How to prevent hydronic piping from freezing?

In_New_England

I have your attention don't I? What a dumb question, right? So here's the scenario:

1400 sq ft colonial near Boston.

We're adding heat pumps to our home. Our lovely home is currently kept cosy and toasty by a traditional forced hot water system.

If the heat pumps pull through on really cold nights, our hydronic heat won't be running. I suspect we have a few runs of piping that go through our outer walls. This raises the possibility of the pipes freezing.

What would be a good way of avoiding that?

1. Glycol or other anti-freeze in the pipes? How does this work with make up water?
2. Run the circulation pumps periodically?
3. Run the system a bit during the cold nights?

Thank you!

EdTheHeaterMan

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.

DCContrarian

You system should not ordinarily be using make up water. If it is, it needs to be repaired, the water brings in oxygen which is quite damaging.

Once you have that fixed glycol is one solution. If you go the glycol route, turn off the makeup water valve, remove the handle and hide it.

Jamie Hall

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.

pecmsg

You could run just the pump but that's no guarantee of a line being air bound.

I agree use the HW when below freezing or drain it.

hot_rod

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

In_New_England

Thank you all! Great advice as usual.

EBEBRATT-Ed

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.

propmanage

You will enjoy having two systems to count on.

My thought is you should only be using the heat pumps Spring , Summer, and fall. Then lower the set point so they stay off use as a backup if temperatures drop below 50 so it comes on during any emergency.

Use your main heating system during the winter months with possible freezing temperatures.

Enjoy!

4Johnpipe

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.

Voyager

I concur with most of the others. Glycol is a pain from a maintenance perspective and also lowers the performance of your system slightly since a glycol mixture transfers heat less effectively than 100% water.

Running the circulators all the time is probably sufficient, but may not be if the boiler is in a location where it also can get pretty cold on a frigid winter’s night (basement, garage, exterior closet/utility room).

Systems like this often live longer if exercised now and then (think of standby generators) and are more likely to work when needed if they have been run regularly. So, I would either get a programmable thermostat that can be programmed to lower the set point at night to run the boiler or, best of all, an outdoor sensor that will run the boiler whenever the temp is 32 or below. The problem with the timer is that occasionally there is a very frigid winter’s day and it just might be cold enough to freeze the pipes during the day.

The simplest way of all, and probably not much more costly in terms of energy use, is to simply turn off the heat pumps in November when the temp gets solidly below 32 and leave them off until March or April when you get several nights in a row above 32. Alternatively, you could set the stat on the boiler for your nighttime setback temp, say 66 degrees and program the stat on the heat pumps to set back to say 62 at night so they don’t run when the boiler is running and then have them jump to your desired daytime temp, say 72, so that they run during the day and keep the boiler from coming on until they setback again at night.

RobertMcRae

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.

Jamie Hall

A comment of two, on the immediate above. Solar panels? Neat. Useless at night when it gets colder. So — solar panels plus batter backup sufficient to power the system for a few days. Most people don't come close to that. Second, it is sufficient to know that the OP has potentially freezable water in the pipes. The fuel used to keep them from freezing is irrelevant. Three choices: drain, antifreeze, keep the house warm. Take your pick.

And your worst case — which can go well into six figures damage, or a total loss, is that "barring a power failure". Bad news. Power failures do happen, particularly in more rural areas.

Voyager

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.

Albany Chris

Others have noted you can run your heat pump when the weather gets cold, but you probably SHOULD run your fossil fuel heating system system when it is cold. Your installer should have calculated the temperature below which the boiler is more cost efficient and set it up with your "hopefully" advanced thermostats. They will use your local fuel costs to calculate that. If you use natural gas its going to be pretty high (40+). If its oil or propane it might be quite a bit lower. Find out how that is (or should be) set up before deciding on glycol.

The heat pump was also designed and sized either to take on the whole load (less common) or only a partial load while depending on the boiler for the coldest temps. You need to know what the design was.

Once you know these things you can make a decision on running the boiler circ full time under certain outdoor temps, or using glycol, or draining it.

Greening

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.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1812820#Comment_1812820

Having solar affects the cost of electricity, which is what determines where the switchover point is. If you live some place with net metering then your cost of electricity is lower whether or not the sun is shining at any given moment.

DCContrarian

https://forum.heatinghelp.com/discussion/comment/1812816#Comment_1812816

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.

Voyager

https://forum.heatinghelp.com/discussion/comment/1812849#Comment_1812849

Not when the person asking the question specifically said it was about pipes not freezing.

PeterOrion

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.

hot_rod

Im not sure the heat pump will provide that same warm cozy feeling as the hydronic systems some heat pumps tend to blow luke warm air when the outdoor temperature drops. The noise of a fan running can be annoying also.

Draining and refilling is not a good option gor hydronics, you just add mire minerals into the boiler and piping with each refill, plus the oxygen that is with the H20, contributes to corrosion of the ferrous metals in the system .

Stew_5

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.

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1812946#Comment_1812946

It's even worse than you think. Most hot water systems have parallel flow paths — that is, the water has more than one way to go. If one of those paths is in a cold wall, as that starts to freeze it will get less water from the pump… and thus freeze more… and get less water… and so on. Until it's frozen shut.

As I said before: there are only three safe options: keep the place heated, use antifreeze, or drain.

akjim

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.

hot_rod

The only 100% non mechanical, non electrical, completely automatic solution is a proper glycol mix and application. The system is always online to be fired up at a moments notice also.

Drain down requires manual intervention and quite possibly compressed air to get 100% of the fluid out of low spots. I'd put that last on the list of options.

A glycol fill tank or glycol PIG is the only fill option for glycol systems.

Many have tried other options, most fail :)

Most all the ICE motor vehicles on the highway are protected by glycol, rarely properly maintained, PG hydronic glycol, it's not as scary as many imagine.

Glycol with a lower surface tension tends to leak out of connections where water doesn't. That can be an issue on old threaded pipe systems that have never been glycoled.

In_New_England

Thank you all for the great discussion and suggestions!

Seeing which way the discussion went, I can add some more detail, though I don't think this alters the key suggestions already made for the original problem posed.

We have an oil fired boiler which is used for hot water as well, as a result the boiler is at a steady temperature year round. I believe the system is quite simple in a way: if the boiler temperature drops, the oil gun fires for as long as it takes to heat it back up. So if I run the circulator pump I believe I will be circulating hot water and keeping it hot, same as if there was a call for heat, though in a somewhat open loop fashion.

If I were to go this route, I would ball park a duty cycle based on the outdoor temperature and run the circulators X minutes every Y hours. As a side effect this would give the heat pumps an assist which they should automatically adjust to.

(Someone mentioned automation - I have some plans sir, involving an ESP32 and a DS18B20 temperature sensor that should not be discussed in public …)

The problem with this, of course, is that it's getting complicated and adds an unknown failure probability.

The glycol solution may be the way to go. All the connections I can see in my basement are brazed copper connections (the house is from the 1960s) but it would be a bad time to find out something behind the walls had a screw joint (since someone observed that glycol can leak through them). This too adds an unknown failure probability.

In the end, the solution that my wife suggested, which some of you also proposed, is genius in its simplicity. When it gets to freezing outside, switch off all the heat pump heads and operate Ye Olde wired thermostat as before.

This naturally led some people in the direction of, what is the optimum switch over point. For the headline requirement, of course, a switch over point less than ~32F is not acceptable. But as further background, the system is a Mitsubishi Hyper Heat system. We have no solar panels though after I receive my next electric bill I may be dialing up solar installers frantically, rather than the other way round as has happened in the past.

Jamie Hall

Ah. Simple enough. If you are already running the boiler for hot water… just use the good old fashioned boiler for hot water heat. Your wife is quite right…

You will use more oil. You will use less electricity. You will lose some of your "green" cred. You will be happy…

akjim

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.

EdTheHeaterMan

I don't know…. maybe keep the pipes above 32°F

🤣🤣🤣🤣🤣

TomS

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)