Glycol vs Antifreeze valves?

[JamesPa]

There seem to be two competing ways to ensure a monoblock outdoor unit doesn't freeze on system failure, namely fill with glycol/water mix or use antifreeze valves.  Both would seem to have potential advantages, and disadvantages.  Has anybody got any experiences and/or reasoning to share for the reasons to choose one over the other?

 

 

[ProDave]

Explain to me how an antifreeze valve works?  Will it shut off the water AND drain down the outside unit completely, and what about re filling and bleeding when it warms up?

 

Antifreeze / inhibitor is not expensive do it properly.

 

Our LG ASHP will automatically turn on the circulating pump when the water temperature gets low to move the slug of cold water into the house.  It will do this even when the unit is registering a fault code that otherwise stops it operating.  So it would need a major power failure to stop that "self protect" function from working.

 

Another thought about the antifreeze valve, that would have to be in some way linked to disabling the ASHP totally so it does not try and do it's turn on the circulating pump thing.

[JamesPa]

As I understand it they drain the system if the pipe temperature falls below 4C.  They are often combined with a pair of normally shut zone valves (kept open by power but which shut under sprig pressure on power failure) so it's only the outdoor unit which is drained.  Refill is manual 

 

Both glycol and antifreeze valves are only really necessary in the case of an extended power or major system failure in cold weather).  When everything is functioning as it should the antifreeze function of the ashp kicks in as you correctly say.  Glycol/antifreeze valves are needed probably once in 10 years!

[Nickfromwales]

49 minutes ago, ProDave said:

Another thought about the antifreeze valve, that would have to be in some way linked to disabling the ASHP totally so it does not try and do it's turn on the circulating pump thing.

Don't ( modern at least ) ASHP's shut down with low pressure warning like a boiler on a sealed system? 

Treating ( or slightly over-treating ) a system with Glycol seems fit and forget, tbh, and is near zero maintenance. Add the ASHP's own safety mechanisms on top of that and you need a doomsday event before this becomes a serious consideration.

[Nickfromwales]

4 minutes ago, JamesPa said:

Glycol/antifreeze valves are needed probably once in 10 years!

Or never, eg if the concentrate of Glycol / other system treatment chemical is monitored and maintained ( as it should be regardless )?

[ProDave]

8 minutes ago, Nickfromwales said:

Don't ( modern at least ) ASHP's shut down with low pressure warning like a boiler on a sealed system? 

Treating ( or slightly over-treating ) a system with Glycol seems fit and forget, tbh, and is near zero maintenance. Add the ASHP's own safety mechanisms on top of that and you need a doomsday event before this becomes a serious consideration.

A total power cut is one such event.  A previous rental property had an outside oil combi boiler so by nature it has raw mains water inside it.  The boilers frost stat will take care of things in normal times but I always worried about a several days long power cut in the middle of a cold winter spell.  I guess running a hot tap frequently would purge the near freezing water out to be replaced with mains temperature water, or even leaving a hot tap on a slow dribble to keep a constant flow.

[JamesPa]

Several ashp manufacturers do suggest antifreeze valves as an alternative to glycol, so they are definitely a recognised alternative solution.  It sounds like everyone on here has gone for glycol? 

 

As several people have said, the additional level of protection offered by either is only really needed in an extreme circumstance such as extended power cut or really major system failure.

[dpmiller]

anybody got a note on how they're supposed to work?

[Spreadsheetman]

The glycol inhibitors are pretty expensive though. The cheapest stuff I have seen is £32 for 5L at Screwfix and that makes 20L when diluted down. A heating system could easily have over 100L of circulating fluid so that is £160+ for a fill.

 

Better hope the system never needs draining down! (or there is a way of collecting and re-cycling the contents - or safely disposing of the contaminated fluid if not)

[MikeSharp01]

5 hours ago, JamesPa said:

as extended power cut or really major system failure.

But only in freezing weather, so about 2 days a decade down here.

[trialuser]

3 hours ago, Spreadsheetman said:

The glycol inhibitors are pretty expensive though. The cheapest stuff I have seen is £32 for 5L at Screwfix and that makes 20L when diluted down. A heating system could easily have over 100L of circulating fluid so that is £160+ for a fill.

 

Better hope the system never needs draining down! (or there is a way of collecting and re-cycling the contents - or safely disposing of the contaminated fluid if not)

I bought 20L on ebay for 94 quid a couple of weeks ago, just seen the same seller is now 120!

There is some for 95 at the moment though.

[JamesPa]

3 hours ago, MikeSharp01 said:

But only in freezing weather, so about 2 days a decade down here.

So are you suggesting not to bother with either glycol or antifreeze valves.  It's definitely a very rare event that they are needed 

[MikeSharp01]

8 hours ago, JamesPa said:

So are you suggesting not to bother with either glycol or antifreeze valves.

No - it is just about your attitude to risk:

• No Glycol / No Valve - Can destroy ASHP, no cost now and won't happen very frequently. (Low likelihood / High Impact)
• Glycol / No Valve - System safe (you can forget), will restart once disturbance is gone, £160 for the Glycol. (Low Likelihood / Low Impact)
• No Glycol / Valve - System safe (you can almost forget), system won't restart after disturbance as it will need a fill and flush, £????? for the valve version. (Low Likelihood / Moderate impact)

Which suits your risk appetite best?

[JamesPa]

2 hours ago, MikeSharp01 said:

No - it is just about your attitude to risk:

• No Glycol / No Valve - Can destroy ASHP, no cost now and won't happen very frequently. (Low likelihood / High Impact)
• Glycol / No Valve - System safe (you can forget), will restart once disturbance is gone, £160 for the Glycol. (Low Likelihood / Low Impact)
• No Glycol / Valve - System safe (you can almost forget), system won't restart after disturbance as it will need a fill and flush, £????? for the valve version. (Low Likelihood / Moderate impact)

Which suits your risk appetite best?

That's a nice summary.  I guess the only other argument one might make is that glycol reduces the heat capacity of the water by about 5percent so the pump will have to work 5percent harder.  That's a pretty marginal increase in energy consumption but of course the cost and carbon footprint does accumulate over time. 

 

For those of us with a dislike for compromising the 'normal' system operation to deal with a rare fault condition it's an annoying choice to have to make.

[dpmiller]

£100 for a one-off fill of glycol vs (say) coming back to a cold  *and unheatable* house after a nice skiing holiday, because you'd turned the heating off and there was sudden cold snap?

 

No-brainer for me, and I don't even do skiing...

 

 

[JamesPa]

13 hours ago, dpmiller said:

£100 for a one-off fill of glycol vs (say) coming back to a cold  *and unheatable* house after a nice skiing holiday, because you'd turned the heating off and there was sudden cold snap?

 

No-brainer for me, and I don't even do skiing...

 

 

Because

DPMiller I agree that £100 vs the risk of catastrophic failure is a no brainer (and I don't ski either).  However...

 

Because heat pumps have built in frost protection, the scenario you cite will only occur if either a)the heat pump fails or b) power fails for an extended period of time, and there is simultaneously a cold snap.  In scenario a) you will return to a house which is unheatable whether or not you have glycol/antifreeze valves!

 

For me this is not about cost, its about system performance.  Glycol increases by 5% the amount of liquid that needs to be pumped round the system, because it has a lower heat capacity.  Furthermore the following post points out that the effect is worse still, because a glycol/water mix is more viscous than pure water:  https://commercial.centralheating.co.nz/assets/resources/The-Effects-of-Propylene-Glycol-on-Pump-Performance-web.pdf  Other sources (eg https://metersolution.com/wp-content/uploads/2013/02/Glycol-Pump-Losses.pdf) also confirm that glycol/water mixes are more viscous than water. 

So we are adding glycol to the system in order to deal with an unlikely (albeit serious) fault condition and by doing so are degrading its  operational efficiency.

 

The degradation is admittedly small.  The NZ paper suggests that the circulation pump might have to work up to 25% harder.  If this is correct then, based on a pump consumption of say 45W, 11W extra is consumed, which equates to 56kWh/Yr based on 7 months operation 24 hrs per day.  Ofgem says that the average house needs 12000kWh/annum to heat so with an scop of 4 that's 3000 and the additional load on the pump is 2% of the total.  Probably not enough to swing the balance in favour of anti freeze valves, but annoying nevertheless.

 

[ProDave]

18 minutes ago, JamesPa said:

Because heat pumps have built in frost protection, the scenario you cite will only occur if either a)the heat pump fails or b) power fails for an extended period of time, and there is simultaneously a cold snap.  In scenario a) you will return to a house which is unheatable whether or not you have glycol/antifreeze valves!

That is a very real possibility here, in the past we have had days long power cuts in winter storms with power lines down, and prolonged sub zero temperatures are common here in winter.  I will keep my antifreeze.

[HarryUp]

Our Ecodan SHP was commissioned a couple of weeks ago and was set up with anti-freeze valves and running with no glycol. Which I wasn't expecting! The increased viscosity was mentioned by the installer, along with the orange staining tint should something leak. Glycol will deteriorate in time and the system flushed and refilled with fresh glycol ££ – not quite fill and forget. So, harder work for the circulation pump and requires more effort on part of the HP to deliver the heat.

On the other hand the system's anti-freeze mode requires circulation at 20° when external temps fall below 5°, which has been quite a chunk of recent nights. This circulation doesn't count as 'heating' apparently and doesn't appear in the data records, though it must be counted somewhere. I'd like to know how much it costs to run the HP to not heat the house.

" No Glycol / Valve - System safe (you can almost forget), system won't restart after disturbance as it will need a fill and flush, £????? for the valve version. (Low Likelihood / Moderate impact) "
Why would it need a flush and fill? It would need filling as the valves will have jettisoned some water but then this just requires adding water using the filling loop, no?

[JamesPa]

Interesting what the ASHP antifreeze mode does. 

 

In fairness my existing gas boiler also switches on when its cold outside, even if there is no heat demand, and it appears to heat to full flow temp, so a similar level of 'inefficiency' (or prudence if you prefer).  So your ashp heating to 20C (at which COP will be >5) is consuming much less energy to keep the pipes from freezing than my boiler.

 

 

 

[Adam2]

Mine comes with this https://coolenergyshop.com/collections/installation-accessories/products/thermox-dtx-25l-inhibited-glycol-ce-tmdtx25 may have better properties re heat transfer + viscosity compared to other options?

[markocosic]

It's Ethylene Glycol; treated to make it less toxic to those drinking it.

 

https://www.hydratech.co.uk/Technical/Ethylene-Glycol/0/34#

 

 

Toxicity isn't otherwise a big deal. (it breaks down fairly readily)

 

https://www.atsdr.cdc.gov/toxprofiles/tp96-c1.pdf

https://pubmed.ncbi.nlm.nih.gov/11302583/

 

 

As such corrosion inhibitors in the "heat transfer fluid" are more of a big deal than the anti toxicity elements. Not uncommon to buy the glycol+additive; then topup additives; rather than dumping the coolant when the corrosion inhibiting additives are consumed:

 

https://www.stafor.lv/gb/heat-transfer-fluids/staterm-concentrate-heat-carrier-coolant#Package:20l

https://www.stafor.lv/gb/heat-transfer-fluids/staterm-long-life-additive-package-for-heat-carriers---coolants-concentrate-1100#Package:1l

[markocosic]

In the UK, in an urban environment, in a house that's regularly occupied, I wouldn't bother with either antifreeze or antifreeze valves. Just some manual valves to drain the outdoor unit down in the event that it fails / electricity fails when it's cold out.

 

Graham's notes are worth a read:

https://www.linkedin.com/pulse/why-do-we-put-glycol-air-source-heat-pumps-graham-hendra

 

If the house will be left empty and unattended for extended periods often enough to worry about the header tank and fill valve in the vented attic space freezing; the electricity is prone to failing in winter; or you're somewhere that actually gets cold then probably antifreeze it.

 

 

Around these parts there are three options:

 

- City district heating = water (and don't for gawd's sake allow it to freeze)

- House in the countryside = -20C antifreeze so that if the heating is out of action for a few days you don't burst all the pipes

- Holiday house in the countryside = -40C antifreeze so that you can leave the heating off without bursting all the pipes

 

The pipes they're worried about...are the pipes INSIDE the house not the pipes in the monobloc. You antifreeze these...even when the heat pump is a split rather than a monobloc. 😄

[SuperJohnG]

Thus is exactly the thread I was looking for? Thanks. 

 

So what did you do in the end @JamesPa? I've got to make the choice of glycol or antifreeze valves in the next few days. Anyone else any input? 

[RobLe]

I’d hoped to be able to cope with water in the underground loop of our small gshp, but had to add glycol as it started to freeze up a year ago.  The flow dropped from 20lpm down to 14 which I had been expecting.  I hadn’t expected that the heat exchangers would be less effective; the propane:water exchanger had a 3C drop which increased to 6C after glycol was added.  When manufacturers quote a COP, is that with water or glycol, as it makes a difference.

 

[JohnMo]

I have gone for antifreeze. But will only apply at about 15 to 20%.

 

Antifreeze valves, if they activate they will allow air in water out, great for protecting pipes, but you would need to bleed the system to get it back online. Antifreeze start up and you have heating. Likelihood of failure on demand, antifreeze is very low.

 

If you live in the sunny south in a town, not sure I would bother with either.  I live in NE Scotland and in the country, so feel some protection is required.

 

Antifreeze pretty low cost 10l about £75 from Screwfix. Two antifreeze valves £200.

 

Antifreeze does have a higher pumping requirement and worse heat transfer quality.  But system will generally be run sub 30 degrees, so will take the hit.

 

Antifreeze valves are generally exposed and wrapped in insulation so you get a heat loss there , so though one loss balances the other losses.