Disconnecting Bosch 2.0 Heat strips

Bobelsass

I have a Bosch 2.0 central heat pump system that was installed in April 2023. During the summer cooling was very efficient with a peak increase of only $65 on my electric bill during a month that it ran almost non stop.
However, I had my installer include a 10kw back up heat strip option when he installed the system. The intent was for it to be an optional 3rd “emergency” heat source if ever needed. (We also have an oil fired boiler w/baseboard heat). 

My installer went through 3 thermostats and could not get any of them to recognize “emergency” heat. (He ended up installing an ecobee 3 lite thermostat) I told him that well he tried and that I’ll live with the Heat(heat pump) and Aux(boiler) two heat source set up.

Well my January electric bill was $385! 

I’ve come to realize that the Bosch is calling for back up heat in certain circumstances such as low temps and when raising the house temp (typically in the morning.) Basically like running a toaster on steroids as one person said. 

I had my installer come out last week to disconnect the heat strips. (They were an option when installed so I figured an easy disconnect). He disconnected a couple of the wires that tie into the system and now although the pump and air handler run in heat mode, I am now getting no heat at all from that system. I’m waiting (for a few days now) to hear back from him on when he will come back for round 2. My question is: Is there a more practical way to disconnect those strips?

(Also FYI…Since getting that January bill I set the thresholds to 30f as the lowest outside temp for the pump and 40f as the highest for my boiler to help control 
heat strip activity…but looking to not use the heat strip at all). 

Any help appreciated

Ironman

We we do this setup regularly and it’s not that difficult for someone who knows controls. Unfortunately, most HVAC installers dont.

You’ll need a relay to make it work.

I can’t post a diagram at the moment. But Ed the heater man is very good with those.

Bobelsass

Thanks Bob. So a relay is needed to disconnect the heat strips? Since the heat strip was an optional add on can it not simply be unplugged somehow? I don’t want to use them. I can see a relay being needed if I wanted it as a separate third source of heat . But not the case. 

pecmsg

Don't the heat strips have a separate breaker?

Ironman

To use a single thermostat to control both the boiler and the heat pump, a relay is needed. 

The heat strips can be disabled by simply disconnecting the white 24v wire and the black wire with a white tracer at the air handler. 

Bobelsass

Re: pecmsg: The heat drips supposedly have an external breaker on the Air handler. (According to Bosch diagrams that I’ve seen) But the only breaker on mine is a 220v breaker for the whole unit. 

Re: Ironman: When my hvac guy returns I will advise him of those two wires. Thank you.
my goal is to let the heat pump run on its own, without supplemental electric heat, down to an outside temp of 30 degrees or so and then let the aux oil heat kick in below that. 

EdTheHeaterMan

pecmsg said:

Don't the heat strips have a separate breaker?

Not usually, the 240VAC to 24 VAC transformer operates off of one of the 240 VAC breakers that also operate one stage of the resistance heater. But the other one or two can be shut off if that is available.

Bobelsass said:

Re: pecmsg: The heat drips supposedly have an external breaker on the Air handler. (According to Bosch diagrams that I’ve seen) But the only breaker on mine is a 220v breaker for the whole unit. 

Re: Ironman: When my hvac guy returns I will advise him of those two wires. Thank you.
my goal is to let the heat pump run on its own, without supplemental electric heat, down to an outside temp of 30 degrees or so and then let the aux oil heat kick in below that. 

It all depends on how the resistance heat kit is configured. If you have one 240 VAC breaker for all the heat then you may find inside the cabinet 2 or 3 fuses. One for each bank of resistance heaters. You may also find only large 240 VAC Breaker for all the heat.

If you could provice me with the model number of the air handler and the model number of the electric heat kit your installed used, along with the model number of the boiler control where the thermostat is connected that does that same zone or space, I will take a stab at a wiring diagram.

@Ironman In the future If you need to get my attention use the @ symbol and edtheheateman as all one word with no spaces, and I will get a notification. It should look line this what you post @EdTheHeaterMan

Bobelsass

@Ironman @EdTheHeaterMan My system is a Bosch model #BOVA-36HDN1-M20 inverter condenser and Bosch BVA-36WN1-M20 air handler. The thermostat is an Ecobee 3 lite and it is powered by the transformer in the Bosch unit. 

I have a Bosch wiring diagram for the thermostat and it indicates that the white and white/black wires (W1 and W2) are for the heat strip. So your earlier suggestion of disconnecting those seems as if it should work. Although I could do so, since it’s new (installed in April ‘23) I will let my HVAC installer check that out when he returns. (Don’t want any issues with him or the warranty). Thank you for your guidance. 

pecmsg

@EdTheHeaterMan

The air handler is rated at 208/230 Volt 15 amp MAX!

The smallest heater Kit is rated 208/230 Volt 30 Amp Max.

You should have 2 breakers!

dko

The heater kit from Bosch comes with pre-wired breakers to be mounted on the AH.

EdTheHeaterMan

In the air handler, there is a control module. You can remove the electric heat completely by removing the plug where the heat kit is connected.



The control in the air handler takes the information from the thermostat, the outdoor unit and the air handler sensors, then determines the fan speed and whether the compressor should operate. Based on this information the outdoor unit will operate in Heat Mode which may cause the outdoor coil to accumulate frost. Before that frost can become a problem, the defrost control in the outdoor unit will operate a defrost cycle. That basically tells the compressor to send hot gas to the outdoor coil to melt the frost off the unit.

At the same time the indoor coil will become cold. Basically it is operating in air conditioning mode for the duration of the defrost cycle. When that happens the outdoor unit sends a signal to the W1 of the air handler to operate stage 1 of the electric heat so the air handler does not blow cold air into the ductwork. If you disconnect all the electric heat completely, you may experience some times when the heating system will blow cold air during the defrost cycle on a call for heat. This may not be an acceptable comfort condition.

You may just want the W1 and W2 from the thermostat to be disconnected but still leave the W1 from the outdoor unit connected. Also you may want only electric resistance heating coil to operate during the defrost cycle, so perhaps the technician can disconnect all but one of the resistance heaters control wires. This way the defrost cycle will use the smallest amount of electric resistance heat to keep the duct work from blowing cold air.

I hope this makes sense, and is helpful in communicating your needs to the technician.

Bobelsass

@EdTheHeaterMan  That all makes sense and I understand and appreciate the thorough explanation. I will pass it on to my technician. My only question is that I purchased the 10kw heat strips separately as an add on. Had I not, and simply had him install the air handler out of the box, what would the system have done to prevent that cold air flow during the defrost cycle? Perhaps it would kill the air handler fan until the defrost cycle was complete and start up when the compressor sends heat to the air handler?? 

Bobelsass

@EdTheHeaterMan One additional comment: I see in the schematic diagram that to the right under SW6-3 there are two "Heat: Anti-cold and Heat fan delay" switches. The lower one is in the "off" position. I would assume that when the heat strip is removed or intentionally disabled, that we would want the switch in the "on" position and that would solve the cold air challeges of the defrost cycle.
@Ironman...I also agree with your comments about there must be a separate breaker of fuses for the heat strips.
The info you guys have provided has been terrific and super appreciated. I will post the outcome after my technician does his service on it.

EdTheHeaterMan

Bobelsass said:

@EdTheHeaterMan One additional comment: I see in the schematic diagram that to the right under SW6-3 there are two "Heat: Anti-cold and Heat fan delay" switches. The lower one is in the "off" position. I would assume that when the heat strip is removed or intentionally disabled, that we would want the switch in the "on" position and that would solve the cold air challeges of the defrost cycle.
@Ironman...I also agree with your comments about there must be a separate breaker of fuses for the heat strips.
The info you guys have provided has been terrific and super appreciated. I will post the outcome after my technician does his service on it.

You may be correct on that. I would need to see the Electric Heat Kit instructions to be sure