Frost protection methods for when supply / return pipes to a monoblock outdoor unit are buried?

[connorbenton]

I can't find any guidelines / best practices regarding frost protection when the supply/return pipes to a monoblock are buried. Quite a few people with units placed close to a building seem to have the scenario I've sketched out in the upper section, where a frost protection valve gets put on the low point of the supply and return (only sketched one line for clarity). However, in my case, the unit can't go against the building, and actually has to go on the other side of a small walkway, so there's pretty much no choice but to bury the supply and return pipes.

 

In theory, putting a frost protection valve at the #1 position I've sketched seems to be just be a waste of money, as in the best case it would simply drain some water out of the unit (provided it's placed lower than the unit's internal pipework). Putting a protection valve at the #2 position seems like it would be the 'safest' option, but would seem to require leaving some sort of culvert / access shaft in the middle of the pipe trench run (sketched the 'ground line' with the dotted brown line, showing the shaft that the #2 valve would have to go in), which seems like a lot of additional complexity.

 

If the pipes are buried below the frost line, could the internal water then be always assumed to be above freezing, and keeping the above-ground outside water in the unit above freezing through natural convection, so that no frost protection valves at all are actually needed? Or at least if the buried pipe water is 'safe', maybe the frost protection valve at #1 would be 'good enough' since it drains out the only above-ground parts of the system (perhaps a mirror would be needed at the house entry)?

 

Curious how you all have seen actual examples of frost protection in systems with buried pipes.

 

[Conor]

Glycol.

[connorbenton]

Given the massive added complexity of glycol system maintenance (mixing, filling, draining, disposal) plus the slightly worse performance of a glycol system, I would by far rather bite the bullet and prep a culvert mid run and install a frost protection valve at #2 if that's what I have to do to avoid glycol. Anybody else with any different setups?

Edited January 29 by connorbenton

[BotusBuild]

Connor, Glycol is the solution to this.

1. It is proven to work. Nit only in HPs, but also in the millions of cars on the road.

2. Once mixed and filled you should not need to drain and refill except during major heating system changes which should be very rare if the system is installed well in the first place.

3. If the system drains via the frost protection, you may end up with a non functioning HP at the very time of year the building needs heat.

[SteamyTea]

11 minutes ago, BotusBuild said:

Nit only in HPs, but also in the millions of cars on the road.

Not the same stuff. The antifreeze for HPs is more environmentally benign.

[JohnMo]

Questions to ask yourself 

 

1. How often do I get a power cut?

2. How long do they last?

3. How long to totally freeze a pipe?

4. Do I have a generator and or battery?

 

My answers are (we have insulated buried pipes)

1. 4 to 6 times a year

2. Up to 12 hrs

3. With no circulation I think to freeze a pipe or the parts inside the heat pump, when tested was about 18 to 24 hrs at -20 (in freezer) when tested.

4. Yes and yes

 

I have NO, antifreeze valve or glycol. In normal use the ASHP circulation pump runs 24/7, even if off the antifreeze running protection in the heat pump will look after the unit and pipe.

 

Observations

My outside cold water pipes have never frozen they start from cold (not warm) they aren't insulated and are buried to a similar depth. We have had -9 for several days over the last couple of winters.

 

You need to get extreme cold and power outage for a prolonged period at the same time.

External oil boilers don't use antifreeze etc.

 

Have read here

https://renewableheatinghub.co.uk/do-air-source-heat-pumps-really-need-glycol

Edited January 29 by JohnMo
Link added

[PhilT]

38 minutes ago, JohnMo said:

1. How often do I get a power cut?

2. How long do they last?

 

1. 4 to 6 times a year

2. Up to 12 hrs

yikes - storms mainly?

[JohnMo]

12 minutes ago, PhilT said:

yikes - storms mainly?

Mostly, longest is 12 hrs, some we don't even notice because the battery hides them, which is good

[Nick Laslett]

4 hours ago, connorbenton said:

I can't find any guidelines / best practices regarding frost protection when the supply/return pipes to a monoblock are buried. Quite a few people with units placed close to a building seem to have the scenario I've sketched out in the upper section, where a frost protection valve gets put on the low point of the supply and return (only sketched one line for clarity). However, in my case, the unit can't go against the building, and actually has to go on the other side of a small walkway, so there's pretty much no choice but to bury the supply and return pipes.

 

In theory, putting a frost protection valve at the #1 position I've sketched seems to be just be a waste of money, as in the best case it would simply drain some water out of the unit (provided it's placed lower than the unit's internal pipework). Putting a protection valve at the #2 position seems like it would be the 'safest' option, but would seem to require leaving some sort of culvert / access shaft in the middle of the pipe trench run (sketched the 'ground line' with the dotted brown line, showing the shaft that the #2 valve would have to go in), which seems like a lot of additional complexity.

 

If the pipes are buried below the frost line, could the internal water then be always assumed to be above freezing, and keeping the above-ground outside water in the unit above freezing through natural convection, so that no frost protection valves at all are actually needed? Or at least if the buried pipe water is 'safe', maybe the frost protection valve at #1 would be 'good enough' since it drains out the only above-ground parts of the system (perhaps a mirror would be needed at the house entry)?

 

Curious how you all have seen actual examples of frost protection in systems with buried pipes.

 

This is basically my configuration.  The pipes leave the ASHP and go straight down under the foundation, then 8 metres later come straight up into the plant room. 
 

I also interpreted the install guidelines for the anti-freeze valves that they were not suitable for my install and would not work. 
 

I was also not prepared to go with a glycol solution. 
 

Although there is a risk that a powercut and heavy frost will coincide, I was prepared to take that risk. 
 

There was an earlier discussion somewhere on the forum about these issues, upon reading a few different opinions I felt happier to take this approach. The Graham Hendra article @JohnMo linked to also helped. But I live in Suffolk, so the weather is not particularly cold. 

 

Possible mitigations and other ideas. If there is the powercut and frost situation, you can still drain down the ASHP so that it won’t be affected. You can install a home battery so that you have power during the powercut. 
 

You can also up rate the insulation on the ASHP pipes, which is what I did. Armaflex do exterior grade up to 32mm thick. I went with 25mm. MCS specifies 19mm. 
 

Is the underground pipe below the frost line?
 

 

Edited January 29 by Nick Laslett

[connorbenton]

11 minutes ago, Nick Laslett said:

This is basically my configuration.  The pipes leave the ASHP and go straight down under the foundation, then 8 metres later come straight up into the plant room. 
 

I also interpreted the install guidelines for the anti-freeze valves that they were not suitable for my install and would not work. 
 

I was also not prepared to go with a glycol solution. 
 

Although there is a risk that a powercut and heavy frost will coincide, I was prepared to take that risk. 
 

There was an earlier discussion somewhere on the forum about these issues, upon reading a few different opinions I felt happier to take this approach. The Graham Hendra article @JohnMo linked to also helped. But I live in Suffolk, so the weather is not particularly cold. 

 

Possible mitigations and other ideas. If there is the powercut and frost situation, you can still drain down the ASHP so that it won’t be affected. You can install a home battery so that you have power during the powercut. 
 

You can also up rate the insulation on the ASHP pipes, which is what I did. Armaflex do exterior grade up to 32mm thick. I went with 25mm. MCS specifies 19mm. 
 

Is the underground pipe below the frost line?
 

 

Thanks for the info on your setup - my proposed setup is pretty much the exact same as what you have (pipes well below frost line, extra insulation on everything that's above). Realistically the only scenario where it seems like I could run into danger with a setup like yours is where there's an apocalyptic combination of:

 

1. Multi day power outage

2. Multi day cold snap

3. I'm not around to manually drain the system myself

 

And I think setting up a generator + battery backup to run the pump would take care of point 3.

[Beelbeebub]

If you go with an AF valve below the HP but out of the ground - ideally just as the pipe emerges then it should protect your HP should there be an extended power cut and cold spell.

 

Ultimately if your bit of pipe sticking up from the ground to you AF valve freezes it's not that big a deal.

 

The goal is to stop freeze damage to the heat exchanger in the HP. If that freezes you HP is scrap. Even if the pump inside freezes it's not the end of the world, so all you need is thr freeze valves to be such that the HP drains down.

 

I would argue a single one on the lowest outlet (doesn't matter if flow or return) is enough. The goal is to get a flow of water through the HP so it can't freeze solid. If the valve opens and some water flows out that achives  the goal.  If enough water flows out to empty the HP, that also achieves the goal.

 

[joth]

13 minutes ago, connorbenton said:

And I think setting up a generator + battery backup to run the pump would take care of point 3.

Don't underestimate the amount of effort it takes to install and maintain a generator that will reliably start and take over supply while the property is unattended in the middle of a storm

 

Personally I'd go with the @Beelbeebub approach of put in antifreeze valve at lowest point you can below the ASHP 

 

But if you want an autostart generator for other reasons too, why not. 

Edited January 29 by joth

[JohnMo]

4 minutes ago, Beelbeebub said:

freeze damage to the heat exchanger in the HP. If that freezes you HP is scrap.

You are just scared mongering now. So now you are worried about a lots of steel plates that are insulated inside an insulated box freezing. If that is the only concern, remove insulation, wrap with 12v heat trace, re insulate, and use simple relay to switch the heat trace on in event of power outage when heat exchanger drops to 3 degs.

[Beelbeebub]

11 minutes ago, JohnMo said:

You are just scared mongering now. So now you are worried about a lots of steel plates that are insulated inside an insulated box freezing. If that is the only concern, remove insulation, wrap with 12v heat trace, re insulate, and use simple relay to switch the heat trace on in event of power outage when heat exchanger drops to 3 degs.

Not scare mongering.

 

*if* the heat exchanger freezes, it's buggered.

 

As you correctly point out, it won't freeze that easily.

 

The only real possibility is a long power cut during a persistent cold snap eg beast from the east (the conditions when the electrical hearing trace won't help)

 

However, in the unlikely event that happens you are onky concerned with draining down the HP. The buried portion prob won't freeze and even if it did it's not that big a disaster.

 

So an AF valve will still provide protection as long as it is positioned to drain out the HP. The rest doesn't matter

[PhilT]

if my calcs are correct a 20% glycol mix will reduce efficiency by around 2%. Important for fully informed choices.

 

Water Cp                             4.18        
Fernox HP-5c @ 20%        4.085        -2.3%
 

[sharpener]

Search for the earlier thread "Are all glycols equal" or wtte.

[BotusBuild]

8 hours ago, SteamyTea said:

Not the same stuff. The antifreeze for HPs is more environmentally benign

Agreed, but the similarity of the principle still holds 🙂

[sharpener]

11 hours ago, SteamyTea said:

Not the same stuff. The antifreeze for HPs is more environmentally benign.

 

 

Depends. Cars use ethylene glycol based antifreeze. HPs use either that, or propylene glycol which is markedly different in several of its properties. Fortunately someone has researched this stuff before <g>, it's all in here.