I’ve got a relatively new 7kW Vaillant Arotherm Plus system, mix of UFH in screed and radiators.
My installer initially filled the system with a 20% concentration of ‘Fernox HP-5c heat transfer fluid’, which is glycol-based. Spec sheet attached: 62544-Heat-Transfer-Fluid-HP-5c-EN.pdf (1.0 MB)
The system had subsequently been drained several times during maintenance, and re-filled with mains water, so the concentration of Fernox present in the system is now negligible. There are no anti-freeze valves fitted, so my system currently has no frost protection.
It appears as though at between 10-20% concentration of the HP-5c product, the specific heat capacity of the resulting fluid is not that far off that of pure water. This to my mind begs the question: do I ask my installer to re-fill with HP-5c, or fit AFVs?
Considerations:
AFVs are a demand-based mechanism, which carry the risk of not working when needed, or deploying erroneously and dumping fluid; HP-5c additive is always present and active.
Reliability of HP-5c’s claimed performance (specific heat capacity when diluted, viscosity)
Long-term effects of having glycol-based additive sloshing around the system
What would be the best approach in my case? I’m inclined to prefer the peace of mind of additive as opposed to AFVs, but not totally convinced of the real-world performance hit of the glycol-based fluid…
For me, glycol, Fernox Alphi-11. If it goes anywhere near a DHW cylinder then must not be ethylene glycol based. It also depends what the ASHP manufacturer recommends, some specifically prohibit its’ use.
Advantages
Pretty much fit and forget. You can buy Fernox water quality test kits, take a sample of water, send it to Fernox, they analyse it and tell you if your glycol and inhibitor are still OK. Also points out any problems from suspicious readings of hardness, copper, etc. Typical cost 34GBP. Much better than some mechanical contraption that is probably going to fail when you need it.
If I need to drain part of my system, I keep what I have drained off and pump it back in when I have finished, so no need to buy new every time you work on the system.
Not reliant on other systems. Go away for a few weeks in winter and you won’t come back to a burst pipe.
Disadvantages
Cost, but that should be one off-ish. Not sure when it needs changing, mine is 5 years old so far and still fine.
Specific Heat slightly lower than water, so you have to pump more to shift the same amount of heat. It means the peak output of your heat pump will be slightly down, but nothing to get excited about.
Viscosity higher than water, so it will cost more electricity to pump the same volume. My concentration is 30%, flow is down a few % when cold, but once up to typical working temperatures no noticeable difference to water.
Pipe Joints. It is good at finding dodgy joints in pipes. A joint that would not be a problem with water can weep slightly with glycol.
Glycol or AFV will only be needed if there is a prolonged power outage. As long as the heat pump has power, it has automatic frost protection by circulation water. If you get regular prolonged outages and cold temperatures in winter, then consider glycol, otherwise AFV are generally the preferred option for increased efficiency and ease of maintenance.
Glycol is a pain for monitoring, since it requires special glycol calibrated Sontex heat meters if you want to accurately monitor the heat output.
AFVs cost about the same as glycol, but never need to be replaced or topped up. The new intatec AFV are very easy to fit, they can go inline on the output pipe of the heat pump before the flexi is connected. This version will fit on Vaillant 1.25" flat-face thread. It’s a 5min job and doesn’t require draining down the system, assuming an isolator valves are fitted outside.
Glycol only if you’re in a properly cold place with regular power outages in a building that lacks sufficient insulation/thermal capacity to ride out the dips and isn’t occupied permanently/with backup heat source and you’re worried about an antifreeze valve dropping the heating system contents leading to all.your potable hot / cold water pipework in the dwelling freezing?
Regular house?
This will never be an issue in a property that is permanently occupied. Have decent internal isolation valves so that you can isolate on the warm side and drain the monobloc AND outdoor pipework for servicing and emergencies. Add antifreeze valves outside to cover ridiculous extremes. Done.
Better still work out how to get thermosyphon from the house into the heat pump (heat pump elevated relative underfloor heating so that cold water falls down into underfloor heating). You then have a couple tonnes of screed coupled to a few tonnes of house at 20C that can maintain the monobloc above 0C for days on end even with zero electricity.
Holiday home in the Highlands and islands?
Add insulation. Add a good solution for draining down the POTABLE water system. Add an air to air heat pump if being cheap (can’t freeze). Add a ground source heat pump if cost no object and have glycol in both loops.
Alcohol (ethanol) is actually better. Less sticky so reduced pumping costs. Non toxic (in the long term). But the revenue people make it impractical to fill ground loops and heating systems with vodka. Thanks humanity!
Our house is in London, hasn’t had a power cut in the 9 years we’ve been here, and winters here are relatively mild regardless.
I suppose my question was more about the specific Fernox HP-5c product specs suggesting a specific heat capacity very similar to that of mains water at a 10% dilution, much closer than ‘regular’ glycol-based additives, thus possibly not affecting performance much…
My other concern is corrosion/limescale inhibition; if switching to the AFV you recommended, would it be necessary to use and maintain an inhibitor additive?
Hi,
I have a Gen 6 Samsung 16kW unit which I installed in 2022. I also agonised over the Glycol v AFV issue. I fitted AFV’s to both the flow and return side and the main argument that caused me to arrive at the decision to fit AFV was the effect of Glycol on the underfloor pipes that are encased in the gypsum floor. The only advice that the piping manufacturer would offer was to use water and would not comment either way on the use of Glycol. So from a risk perspective, the AFV’s are an easy fit and as the ASHP has proved to be very reliable. The use of Glycol is to protect the ASHP from a risk that is unlikely to occur and in any rate if it did happen, at worst you could replace the ASHP. Alternatively if the Glycol causes an issue with the plastic pipes that are embedded in the floors of the property that would be very difficult to resolve.
I’ve decided to go with the Fernox HP-5c, instead of anti-freeze valves.
The recent cold snap reinforced this decision; if power had ceased for a while and the AFVs triggered, it would have been a major ballache in -1deg weather.
Our external flow/return primary pipes are also mostly buried, under a 2.5m long paved patio, so the AFVs would be unlikely to protect against the liquid in the buried pipes freezing; if this were to happen, it would be even more of a ballache waiting for the water to defrost before being able to fire up the ASHP again.