Hey, it’s me again. Hang on for this one!

[Mikebarone]

Hey, it’s me again. Hang on for this one!

I am in Phoenix Arizona. A close friend of mine that has a 5 ton A/C unit, (that heats with propane gas) on her house. Her house is 2700 square feet, and the wall insulation is just moderate. I think the unit is under sized for the house. I would like to add an additional 3-1/2 ton heat pump on the house, but I would like to hook them up to run like a two stage compressor would run. I would like to have the 3-1/2 ton heat pump run as the primary unit, and then when it’s not keeping up with the set temperature setting on the thermostat, it would shut off the heat pump and turn on the existing 5 ton unit.
Could I do this with a two stage, (heat/cool) thermostat? Even if I had to install like a low voltage relay to the second temperature setting on the thermostat that would shut off the 3-1/2 ton heat pump while the existing 5 ton unit is running.
I know this sounds, a little “off of the norm”, but I got the heat pump for nothing and her propane bill is out of site. Before I do any of this work, I am going to do a leak test on her duct work system, and see if that is why her propane bill is so high.

Thanks, Mike

[hj]

If the existing unit is undersized for the house, why would you turn off the auxiliary unit, just at the point where it might be needed? If the distribution ductwork is adequate for both units, then all you might have to do is add an additional return duct to the new heat pump. Do not tie it into the existing unit's return duct.

[nhmaster]

Yea, I don't think I'd want to shut one unit off, I'd rather stage them both.

[Furd]

He stated that the cost of propane for the absorption unit was "out of site" [sic].

There are several reasons why the absorption unit may not be performing as well now as when new. Short of doing a heat gain calculation on the house nobody knows if the 5 ton rated unit is sufficient to properly cool the house. If the cost of operating the absorption unit is significantly higher than the 3.5 ton then it may make very good sense to operate the smaller unit at all times that it can handle the load and only "light off" the larger unit (by itself) when needed.

Anyway, to answer the question, yes, you could use a two stage cooling thermostat to do this. It would probably require a few control relays to accomplish the interface but I'm just going out the door so don't ask me how tonight.

[Mikebarone]

If the existing unit is undersized for the house, why would you turn off the auxiliary unit, just at the point where it might be needed? If the distribution ductwork is adequate for both units, then all you might have to do is add an additional return duct to the new heat pump. Do not tie it into the existing unit's return duct.

Yeah, I wasn't going to shut off the existing 5 ton unit. I was going to just make the 3.5 ton the primary unite, and if need be, the two stage thermostat would shut off the smaller unit and let the larger existing unit come on until the immediate need was met, and then the larger unit would shut off, and let the smaller unit run again…like a two stage compressor would do.
Thanks for that suggestion of not tying the new unit into the existing air returns. To tell you the truth, I didn’t even want to think that far ahead, until I could get both units to run "in stages".
Even if I tie the newer unit into the existing delivery duct work, I think I would have to install a pressure damper, (at the existing air handler) so the new unit wouldn’t push are backwards through the existing air handler and out the existing air returns. I guess that would be one way to keep the filters clean…..LOL!

Thanks again!

[Mikebarone]

Yea, I don't think I'd want to shut one unit off, I'd rather stage them both.

Sorry if I made it sound like I was going to shut the existing unit off. I do want to stage them both, (as you said).

Thanks, Mike

[Mikebarone]

He stated that the cost of propane for the absorption unit was "out of site" [sic].

There are several reasons why the absorption unit may not be performing as well now as when new. Short of doing a heat gain calculation on the house nobody knows if the 5 ton rated unit is sufficient to properly cool the house. If the cost of operating the absorption unit is significantly higher than the 3.5 ton then it may make very good sense to operate the smaller unit at all times that it can handle the load and only "light off" the larger unit (by itself) when needed.

Anyway, to answer the question, yes, you could use a two stage cooling thermostat to do this. It would probably require a few control relays to accomplish the interface but I'm just going out the door so don't ask me how tonight.

I have had some experience on using a two stage thermostats. On the same house, about two years ago, the original installers put the air handler, (in the attic) at the far West end of the house, (where the bedrooms are). But the large family room, laundry room, bathroom, and small office, (all on the far East end of the house) that got hardly no air at all. So, about the middle of the house, (up in the attic) I cut into the 14” delivery duct, and I installed a 1500 cfm attic gable fan. Then, at the end of the east run, (just before it branched out to all of the room) I installed another smaller a.c. booster fan. I tied both of the booster fan, to a two stage thermostat and I had to hook up a relay to run the 110 volt attic fan. I also installed a switch in the hallway, so when they go to bed, they can manually shut off the booster fans, so they won’t be conditioning the rooms they aren’t using for the night. It works trick….when both booster fans are on, it actually suck air from the delivery vents in the bedrooms on the West end of the house to cool the East end of the house. I’ve done this to two houses so far, and both of them were success stories.
Yeah, any ideas you have to help me out on the two stage thermostat and the relays, I sure would appreciate!

Thanks, Mike