Wiring old hydronic gas boiler as backup for new heat pump

litedesign

Hi!

I am getting a new central Bosch heat pump system installed, and my contractor and I are a little mystified on proper wiring for the backup heat.

An overview of the system:
- New Bosch IDS 2.0 central ducted heat pump
- Existing gas-fired boiler with hydronic baseboard radiators
- Honeywell T5 Smart thermostat
- NO heat strips
- NO outdoor temperature sensor (EDIT: The thermostat gets outdoor temp from the internet)

The plan is to use the existing boiler system as a backup for the heat pump. Supposedly, the latest (in 2023) Bosch can handle outside temperatures down to 5ºF. I live in New York, where it reaches that temperature only a handful of nights each year.

So the basic questions are:

1.) Should he terminate the W wire from the boiler to AUX or E on the Honeywell?

2.) Should he put a jumper between AUX and E?

3.) What settings should we look at on the Honeywell, so the system knows it's using hydronic radiators, and not forced-air heat strips when the heat pump is underperforming and needs a boost?

Thanks!

pecmsg

BOSCH heat pumps CAN heat down to 5*f but what do you do when they’re down?

honeywell T8 t-stat can control that with a relay and good controls technation. 

pecmsg

You don’t say where you’re at. 

My heat pump on the east end of the Isl of long switches over at 30*F. I can run lower but don’t like the defrosting cycles. Yes I’m wasting a little energy but am more concerned with comfort. 

Hot_water_fan

The plan is to use the existing boiler system as a backup for the heat pump. Supposedly, the latest (in 2023) Bosch can handle outside temperatures down to 5ºF. I live in New York, where it reaches that temperature only a handful of nights each year.

This is not the relevant detail from the installer: it might be able to output down to 5F but what is that output and how does it compare to your heat loss? You need to know the balance point.

Ex. Say you have a heat loss at 5F of 30,000 btu. If the Bosch can output 32,000 btu at that temp, you're done. If it can output 25,000btu, you need the boiler. You need to know the balance point, the point where the heat pump's output intersects the heat loss.

JUGHNE

IMO, the simplest way is to install a simple tstat for the boiler. Set it to 65 or so. It will come on when temp drops. You want one that will adjust CPH for hydronic heat. It will be "smart" enough for this purpose.

In the future when the HP is acting up, (and guaranteed that it will) the tech, who will not be the one doing this install, will be confused by the 2 stage set up and at a loss.

KISS method.

unclejohn

Your going to need a outdoor sensor unless there is one built into the BHP. 

ethicalpaul

JUGHNE said:

IMO, the simplest way is to install a simple tstat for the boiler. Set it to 65 or so. It will come on when temp drops. You want one that will adjust CPH for hydronic heat. It will be "smart" enough for this purpose.

In the future when the HP is acting up, (and guaranteed that it will) the tech, who will not be the one doing this install, will be confused by the 2 stage set up and at a loss.

KISS method.

It's sad that we can't count on HP or burner/boiler techs being able to understand such a simple thing.

My mom has a relatively simple thermostat that has "heat" and "emergency heat". When the "heat" device can't hold the temperature (or if the temperature is below a threshold set in the thermostat settings), then the thermostat calls for heat on the "emergency heat" device.

it's so simple even I can understand it. But every EVERY tech who has ever come to the house to work on the oil burner (the "emergency heat" device) either screws it up or has to have my mom call me to explain it to them. How is this possible?

GGross

@ethicalpaul

Stubborn people who refuse to learn anything new and instead blame the equipment for their own lack of knowledge to protect their own egos. Its unfortunate but quite rampant

litedesign

Hot_water_fan said:

You need to know the balance point, the point where the heat pump's output intersects the heat loss.

How does one measure that intersection point? The thermostat takes the outdoor temperature from the internet. My contractor suggested setting the balance point at 32ºF for now. It's late April, so we're not going to see freezing temps again for a while.

pecmsg

litedesign said:

Hot_water_fan said:

You need to know the balance point, the point where the heat pump's output intersects the heat loss.

How does one measure that intersection point? The thermostat takes the outdoor temperature from the internet. My contractor suggested setting the balance point at 32ºF for now. It's late April, so we're not going to see freezing temps again for a while.

Thats called the balance point. When is it cheaper to operate the heat pump or boiler? My cut off point is 30°F. I dont like the defrost cycle(s) below that.

Hot_water_fan

@litedesign you'll need your heat loss. Usually 32F is picked by contractors because it's when water freezes, not because it's a relevant balance point. Basically, you first plot your heat loss. It's a linear function of outdoor temp, as it gets colder it increases consistently. Then you plot your heat pumps operating envelope, which works in reverse - as it gets colder, it outputs less. It's usually not linear though. Idronics describes it better than I can.



If you're interested, you can probably find your Bosch's on the internet. Where the heat loss = heat pump max output, that temp is the balance point. You can try trial and error - lock out the boiler and see at which temp the heat pump can't maintain the thermostat. There's another topic called the "economic balance point", which is the point when the heat sources are equal in cost. That's much harder to find and often isn't worth the effort since gas prices changes frequently and efficiencies vary by temperature.

https://www.caleffi.com/sites/default/files/file/idronics_27_na.pdf

litedesign

Thanks for the advice everyone. I don't know how this got into a conversation about balance points and outdoor temperatures... it's helpful, but it doesn't answer any of my original questions.

We've done some experimentation and discovered the following:

1.) There appears to be no difference between AUX and E terminals on the T5 thermostat. We tried wiring it both ways, and on both AUX and E, the thermostat fires the boiler manually in Emergency Heat mode, and also fires in regular Heat mode when the temp is set higher than 2-3º above the ambient temp. So what's the actual difference between AUX and E?? I guess that's going to require a call to Honeywell tech support.

ethicalpaul said:

My mom has a relatively simple thermostat that has "heat" and "emergency heat". When the "heat" device can't hold the temperature (or if the temperature is below a threshold set in the thermostat settings), then the thermostat calls for heat on the "emergency heat" device.

Interestingly, the T5 thermostat doesn't allow you to adjust that threshold. It appears to be locked in at 2-3º from wherever the thermostat is set.

The T6 (upgrade from T5) thermostat has an additional setting, "Droop," to allow you to set the differential between ambient and the user setting before it fires up the AUX. It's funny, I looked all over the internet for the differences between the T5 and T6, and mostly all I could find was "The T6 is a different color," and "The T6 has support for 3-stage heating." But the correct answer is, "The T6 supports outside Temp sensor and ventilator wires and it also has dozens of additional setup options for heat pump configuration." Go fig.

JUGHNE

You have sought advice here, to help your installer figure out a 2 stage tstat to bring on your boiler when

his heat pump will not maintain comfort temps for whatever reason.

One would assume that your contractor on site would be the sharpest knife in the drawer for his company.
And he is having issues figuring this out?

So as time passes the next tech they, or any other company, sends out for troubleshooting will most likely not be the top tier of techs......often what can happen when things go south, the unschooled will point at the boiler connection and assume that is the problem....we need more parts etc.

IIWM, I would let the "Smart" WIFI tstat operate the heat pump only. This will let you monitor the indoor temp from anywhere.

A second simple "Dumb" tstat for the boiler would bring on the heat as needed.

Having 2 separate control systems gives you redundancy in the event that some part of the Smart system falters.

The HW PRO 5000 has cycles per hour adjustments to cover hot water systems.
Also max temp range stops to keep someone from bring it on too early.

A 3 wire cable from this to the boiler will eliminate needing to rely upon its battery.

And most likely not need an isolation transformer/relay (another failure point) to achieve this.