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Heat pump sizing

Posted by Hotrod1952 (My Page) on
Sat, Feb 9, 13 at 19:38

I have a 1500Sq ft Home built in the late 60's. Currently has a 2.5 ton A/C central air and 60,000 Btu propane furnace. When propane went to $2 a couple years ago I added a 60kw electric duct heater and switched my home to all electric. Saved a bunch of money on heat bills. My question is the A/C unit is really old and I want to upgrade the entire system getting rid of all propane etc. With a Air to Air heat pump do I need to size it for the 60,000 BTU or for the 30,000 Btu A/C load? I live in Northern Iowa. Why would there be such a large difference in the heat/cool sizes?


Follow-Up Postings:

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RE: Heat pump sizing

60kw electric duct heater? Are you sure about that? Thats about 6 times larger than you need... im not even sure if they make that size. It might possibly be a 15kw heater thats pulling around 60amps. As far as your heat pump sizing goes, it is always best to have a company do a load calculation on your home.

Hope this helps


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RE: Heat pump sizing

I would want to see a load calculation both cooling and heating at correct design temps both inside and out first to be sized correctly.

The general rule is you size a heat pump system to cooling with backup heat strip for the additional BTUs required for heating.

One thing to take into consideration is the possibility of upgrading your electric panel service for the heat strips. The heat strips should be wired
staged and be placed on their own circuit.

Just for info, heat strips are sized by KW. Each KW of heat strip yields
about 3400 BTUs.

For your location, I would not purchase a new HP system that rid not have electronic demand defrost. You might be a good candidate for Carrier's relatively new Greenspeed HP system depending on your budget.

IMO


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RE: Heat pump sizing

If you have a 60kW heater you must also have a 400-amp service entrance!

I too would recommend looking into a Carrier Infinity Greenspeed system.

SR


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RE: Heat pump sizing

Of course this duct heater would need to be decommissioned.

IMO


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RE: Heat pump sizing

My bad it is a 20kw duct heater!


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RE: Heat pump sizing

I went to http://www.alpinehomeair.com/Furnace-choosingsize.cfm#top and used there calculator to get a rough estimate of unit size etc. (I realize a heat/A/C load calulation is a good idea prior to buying.) The calculator comes up with 2.5 ton Cooling load and 60,000 Btu heating required. So does that mean I need a 5 ton Air handler and use a 2.5 ton heat pump so I can move enough air for 20 kw electric heat load? As to the other part of my original question my guess is that during A/C season I would have a Max delta T of say 30 degrees and during heat season I would have a max delta T of say 80 degrees so that is why the size difference. Am I right on that?

This post was edited by Hotrod1952 on Sun, Feb 10, 13 at 12:43


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RE: Heat pump sizing

Hot rod,

You need a 2 1/2 ton HP condenser with best matching AHRI air handler. You do not need a 5 ton air handler. Don't get confused on this issue.

And you certainly do not need a 20 KW heat strip. I would recommend no more than a 15 KW staged in three 5 KW increments.

If not going with the pricey Greenspeed HP, I would strongly recommend a brand/model that has electronic demand defrost. EDD eliminates/reduces unnecessary, nuisance, and expensive defrost calls.

IMO


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RE: Heat pump sizing

Thanks for you answers. Just trying to learn a bit before getting some quotes. Goodman and Rheem are readily available in my area. Do either of those use Demand defrost? Also ran into some reading about a "2 stage" what does that mean?


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RE: Heat pump sizing

Rheem is fine depending on model.

Stay away from Goodman HPs.

IMO


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RE: Heat pump sizing

2-stage means that the unit can run at two power levels depending on the load. Usually this is 50% - 100%, but some are more like 75% - 100%. The idea is that it cuts down the power usage during moderate temperatures as well as short cycling.


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RE: Heat pump sizing

Hey Tigerdunes what Rheem do you reccomend and why?


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RE: Heat pump sizing

Here is my general spec sheet for new system.

both outside and inside units should be replaced to have a properly matched system.

15 SEER, 12.5+ EER, 9 HSPF
best matching VS air handler
full BTUs in both cooling and heating for your rated size
R-410a refrigerant(same as Puron)
scroll compressor preferred
electronic demand defrost preferred
thermostat with "dehumidify on demand" feature
staged backup heat strips
new and correctly sized refrigerant lineset
10 yr warranty on parts and compressor

you want a thorough inspection of your ductwork system. size, overall condition, supply and return lines, insulation qualities, leak test, etc.

any hot/cold spot issues in your home should be addressed.

My personal recommendation is Trane/AmStd, Rudd/Rheem, and Carrier/Bryant.

Depending on your location, I would not purchase a new system that did not have electronic demand defrost.

As far as Rheem, any of the models other than the value series. No need for the two stage models. Stay with a good single stage model like RPNL model or RPQL.

I have attached a link.

IMO

Here is a link that might be useful: Rheem Heat Pumps


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