Heat Pump - Twin Air Handlers

[Bill Kibbel]

I inspected a large farmhouse today that had heat pumps retrofitted in the mid '90s. There are 3 condensing units and 4 air handlers/evaporators. Back-up is a propane boiler for hydro. The "twin" air handlers in a basement are connected with a "T" to a single lineset that connects to a single condensing unit.

I've never seen this set-up before. I've seen many split ductless ACs and HPs that have 2 or 3 indoor units running off of a single condensing unit, but they all have separate linesets. Anyone else?

I was looking forward to seeing this system in action, but the right system wouldn't work on any mode. The condensing unit was 2 tons and the evaporators were each 1.5 ton. Any comments?

[Chad Fabry]

My brain won't wrap around how that works. Splitting what has to be the liquid line (at least 50% of the time) would cause a refrigerant phase change in an undesirable location.

[Bain]

The condenser can never be larger than the evaporator coil. The system won't operate properly unless it's undercharged. I ran into something similar several months ago and called a master HVAC contractor/friend to make certain I didn't screw up, and that's what he told me.

[Phillip]

I have never seen that set up there. I cannot see how it would work.

I have installed two furnaces side by side and tied them into a coil that was as large as the top of both furnaces. I don't know why they did it, all I was told to do was to connect them together and connect them to the ductwork and make sure they did not leak.

[Bill Kibbel]

Thanks for the replies, guys.

Where's Terry? He seems to always have the final word HVAC issues.

[Terence McCann]

Hey Bill, just got back from a weeks vacation. The wife and I went hiking in Hocking Hills, Ohio. Great scenery - Old Mans Cave, Ash Cave & Cedar Falls to name a few. Beautiful terra-forming caused by retreating glaciers.

You were right when you called it "twinned". If you Google "Carrier Twinning Kit" you'll come up with quite a few good links. Two furnaces with one condensing unit. I've only seen two of these systems in action and that was many years ago.

Off the top of my head: As long as you have a full column of liquid to the TXV (expansion valve) and as long as the super heat is correct everyone should play nicely (however flow rates through conventional tee's is the problem). I'll keep digging to see what I can come up with for a piping diagram. The twinned systems do work though.

[hausdok]

Hi,

I've been pondering this for a day or two. Now, I could be wrong, since I'd only been trained on auto AC systems and that was 39 years ago and the last time I touched one was in 1975, but it seems to me that, if the combined capacity of the two evaporator coils were to equal that of a single evaporator designed for the same condensing unit, the only thing preventing this from working right is flow rates to each.

You stated that the one on the right isn't working correctly. Well, if the flow rate to that A-coil doesn't meet what's needed, it's not going to work, is it? I don't know much about fluid dynamics, but that line branches with a 90° Tee, which I believe will make for some cavitation and prevent proper distribution to one of the two A-coils - in this case, the right one. Common sense is telling me that, unless you have a means to ensure that distribution into each of those lines is equal to exactly what each evaporator needs, one or both is going to suffer.

It's like the brakes on a car; if you don't have an equalizing valve ensuring that pressure equalizes at every wheel, you're brakes are going to be more effective on one wheel than another and you'll pull left or right or find yourself diving every time you put on the brakes.

So, what's balancing this system? It seems to me there should have been some kind of manifold instead of a Tee, so that fluid can flow into the manifold and then from there flow equally to both evaporators, or, if one evaporator is a different size from the other, some type of regulator should have been installed to ensure that only the proper amount of refrigerant flows to each.

Does that make sense to anyone?

ONE TEAM - ONE FIGHT!!!

Mike

[Bill Kibbel]

Terry,

I've heard of Hocking Hills. It's near Columbus right? They're probably the only hills in the state. [:-tong2] I spent some time in OH; Grandma was in Shaker Heights, Niece & Nephew were at OSU, Sister is in Cincinnati & I went to Wittenberg U for a short time. We'll check it out the next time we drive through.

Terry & Mike,

I'm not up on the science behind refrigeration, but the flow of refrigerant through a "T" is what came into my mind first. I even joked with the buyer and said "how does half the refrigerant know it needs to make a left?"

I thought twinning referred to installing 2 systems into a single zone duct system and the "kit" would be the new controls.

[Terence McCann]

Originally posted by inspecthistoric

I thought twinning referred to installing 2 systems into a single zone duct system and the "kit" would be the new controls.

That's correct.

[JEuriech]

If I was to guess, I would think that the type of metering device would determine how well the refrigerant was balanced going to each evaporator.

If both metering devices had a fixed orifice, then I would suspect that you would have great difficulty balancing the refrigerant.

If both metering devices had thermostatic expansion valves, I don't think you would have a problem balancing both units.

I would also be concerned with the size of the evaporators versus the condenser. You have two 1 1/2 ton evaporators (3 tons total) and only a 2 ton capacity condenser. You have the ability to absorb more heat then the condenser can get rid of. I would think that you would now have an unbalanced system that would not run very efficient.

Jeff Euriech

Peoria Arizona

[Terence McCann]

Rule of thumb is you can oversize the evap by 1/2 ton. This use to be standard practice for better dehumidification. If you go much larger you run into the problem of not boiling off all the refrigerant and bringing back liquid to the compressor (3 ton evap, 3 ton expansion valve, 2 ton condensing unit). If you used a 3 ton evap with a 2 ton expansion valve then there would be the problem of to high of a superheat coming back to the compressor (to much coil and not enough refrigerant).

[dseletsky]

Originally posted by Terence McCann

Rule of thumb is you can oversize the evap by 1/2 ton. This use to be standard practice for better dehumidification. If you go much larger you run into the problem of not boiling off all the refrigerant and bringing back liquid to the compressor (3 ton evap, 3 ton expansion valve, 2 ton condensing unit). If you used a 3 ton evap with a 2 ton expansion valve then there would be the problem of to high of a superheat coming back to the compressor (to much coil and not enough refrigerant).