Controlling cold climate heat pump with gas fired hydronic baseboard boiler backup heat

bobtee

I have a newly installed Bosch cold climate heat pump controlled by a Honeywell T-10 thermostat. When I connect my existing gas boiler hydronic hot water baseboard system to the T-10 aux terminals to provide low temp aux heat, the Bosch air handler fan continues to run even though the outdoor Bosch HP properly locks out. I guess the air handler thinks that aux heat is being provided by electric strip heaters or gas fired duct burners and continues to run. What’s the workaround to shut off the air handler fan in aux mode when heat is being provided by my old hot water boiler baseboard heat?

GGross

It could be that the thermostat has a minimum fan run time (Pretty sure these do have that funciton, but not sure if that's the issue) The minimum run time is usually a percentage of each hour (30% in many cases) they do this to help with ventilation requirements (if a fresh air intake is piped to your duct system it will bring in fresh air) and to promote air movement for indoor air quality reasons (filters the air etc)

Usually this runtime is met by the length of time the unit is running for heat, if however you turn off the heat the blower will still run for 30% of each hour. Are you certain that the air handler blower comes on with the call for heat and is not just running to maintain a minimum blower run time?

pecmsg

What setting is aux heat?
electric or gas/oil

bobtee

Aux setting is for gas/oil but it seems that this is taken as gas fired duct or furnace firing so fan stays on thinking that the warm gas/oil fueled air must be pushed through the ducts. The blower isn’t running to satisfy minimum run time after heat. It simply stays on blowing cold air which is counter productive with the gas fueled boiler for the baseboard loops.

ratio

I'm not terribly familiar with the new T series of Honeywell stats, but IIRC the previous generation has a setting for fan-on-during-aux-heat. You might review the setup.

pecmsg

bobtee said:

Aux setting is for gas/oil but it seems that this is taken as gas fired duct or furnace firing so fan stays on thinking that the warm gas/oil fueled air must be pushed through the ducts. The blower isn’t running to satisfy minimum run time after heat. It simply stays on blowing cold air which is counter productive with the gas fueled boiler for the baseboard loops.

When set to oil/gas the furnace controls the fan. Elec the fan runs. 

HVACNUT

Are you using one power source (R) at the thermostat? If so the boiler side is through an isolation relay and would normally connect to W, not aux. Maybe the T10 is different?

bobtee

Yes, there is only one power source to the thermostat. Maybe connecting to W will work. Thanks.