Over-voltage and Mitsubishi Heat Pumps

[WattsGood]

I installed the branch circuit, disc, etc to supply power to a Mitsubishi brand hybrid electric heat pump (hybrid as in the gas heat stays and is used to supplement if the heat pump alone isn’t enough).
At any rate, the incoming power from the utility company is around 253.2V phase A-B and around 126V phase-ground. The manufacturer, and the HVAC company I worked along side of, state that the incoming voltage is too high and they are having an influx of control boards burning up. They state that a surge protection device should help and that I can install one on the load side of my disc.
I'm sure vocabulary between everyone is slightly askew, but they are asking for a surge protection device, but I am thinking I need something like a rectifier or buck/boost transformer so I can lower my branch circuit down to 240V to prevent, what I'm assuming the issue is, overvoltage and not a surge.

Any one have thoughts one what is feasible? The customer is having a Mitsubishi tech coming out and I'm sure I'll have more info after they look at their equipment.

Thanks!

 

[don_resqcapt19]

That voltage is within the permissible limits in Illinois based on the requirements of the Illinois Commerce Commission, and expect that the limits are pretty much the same in other states.
If there are issues, it is because of incorrect design on the part of Mitsubishi.

 

[Tulsa Electrician]

Here is one diagram says.
I would do a voltage check with it under load.

 

[WattsGood]

That voltage is within the permissible limits in Illinois based on the requirements of the Illinois Commerce Commission, and expect that the limits are pretty much the same in other states.
If there are issues, it is because of incorrect design on the part of Mitsubishi.

I agree. It’s within the voltage rating of the compressor and blowers. It’s their control board that is frying. And their customer service center is apparently working on this influx of board issues in the area. Digging a bit more on the internet, supplyhouse.com has some HVAC surge protection and voltage monitoring devices that might help me out. But I do agree that the design of the equipment should be able to handle a better range of over/under voltages. But alas, the electricians will save the day. Lol

 

[WattsGood]

Here is one diagram says.
I would do a voltage check with it under load.

I overlooked that part. I'll check it under load as well.

 

[LarryFine]

The equipment should have the acceptable voltage range on the nameplate.

Example (you're 0.2v over):

 

[winnie]

A surge protection device cannot reduce the high steady state voltage. (Well, not without burning up...)

It is plausible to me that the high steady state voltage brings the entire system closer to a damage threshold and thus more sensitive to surges.

I buck transformer would be needed to reduce the steady state voltage.

I don't know if such a transformer is necessary to solve this customer problem, but the transformer would both reduce voltage and act as a line impedance reducing surge intensity.

A 120/240V to 12/24V transformer used in buck configuration would do the trick.

 

[ptonsparky]

Maybe a line reactor?

 

[qcroanoke]

try a multitap control xfmr?
If the control board is frying.

 

[WattsGood]

Maybe a line reactor?

I’m not sure what that is.

 

[ptonsparky]

I’m not sure what that is.

Reactors can be used on VFDs. Line and Load. We used them on the line side to filter the spikes seen by POCO capacitor switching and on the load side to smooth the output. FWIU they will reduce the voltage somewhat when used on the line side. 3% would get you down to about 245.
I have never measured it myself, hence the question mark. They are fairly reasonable in price but I do not know how one would compare to a small buck/boost transformer in price. The BB would definitely work for you.

 

[kwired]

try a multitap control xfmr?
If the control board is frying.

We have a winner. The control board is very likely operating on the 24 volt control circuit, the compressor and blower motor(s) can likely take the supply volts and probably even more with little problems.

Could even be something as simple as selecting correct 208 or 240 input tap of control transformer you already have that can make a difference here.

 

[winnie]

This is a minisplit. Very likely it has an inverter controlled compressor and the 'control board' carries power to the motors.

 

[kwired]

This is a minisplit. Very likely it has an inverter controlled compressor and the 'control board' carries power to the motors.

Minisplit that integrates into an existing gas fired heating system?

I presumed it was a more conventional heat pump to add onto an existing forced air system and use the original heat source as backup heat when needed.

 

[winnie]

Minisplit that integrates into an existing gas fired heating system?

Whoops. My misreading.

There are systems where the outdoor unit looks like the minisplit box rather than the traditional central air drum, and there are DC inverter systems with furnace integrated coils.

But on re-read I agree that an inverter fed unit is less likely.

Jon

 

[synchro]

If this is a Mitsubishi PAA series, below is an excerpt from the specs of a 36,000 BTU/HR model:



And so with the measured 253.2V, perhaps using a 24V winding of a buck-boost to drop the voltage to near 230V could be considered. As Jon (winnie) mentioned, a buck boost would also add some impedance to to the line. That could make a surge arrester after the buck boost more effective, and an arrester could also reduce any "flyback" voltage spikes if a load on a series inductance is suddenly interrupted.

The 24VDC shown above for the S2-S3 lines between indoor and outdoor units suggests that a switching supply and not a transformer is being used to develop the control voltage from the line voltage. And perhaps such a switching supply is what's actually failing.

The spec lists an MCA of 0.2A for the indoor unit, presumably for the AC line voltage supply that it receives from the outdoor unit. A small isolation transformer could be considered to drop this voltage, but then there could conceivably be listing issues. And Mitsubishi would need to be consulted to OK it for warranty considerations.

 

[WattsGood]

Minisplit that integrates into an existing gas fired heating system?

I presumed it was a more conventional heat pump to add onto an existing forced air system and use the original heat source as backup heat when needed.

All new. They never had FHA before everything was hot water baseboard heat. With the hybrid heat pump they are using the furnace to supplement should the heat pump not get it hot enough.


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[David Castor]

As others have said, the surge protection will not help reduce the steady-state voltage. All of the local hvac guys install surge protection at the disconnect. Seems to be treated as a cure-all. Not a bad thing to do, but it isn't doing as much as they think it is.

 

[qcroanoke]

As others have said, the surge protection will not help reduce the steady-state voltage. All of the local hvac guys install surge protection at the disconnect. Seems to be treated as a cure-all. Not a bad thing to do, but it isn't doing as much as they think it is.

It's a warm fuzzy for the homeowner to know it's there and it MAY help but I would like to see the install.
Not that I wouldn't install it the same way as I suspect they install it.......

 

[WattsGood]

As others have said, the surge protection will not help reduce the steady-state voltage. All of the local hvac guys install surge protection at the disconnect. Seems to be treated as a cure-all. Not a bad thing to do, but it isn't doing as much as they think it is.

I can see them doing that just to honor a manufacturer warranty, maybe? But that is exactly what I was told by the HVAC tech that’s what his recommended fix was. The homeowner is getting in touch with the POCO, maybe their pots need to be retapped in the town before I look into a buck/boost for the heat pump or their whole house. We’ll see what’s budgeted and who’s paying for what fix before I go back.

I’ll keep checking in for the continued and useful help from everyone, but I’m on hold until I get more answers from the homeowner.


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