Gas and Oil Home Heating Furnaces - Increase Size of Emergency Heat

Hello: Replacing our worn-out 3 ton heat pump system with a new 4 ton (sq footage has increased 800+ since the original 3 ton was installed)

Question on sizing the electric resistance heat in the indoor AHU, the current unit is 15KW, supply house suggests an increase to 20KW.

How do we determine if this increase requires a new circuit breaker/wire size to the electric heat?

Existing breaker size is 50 amps. The spec sheet on the new 20KW heat calls for: 1) Circuit 1 Min Ampacity of 48 and Max Overcurrent Protection Amps of 60. 2) Circuit 2 Min Ampacity 43 and Max overcurrent of 60.

Do we need to increase to 60 amp breaker and appropriate wire size?

If the home is bigger and they ran a heat load on it Yes you need more heat. First off if you put a new unit outside you also have to put a new blower coil unit inside and it will have more heat in it. So

Yes you have to rewire for the new unit to the spec's it calls for.
Wire and all. ;) ED

I'm not a pro, but running the numbers, I don't see how you have enough amperage to run the 15KW system with only 1 circuit. Isn't there already a second circuit? Am I figuring this correctly - 15000 divided by 240 = 62.5 amps?? Either way, the first circuit you have will work, but you'll either have to add or upgrade an additional circuit.

Doug M.

add another circuit or upsize the supply wire and breaker to size rated for new load

My mistake, there are already two circuits to the exisitng air handler...


Ckt 1: 50 amp breaker in the main panel, AWG 6-2 w/ 10 ga ground

Ckt 2: 30 amp breaker in the main panel, AWG 10-2 w/ 10 ga ground

Both wire runs are less than 10 feet

Opened up the new air handler tonite, both circuits have an internal 60 amp circuit breaker...my guess is I will need to upgrade the existing 30 and 50 amp panel breakers to 60 amp, and size wire accordingly....

Thanks!

Yes, you'll have to change/upgrade all the electrical from the breaker panel to the air handler. I trust you are also replacing the ductwork?

I would upgrade the 30amp circuit to a 60amp and use the 50amp circuit as is. It's still within spec, is running 10KW of the 15KW total now and will be running 10KW of the 20KW total with the new system. It should only draw around 41 AMPS. More than enough margin of error. Why spend more than is needed?

New ductwork too? Great idea if it it's feasible.

Doug M.

Most installers I have spoken too seem to feel the ductwork is large enough.

House is 2800 sq. feet, but 800 of that was unfinished at the time the heat pump was first installed. I think the builder/owner sized the ductwork for the future expansion but went a little small with the 3 ton unit.

What's a good air velocity number for a 4 ton heat pump?

Need about 400 to 500 cfm per ton.Or old rule 60" to 80 "per ton for the duct.;) ED

Just wanted to close the loop, and say thanks to all who offered advice on this one.

Got the new system installed last week, and everything is working great!