underfloor heating and tandem breakers

[bob715]

Hi,

this is a bit complicated.

We installed two separate underfloor heating system in our downstairs area.
One at 240V and according to the manufacturer at 11.9Amps. The other one also at 240V and 9.7Amps.

Because of the lack of space in the 200Amp panel, we connected them to two separate tandem breakers, each one at 2*15Amps. The panel is new and all electricians involved in this agree that it can handle tandems.

There are two thermostats one for each circuit. The thermostats can handle up to 15Amps, that's why we have two.

Here is what happens: Only one circuit will heat up! And it really depends which ones of the thermostats I crank up first.

My installer thinks it's because of the tandems. We checked and they are installed properly. Still he says it's because of the tandems (without providing further evidence).

We also talked to the underfloor heating provider. They said the tandems should be at least 20Amps. But my installer disagrees because the thermostat can only handle 15Amps each...

I was wondering what you guys think about this. Why could it be that tandems "generally" don't work for this type of installation or why 20Amps tandems might work?

Thanks

 

[Lucien Nunes]

UK readers - the OP appears to be in the USA, perhaps Lancaster PA or CA, not Lancaster in Lancashire.
OP - this is mainly the UK section of the forum.
It seems to have been moved to the USA section

Are you saying that you fitted one tandem on each phase in the panel, and connected each heating zone from one side of one tandem to one side of the other tandem to get 240V? I'm not sure that meets code but let's wait for confirmation on that, it won't stop it working it's just about isolation safety.

Are the breakers tripping? If not, and both zones work OK independently, I can't see that the breakers have anything to do with the problem. There can be issues if your panel is packed with tandems running heating circuits as the heat dissipation in the breakers is more concentrated than with ordinary SP or DP breakers, but the problem manifests as premature tripping and you haven't said they're tripping. I can't see a problem with the breakers being 15A, especially if they are not tripping at all.

I think you would need to take some measurements. If one zone won't heat, where is the power actually getting to? Is there 240V leaving the breakers and is the thermostat closed i.e. voltage reaching all the way to the element? Actually as it's resistive heating elements it could be tested more safely with the circuit off using resistance tests, and the resistance of the element can be checked at a time when it refuses to heat. Without tests we'd just be guessing how the two zones are interacting. Is there any possibility that the wiring of the two thermostats and two elements has got badly muddled up in a j-box somewhere and the two are effectively in series?

 

[bob715]

UK readers - the OP appears to be in the USA, perhaps Lancaster PA or CA, not Lancaster in Lancashire.
OP - this is mainly the UK section of the forum.
It seems to have been moved to the USA section

Are you saying that you fitted one tandem on each phase in the panel, and connected each heating zone from one side of one tandem to one side of the other tandem to get 240V? I'm not sure that meets code but let's wait for confirmation on that, it won't stop it working it's just about isolation safety.

Are the breakers tripping? If not, and both zones work OK independently, I can't see that the breakers have anything to do with the problem. There can be issues if your panel is packed with tandems running heating circuits as the heat dissipation in the breakers is more concentrated than with ordinary SP or DP breakers, but the problem manifests as premature tripping and you haven't said they're tripping. I can't see a problem with the breakers being 15A, especially if they are not tripping at all.

I think you would need to take some measurements. If one zone won't heat, where is the power actually getting to? Is there 240V leaving the breakers and is the thermostat closed i.e. voltage reaching all the way to the element? Actually as it's resistive heating elements it could be tested more safely with the circuit off using resistance tests, and the resistance of the element can be checked at a time when it refuses to heat. Without tests we'd just be guessing how the two zones are interacting. Is there any possibility that the wiring of the two thermostats and two elements has got badly muddled up in a j-box somewhere and the two are effectively in series?

1) Indeed that's how we fitted the wires in the panel to get to 240V
2) no breakers are tripping ever.
3) we have a lot of tandems as the electrician put in a smaller panel than I wanted him to put (but I didn't pay enough attention...learning for next time)
4) we did these tests below and they all passed:
Test 1: Conductor resistance (required)
Test 2: Conductor and ground braid continuity (required)
Test 4: Thermostat Floor Temperature Sensor Test (required)
Test 3 "Insulation resistance (recommended)" wasn't done because the guy didn't have a megohmmeter.

Are there any other tests that you can think of?

5) there is absolutely the possibility that they messed the wires up. We have the thermostats next to each other and the heating floor wires coming from the same direction. If the wires were crossed at the thermostats however, would the system work at all?

 

[Megawatt]

Hi,

this is a bit complicated.

We installed two separate underfloor heating system in our downstairs area.
One at 240V and according to the manufacturer at 11.9Amps. The other one also at 240V and 9.7Amps.

Because of the lack of space in the 200Amp panel, we connected them to two separate tandem breakers, each one at 2*15Amps. The panel is new and all electricians involved in this agree that it can handle tandems.

There are two thermostats one for each circuit. The thermostats can handle up to 15Amps, that's why we have two.

Here is what happens: Only one circuit will heat up! And it really depends which ones of the thermostats I crank up first.

My installer thinks it's because of the tandems. We checked and they are installed properly. Still he says it's because of the tandems (without providing further evidence).

We also talked to the underfloor heating provider. They said the tandems should be at least 20Amps. But my installer disagrees because the thermostat can only handle 15Amps each...

I was wondering what you guys think about this. Why could it be that tandems "generally" don't work for this type of installation or why 20Amps tandems might work?

Thanks

The tandems will withstand the amperage that’s marked on the breaker. I’m my opinion you can use 20 amp tandem breakers. We are talking about 5 more amps so no problems. Don’t listen to your installer because the provider knows what it requires. I hate to say this but your electrician is wrong.