Electric Heat kit minimum CFM

[reh22]

I have a 2 ton Heat Pump paired with 15kW of electric heating. Manual J shows 10kW total required in the winter. The extra 5kW was installed as emergency heat and for when the outdoor temps drop below design.

My air handler specifies a minimum of 875 cfm for 10kW strips and 1050 cfm for 15kW. Unfortunately, my options with the multi speed fan are 880 cfm or 1385 cfm at 0.5” wc. The high speed definitely makes a lot of noise and I’m concerned about it overloading the ductwork or tripping out the fan on high static (it has never done this yet). It’s also noisy and doesn’t heat evenly at that high speed.

Would it hurt the system to run the full 15kW at the lower 880 cfm? Or is the 1050 cfm suggested a hard minimum? I do not want to make the system cycle on the thermal limit switches (they are 150F-50)

For reference, the air handler is a Goodman ASPT29B14.

Thanks in advance for your expertise.

[pecmsg]

Whats the actual Static Pressure on the unit?

[James Colver]

Well I'd expect a 2 ton air handler to be running at it's highest speed if it's using a 15 heat strip. Second I would not suggest oversizing your heat strip. You'll spend hundreds more every winter.......

[garyed]

I'm not a fan of staging heat strips but I think in your case it would be the best solution.
I would just set the system up to run normally at the 880 or so CFM's with 10kw & then to go to the higher speed only if EM heat is called for along with the other 5kw.

[reh22]

I was assuming 0.5” static, but have to double check it.

Part of my problem is my dumb ecobee thermostat keeps calling for the second stage of Aux heat when the heat pump locks out. I’ve even got it set to manual staging but for whatever reason it goes for it even when the temperature requirement isn’t met yet.

[beenthere]

I was assuming 0.5” static, but have to double check it.

Part of my problem is my dumb ecobee thermostat keeps calling for the second stage of Aux heat when the heat pump locks out. I’ve even got it set to manual staging but for whatever reason it goes for it even when the temperature requirement isn’t met yet.

The static pressure can't be .5" at 880, 1035, and 1385 CFM.

What temp are you locking out the heat pump, and or aux heat?

Moving 880 CFM with 15KW of aux heat, will give a temp rise of 54°F.

[reh22]

Right now I lock out the heat pump at 20 degrees.

Is temp rise the driving factor behind these minimum air flows provided in the air handler specification, or is it minimum to prevent the heat kit from cycling on high temperature?

[VTP99]

Both

[garyed]

Right now I lock out the heat pump at 20 degrees.

Is temp rise the driving factor behind these minimum air flows provided in the air handler specification, or is it minimum to prevent the heat kit from cycling on high temperature?

There is really no reason to lock a heat pump out at 20 degrees unless you have a dual fuel system. A heat pump is still more efficient than heat strips even down to 0 degrees. The only down side is the efficiency loss during defrost but the Heat Pump is still more efficient at colder temps than strips. Just make sure the defrost is set to 90 minutes. What you might consider is to have the outside thermostat wired to activate the strips along with the heat pump at colder temperatures. That would give you more heat & still get whatever efficiency gain you can from the heat pump.

[reh22]

Thanks. I actually don’t have an outdoor air temp sensor on the unit, we never had it put in since the Ecobee has the lockout function built in. I might go ahead and have one installed if I can use it to activate W1 when temps get cold and leave the lockout to the ecobee when it’s down below 0.

As for the full 15 kW of heat, I’ll leave that on the high fan speed to prevent the unit from cycling. Right now I have W2 jumpered to speed 5. It’s an X-13 type motor. Not sure if the speed taps could possibly back feed the contactor and run the heat strips? If so, I’ll switch over to a relay for this function.

[VTP99]

What's the condenser model number ?