. And ditto the high electric rate comment above. If Man J says you need 8 tons, then you know where you are at. But if the house stays cool in the summer and you are using NG, I'm not sure why it matters. If you are undersized on the heating size, that is fine. You have a good backup.

This is the way I look at it (that doesn't mean it is right), you want to use HP, when it is above 30 degrees or so. Your 5 tons should do a good job down to that. If your backup was expensive, you'd want to HP down to 15 or so. Then, your undersized system would be a problem. But since you are just using HP for milder weather, you are fine - on the heating side. The HP gets less efficient as it gets colder regardless of size of unit. So you never really need the extra capacity, because you switch over to your good backup.

As Tigerdunes said, if your electric rates are high, your backup might make sense at 40 degrees. In that case, your HP is doing very little.

By my math, with my system, 35 degrees is my switch over at $.10 a KWH and $1 a therm. If you are $.15 a KWH and $1 a therm (therms are pretty constant nationally), then your switchover will be closer to 40 degrees.
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smackboy1
Original Author
13 years ago
Thanks for the replies so far. I have a Lennox system: 5 tons 10 SEER AC and 81% AFUE gas furnace. I just got HVAC-Calc and I'm playing around with the dimensions of my house. It seems like it's pretty complicated and it's apparent that my original equipment was probably incorrectly sized for my house. The correct size is eluding me right now. Methinks I will have to find better contractors.
neohioheatpump
13 years ago
get highest single stage heatpump system in 5 tons. I would also get a 95% gas furnace thats 2 stage. If you want to get extra fancy you can get a 2-stage 5 ton condensor which will run in its low stage most of the time.

4,000 sq. feet is alot of finished sq. feet. I think 5 tons is the right size.
funnycide
13 years ago
The one thing you didn't mention is how happy are you with the current system? Do you have hot or cold spots? Are you comfortable on the hottest and coldest days of the year?

Help w/ Sizing Dual Fuel Heat Pump

Comments (5)

tigerdunes

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Help w/ Sizing Dual Fuel Heat Pump

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tigerdunes

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