ASHP glycol

[Triassic]

I’ve been reading up about ASHPs and a number of people have mentioned the cost of glycol. Could someone enlighten me, what is the glycol for?

[ProDave]

The "water" that circulates around the primary loop and out to the heat pump, has to contain antifreeze as it's going to encounter sub zero temperatures in winter if the HP is off and you don't want it to freeze.

 

I have just used standard central heating inhibitor / antifreeze that is glycol based at a concentration that will not freeze at -10.  I didn't find it that expensive.

 

If you think this is expensive, look at the cost for the ground loops of a Ground Source Heat Pump.

 

I think those surprised by the cost may be using a buffer tank without a heat exchanger, so the volume in the buffer tank also needs to be glycol.

[ragg987]

You do not need glycol witha split ASHP. The primary loop is filled with refrigerant, in our case this was already inside the unit and once pipework was in place the pressure released to fill the system.

[PeterW]

As @ProDave says - you can use ordinary inhibitor based antifreeze which isn’t that expensive and the volume is about 15-20% to get the cover down to -10c. It’s only really an issue if the ASHP isn’t in use and the outdoor pipes freeze - it takes water a fair while to drop to freezing point if they are insulated with 15mm of nitrile rubber insulation (35c to -2c is 9 hours from memory) so this is only really needed if the heat pump is not on for a long cold period or the power fails. 

 

I plan to use a mix of decent insulation, an amount of inhibitor and also a timed frost programme on the heat pump to run the pump every couple of hours if the outside temperature is below zero. All those should resolve any issues. 

[ProDave]

I notice my ASHP, if not running and the outside  temperature gets low, it starts the pump for about 5 minutes (just the circuulating pump not the heat pump) presumably to mix the water around and get some less cold water into the outside bit.

[ragg987]

21 minutes ago, PeterW said:

It’s only really an issue if the ASHP isn’t in use and the outdoor pipes freeze

What about water inside the heat pump? I would think that is going to freeze sooner than water in an insulated or underground pipe.

 

[PeterW]

I’d included that - sorry ..! The only component at risk is the plate heat exchanger and they are partially protected by being inside the casing. Either way, protecting to -25c in UK climates seems a bit overkill. 

[Guest Alphonsox]

18 hours ago, ProDave said:

I think those surprised by the cost may be using a buffer tank without a heat exchanger, so the volume in the buffer tank also needs to be glycol.

 

That's my problem - With the current set up I'd need to have the buffer tank and UFH loops filled with a glycol mix. I guess the answer could be to add a heat exchanger between the heat pump circuit and the buffer tank+UFH.

Any ideas as to the best way to design and spec such a system ?

[PeterW]

Glycol is cheaper than 2 pumps and a a heat exchanger..!!

 

I’ve got a spreadsheet that works out hon much you need somewhere if you know the loop lengths etc

[Alexphd1]

Thinking the other way, do we actually need glycol? We never installed any special antifreeze (other than off shelf inhibiter) in an external oil boiler?? 

Only problem we have seen with a heating system freezing is when the system failed when a family was on holiday for a few weeks on a really cold snap of weather. This was a gas boiler in the garage but it was pipes in the eaves with poor insulation directly beside the vents that burst. 

 

[ProDave]

As I have said I just used cheap Inhibitor / antifreeze from screwfix.

 

I would not want to leave it out. Imagine a winter storm that brings down the power lines so no power to keep the HP working or even the circulating pump going.

 

If you freeze and split the heat exchanger that's an expensive professional repair as the refrigerant gas would need removing, heat exchanger replacing (brazing) and the gas re filling.

 

I always worried about outdoor combi boilers. You can protect the heating circuit, but not the cold water in / hot water out circuit, so again prolonged winter power cut could be a big problem.

[Jeremy Harris]

I was grateful that I was accidentally supplied with a buffer tank with an indirect coil in it by mistake, as that massively reduced the amount of inhibitor/antifreeze needed. Like @ProDave, I used a 5 litre can of the stuff from Screwfix that's about £20, IIRC.  That was plenty to give 25% dilution for our UFH loops, ASHP volume and indirect buffer coil volume.  Would have been expensive if I'd used a direct buffer, though, as the total volume of our system would have been up around 90 litres then, I think, so for 25% dilution I'd have needed to buy five (may have just about got away with four) 5 litre cans of the stuff, so around £100 rather than about £20.

[jack]

Genuine question: why do ASHP manufacturers require the use of glycol if Sentinel-type inhibitor is good enough? Are you sure that a couple of bottles of inhibitor is sufficient to prevent against freezing to a low enough temperature? 

 

I should say that we had a circulation pump failure on our ASHP after just over two years. It failed after having been off for several weeks, and possibly up to three months, over summer last year (our PV diverter basically kept the tank hot for most of summer). At the time, we weren't running glycol, because we were trying to get around to finalising the installation. I had it from a couple of sources that this shouldn't have made any difference, but it cost me hundreds to replace the pump and board (it's strongly recommended that they be replaced together). The only thing I did that was out of the ordinary was not to use glycol, so I'm reticent to recommend running without.

[Jeremy Harris]

I think the combined inhibitor/antifreeze stuff is glycol based.  I strongly suspect that it's just a mix of ethylene glycol plus a small amount (0.1% to 1%) of sodium nitrite as a corrosion inhibitor. It's no cheaper to just make a DIY mix up though, at least not for just 5 litres of the stuff, as ethylene glycol is around £20 for 5 litres on it's own.

[Alexphd1]

Is there any efficiency drop in running a high antifreeze? 

 

For the record I will be running a anti freeze mix when i fill the system.

[pdf27]

9 minutes ago, Alexphd1 said:

Is there any efficiency drop in running a high antifreeze? 

 

For the record I will be running a anti freeze mix when i fill the system.

It’ll take a minor hit - glycol is a bit more viscous so the circulating pump has to work harder, and the heat capacity is a little lower which will push the flow temperature up marginally and so the COP down. The impact will be very small though - a couple of percent I’d guess.

[Nickfromwales]

External boilers ( combi / system ) have a manufacturer required frost stat. At the set temp the boiler will fire and the heat from the exchanger will keep hot / cold etc from freezing. That assumes the pipe work has been insulated accordingly of course. 

In adverse installs an electric driven ‘heat tape’ should be laid with any susceptible pipe work to give an offset heat input, ( the colder it gets the higher the output, and vice versa ). 

In cold weather any system should be designed to stave off frost attack. It’s only in power fail that you cannot mitigate against such a risk and then have to rely solely on antifreeze measures in the liquids. 

[Guest Alphonsox]

So in my case the total system volume is around 160ltrs so I'd need 40ltrs of Screwfix antifreeze @ 25% dilution. So around £170 all in with replacement every 5 years IIRC.

 

[Triassic]

40 litres costs around £50

 

 

 

 

Edited September 11, 2018 by Triassic

[Guest Alphonsox]

18 minutes ago, Triassic said:

40 litres costs around £50

 

From where ? Glycol seems to be £4/ltr from just about anywhere.

[Nickfromwales]

2 hours ago, Alphonsox said:

So in my case the total system volume is around 160ltrs so I'd need 40ltrs of Screwfix antifreeze @ 25% dilution. So around £170 all in with replacement every 5 years IIRC.

 

I'm still struggling to find anything that backs that timeframe up tbh. Google doesn't exactly offer anything up on a cheeky scan. Some professionals seem to think it will last much longer than 5 years. 

[Triassic]

Surely you would use an antifreee tester to determine if the antifreeze was ok or not.

[Nickfromwales]

2 minutes ago, Triassic said:

Surely you would use an antifreee tester to determine if the antifreeze was ok or not.

No idea.......and don't call me Shirley  

[dpmiller]

Certainly motor antifreeze can be "lifetime" rated now and that's more about the inhibitors than the glycol. I'd contend that the vehicle application is more hostile too.

 

But is there anything in a heating / ASHP system that might be sensitive to OAT or HOAT inhibitors?

[PeterW]

You can’t use a standard motor antifreeze as it’s propylene glycol which is harmful so isn’t licenced for use in potable systems. If you only use it for UFH then it may be acceptable, not sure how a sunamp would work with it - one for @AndyT I expect as there are two layers of separation between the heating medium and the potable water.