Keeping a heat exchanger from freezing when there's no fire....

  • Active since 1995, is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.


New Member
Oct 9, 2022
Bristol, VA
We bought a foreclosure two years ago, which came with a CL4030 boiler. The house has a package heat pump with a HX bolted to the exhaust duct between the heat pump and the house. It's our second home, primarily use it on weekends, so its not permanently occupied. During the winter we usually set the heat pump to 45 degrees and don't run the OWB unless we're out there. Last winter, the heat pump malfunctioned during a two day cold spell, we didn't know it and the heat exchanger froze and took out 6 tubes. $160 for a new HX.

Don't want to really use antifreeze so my idea is to split the hot wire going to the circulator into two parallel legs. One with a thermostat, the other with an on/off switch. If we fire the boiler, the switch is on and the circulator runs. If it's off, then if the temperature drops below say 40F, the circulator kicks on and runs keeping the HX from freezing. This would only be for those days where it's like 50 in the day and 25 at night. Thoughts?
I live in a part of the world where running without glycol (antifreeze) in water is simply not heard of.

That would be my solution, 16% ethylene glycol would give you a freeze point of 20f.
  • Like
Reactions: sloeffle

I'd run the circulator 24x7. Like any electric motor, turning them off and on is the worst thing you can do to them. Setup a ranco for the thermostat so the heat from the heat pump will keep the exchanger from freezing and you will only need one thermostat. If you are worried about the HP going out again, I'd definitely add glycol to the loop.
Last edited:
I had a similar situation. My EKO 40 is located in an outbuilding. I only batch burn and travel quite a bit, so I was worried about the outdoor parts of the system freezing. I priced glycol for my 1400 gallon system, then used the solution you proposed. I installed a snap switch that closes at around 35F and open around 40F. Have had no issues in 6 years.