heat pump with hydronic coil in air handler

Pat McEnroe

Having a strong technical background but nearly no experience with HVAC, your reply was perfect - get down to the fundamental theory of operation instead of a buch of tech talk peppered with insider terms. Thank you for making this clear to me. Now if someone could just tell me if the compressor output is turned off on the Honeywell stat if I switch the configuration to 'equipment controls fan' instead of 'electric heat' I'll be all set. Then I can buy a relay and be done. Otherwise, I need to look for a different stat or a dual fuel kit and disable the outside air temp lockouts on the existing stat.

Pat McEnroe

heat pump with hydronic coil in air handler

I've installed a hydronic coil in my air handler (spacepak system). I've installed a Honeywell VisionPro 8000 thermostat, along with an outdoor air sensor. I've set the lockout points, but their is a range where both the heat pump and the auxillary (hydronic) will run at the same time. My concern is this: if the stat calls for both stages of heat in the temperature range between the auxillary lockout and heatpump lockout, both systems will be running. Since the hydronic coil is ahead of the heatpump coil, the air will be preheated and I'm worried that this might cause some damage to the heat pump. Should I be worried or will I just be wasting some energy in that temperature range?

Wayco Wayne_2

I would worry

If the hydronic coil is heating the air before you caould go off on high head pressure. I like to go downstream from the heat pump coil to avoid this problem. WW

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Pat McEnroe

The hydronic coil is heating the air ahead of the heat pump coil (this is the design of the spacepak system). The water temperature is on a reset controller. The heat pump lockout temp. is set for 35 deg, and the boiler reset curve puts the water temp to the hydronic coil at 135 deg, so that would be the worst case - maybe 120? deg air going to the inlet of the heat pump coil. Please explain 'going off on high head pressure'.

Thanks!

Wayco Wayne_2

In the heating mode

of the heat pump the air across the refrig coil is filled with hot gas from the outside unit looking to be cooled, and condensed by the air stream from the duct system. With out that cooling, the head pressure will build until it goes off on the high pressure control located in the outside unit. This frequently happens when folks forget to change their filters in a heat pump air handler and not enough cool air goes across the coil and the outdoor unit goes off on safety. If you have hydro air heating the air at the same time as the heat pump is running, it will be even worse. WW

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Techman

Ouch!

Pat! You are hurting the refrigeration system running it this way, the compressor will be damaged!I think the lockout setpoints overlap allowing this to happen. Whenever 2nd stage is called for it locks out the 1st stage.

Pat McEnroe

Thanks! I am 8000 miles from home, so I had my wife open the breaker to the heatpump for now. The honeywell stat does have a configuration that says if the furnace will control the fan. Since I don't have a true dual fuel, I set it for the stat to control the fan (i.e. electric strip heater backup). I'm wondering if I change that setting to let the furnace control the fan (and put a relay in on the pump controller output) then the stat will not call for both at the same time? Not clear from the stat documentation, but seems to stand to reason. The outdoor unit does not have a high pressure switch (next time I won't buy Goodman).

Eugene Silberstein 3

Heat Pumps

When a heat pump system is operating, the head pressure (condenser saturation pressure) depends on the temperature of the air moving across the condenser coil. In the case of a heat pump operating in the heating mode, the condenser coil is located indoors, so that it can reject system heat into the occupied space.

To give you a quick example with random numbers, let's say that the space temperature is 70 degrees and there is a 40 degree temperature rise across the hydrocoil and a 35 degree temperature rise across the indoor (condenser) coil of the system.

Scenario #1 - Heat pump coil upstream of the hydro-coil:

Air at 70 degrees passes through the heat pump coil and leaves at about 100 degrees. The condenser saturation temperature of the refrigerant is about 105 degrees, which equates to a pressrue (for R-22) of about 209 psig.

Scenario #2 - Heat pump coil downstream of the hydro-coil:

Air at 70 degrees passes through the hydro coil and leaves at about 110 degrees. Air at 110 degrees then flows through the heat pump's condenser coil. The condenser saturation temperature of the refrigerant will now be about 145 degrees, which equates to a pressure (for R-22) of about 355 psig.

Now for the technical stuff. Since the head pressure will be much higher than intended, the compression ratio for the system will be excessively high. This, in turn, will cause the heat of compression to go up, and the coefficient of performance for the sytem to go down.

Since EER is directly related to COP by a factor of 3.413, reducing the COP by a unit of 1 will have a negative effect of 3.413 on the EER of the system.

So, in a nutshell, both stages should not be energized at the same time or, if they are, the heat pump coil should be located downstream of the hydro-coil.

Whew!

Techman

Whew Yourself!

I sure wish I could of said it like that!Thanks Professor!You make want to be more "speakable" like you! Enjoy Your Day!

rucomfy

Thermostat Fan Management

It is my understanding that a "dual-fuel" digital thermostat of this type has a dip switch or sometimes a jumper clip to allow for field set-up for the fan/blower management.

In the electric heat mode the blower will be activated by the thermostat during any call for heat.
In the gas or equipment mode, the furnace or AHU will control the blower operation.

Configure to meet the needs of the install or per mfg. I/O manual. Some hydro heat systems like the older Rheem units needed the thermostat to turn on the blower with the HW circulator. Some AHU have relays on the board to control blower operation. It depends on the AHU, hence the term "fan management" either the thermostat or the AHU.

Compressor operation is not affected and is solely a fuction of the R to Y1 call for heat. The manual will normally show the wiring set-up and often the internal switching, such as the Reversing Valve.

Pat McEnroe

I guess I'll need to test it to be sure, but seems like my only choice is to install a dual fuel kit to get around the problem (or maybe a different stat). For now, my wife is taking the place of that kit - turning off the breaker for the heat pump and turning on the switch for the boiler or vice versa. Works, but I'm sure that I'll have to pay a price for her service!

John Mills_5

8320?

If using the 8320 with outdoor stat, set it as a dual fuel thermostat which is options 200+210 I think. That way it won't run the pump and the hydronic heat at the same time.

Pat McEnroe

8220 commercial. From the manual, I selected for 170 - 7 (2H/1C HP), and for 180 I would select 0 for fossil fuel (right now I have 1 selected for electric). Then I would need to add a relay on hydronic system to pull in the fan, correct?