2 A/C condensers in "tandem"?

[sw_ross]

I have a customer that has a small cafe, he is having issues with the condensing unit for the a/c. The Hvac guys came out and put a hard start kit on it and told him it was getting old and might be on its way out.
They proposed that they install a second condenser and "tandem" them up.

I've never heard of this being done...

 

[hbiss]

My question would be why would you want to do that and not just replace the first?

I assume adding a second unit would involve additional wiring and a disconnect.

-Hal

 

[Dennis Alwon]

We have installed many units where the blower unit was double upped but never the a/c unit. I also think this is a strange comment not because of doubling up but if one was bad why would you leave it there

 

[480sparky]

I got a nickel that says the owner misinterpreted what the tin-knockers said. It's possible to replace one large AC with multiple smaller units. In most cases, they are staged and each condenser will start up in a predetermined sequence. The tinners I work with call this 'twinning'.

 

[sw_ross]

That was the first thing I asked him! If it's going bad, why not replace it with a new one! I'm not sure if the Hvac company was good at explaining why this proposal was the best option...

The owner wanted me to look at the option of feeding the second condenser off the same disconnect...
I told him no, they each need their own disco and each needs to be protected with the properly sized OCPD.

He seems to think the existing condenser is undersized. I told him the conductors can handle a larger unit if the Hvac guys wanted to do that. He said they told him the air handler (fan motor? A coil?) couldn't handle a larger unit. I'm not sure how that differs from adding a second condenser, unless he (and maybe me) isn't understanding what the Hvac guys are talking about.

Changing out the condensing unit doesn't require additional wiring, maybe just changing out the breaker to proper sized breaker.
Obviously, Adding a second condenser requires additional wiring.

 

[480sparky]

You can still add more condensers without bringing an entirely new circuit in for it. I rewired a massive 4-pump compressor system in a church that was fed with a 200a branch. I replaced the 200a non-fused disco with a 12-space panel and put three 40amp breakers in it to feed three small condensers.

 

[drcampbell]

... He seems to think the existing condenser is undersized. ...

That's almost never the case, especially in an older building.

First, they estimate the building's cooling load. The result of that calculation is never a nice cardinal number; it's necessary to select a condensing unit in the next standard size up. But they usually select one that's two sizes up, "just to be sure". Customers get really annoyed when an air conditioner can't cool the building on the hottest days, but not so much when the energy consumption is higher than it needs to be or the humidity control is poor.

Some years later, the original unit reaches the end of its life, they read the capacity on its nameplate and install the next size up, "just to be sure".

Some more years later, the second unit reaches the end of its life, they read the capacity on its nameplate and install the next size up, "just to be sure".

et cetera ...

(the same thing happens with heating equipment)



Twinning can provide better comfort, humidity control and energy efficiency when two different size units are installed. For example, if a 10kW and a 20kW unit is installed, you can have 10, 20 or 30 kW of cooling using only simple on-off contactors and without resorting to a VFD.

 

[kwired]

He said they told him the air handler (fan motor? A coil?) couldn't handle a larger unit. I'm not sure how that differs from adding a second condenser, unless he (and maybe me) isn't understanding what the Hvac guys are talking about.

Generally the more cooling capacity you have the more CFM of air you need to move over the coils. Not enough air over coil is as bad as having plugged filter, gets to a point where it can ice up, or even have freon not evaporate when it passes through the evaporator. You don't want liquid to return to the compressor.

Twinning can provide better comfort, humidity control and energy efficiency when two different size units are installed. For example, if a 10kW and a 20kW unit is installed, you can have 10, 20 or 30 kW of cooling using only simple on-off contactors and without resorting to a VFD.

So can two stage compressor. Twinning I think is often chosen when a single unit would be larger then 5 ton unit, or over a certain BTU of heating or both.