How to prevent hydronic piping from freezing?

The lowest cost way to do this is to select a temperature around 30 degrees and manually turn on the hydronic heat by raising the thermostat of lowering the heat pump thermostat so that the compressors do not operate at all. I believe you will find the fossil fuel will cost about the same as the reverse cycle compressor of the heat pump. This means that you need to manually make the change, so going on vacation or getting laid up in the hospital in the winter, may not be a good idea.

An outdoor sensor may be a good automatic idea. Open all the zone valves (if you have them) and turn on all the pumps when ever the outdoor temperature goes below 32°F

Glycol is the best idea. but you must check how potent it is every heating season. You may need to add some each year based on age and how much water may have been added over the previous year.

Finally, Have a plumber remove all the water from all the pipes and then you dont need to worry about it. But you won't be able to use that system until you put the water back in.

Yep. Glycol or deactivate and drain it — or run it; it will cost less to run in cold weather than the heat pumps.

Oh and one other thing: If you don't run it or deactivate and drain it, or don't use glycol, it is not a question of "a few runs of piping that go through our outer walls. This raises the possibility of the pipes freezing." Those pipes will freeze. Not a matter of possibility — that's a certainty.

is it a single zone system? If so you could run constant circulation, the call for heat fires Hp, or buffer tank.

What about power outages?

A small battery backup could run an ECM for a few days

First thing to find out before doing anything that will cost $$$ is find out if the heat pumps will heat the house during cold weather. You may find you need the boiler anyhow. Glycol can cause issues and with a "few runs of pipe through the outside walls" I wouldn't use glycol due to the expense and trouble it may cause including it turning acidic over time.

I would run the HW system occasionally during cold weather to make sure all the piping and radiators heat. Make sure you have a functioning low water cut off mounted above the lowest boiler safe water line.

Then if the heat pumps prove to be adequate install an outdoor air control to run the circulators when the OA is below 40 degrees.

Your not protected by glycol now so you will be in no more danger from freezing than you presently are now when using the heat pumps.

You could set it up on constant circulation or glycol mixture. I would stress that the cost to heat your home with forced hot water on colder days / nights will be less than the heat pumps.

No one has asked if this house has solar panels, what the heating fuel is, or what type of heat pumps are being installed. To me, this is all necessary information to give this gentleman accurate and reliable advice. I f he has solar power generating ability, well then the heat pumps are very likely the most cost efficient way to heat the home, even well below freezing. If his fuel is oil or propane, that also affects the calculation. If his heat pumps are ductless splits, they’re capable of heating efficiently quite well down near zero degrees. Absent an exotic control system (which is what I would do because that’s my specialty) I would recommend an outside air temp control that simply operates the circulator continuously whenever it’s about 35 or below. It’s also a prime example of the KISS philosophy, which is always the most reliable as well. I wouldn’t change to glycol, due to the maintenance headaches. You’ll have two heating systems as it is, barring a power failure, that’s more protection than most people have.

Regarding “accurate and reliable”, I tend to respond to a question in the context it is presented. The OP did not mention solar or even energy efficiency. He mentioned being concerned about pipes freezing. So, I replied in the context of preventing the freezing of pipes, not in providing the ultimate in energy efficiency. Most folks who have energy efficiency as a high priority will mention that as part of the question or concern.

And if energy efficiency is paramount, then the KISS principle rarely applies. Gaining efficiency almost always requires more complexity and less reliability. Rarely do you get both together.

A few neighbors had water pipes on outer walls burst after a multi-day power outage / cold spell. They were on holiday.

I don't remember if the burst pipes were hydronic or water service, or both. The bursting was on top floors so damage was "extensive".

I don't know if they had temperature monitoring systems (with phone app warning). Anyways, those systems require a bit of planning. If the monitor and "home network" were powered by mains, I don't know how any low temperature warning would have been sent to the phone app.

I'm not an expert or pro.

Exactly.

The switchover point between heat pump and combustion is an economic decision, it is not a technical decision based upon inherent properties of the technology. The switchover point in entirely determined by the relative cost of electricity versus the combustion fuel, and the total cost is determined by the cost of the underlying power plus the efficiency, all of which can vary tremendously depending on region, climate and choice of technology.

So blanket statements about specific temperatures always being the optimal switchover point are at best not helpful and at worst harmful.

The above assumes the heat pump was properly sized, if it's undersized you're going to have to switch over when it's no long capable of maintaining indoor temperature. That can be easily implemented with a two-stage thermostat.

Hello Folks, I have a similar situation in Climate Zone 6 / Central NY for a slab on grade cape with distribution running up to the attic knee wall areas and then down a few exterior wall bays. We are a weatherization agency looking to cut down the hemorrhage of natural gas usage with insulation and air sealing measures. Others have warned about insulating the wall bays with piping arguing that hot water freezes faster that cold. An internet query offers that is more the case with warm or ambient temp water and not significantly faster than if the water is cold or hot. It seems more sensible to prepare the client to shut down and drain the system in the event of an extended power outage rather than the permanent costs associated with the degraded distribution efficiency. In short, we are weighing not insulating the wall bays with piping vs. changing the system to glycol. Any feedback would be greatly appreciated.

I would not rely on circ pumps preventing freezing. I’ve watched considerable 40 psi flows gradually choke down and freeze solid. And circ pumps don’t move nearly the water.

I'd go to glycol …. but don't keep the water make-up, change it to a small glycol make-up reservoir. These small units are available with pump included to address this problem.

A word of caution, if there is any interface between your domestic hot water and the circulating heat system, then you must use only propylene glycol, never ethylene.

There is a timer called ThermGuard that connects across your thermostat and will turn on your boiler for a programable set amont of time and a programable number of times per hour. It has a defult setting of turning on the boiler for 5 minutes every half hour. I purchased one for an area in the home that I didnt plan to heat but never installed it so I cant say anything about its reliability.

ThermGuard (bearmountaindesign.com)