Frost valves don't strike me as a particularly elegant solution. Their protection will I suspect be very local, and when I looked for an answer to what happens after your frost valves have de-pressurised the system, I didn't really find one.
There are arguments both ways on this and each has its following. Glycol has a higher viscosity and a lower heat capacity than water so may slightly degrade system performance. Also some don't want their system entirely filled with glycol so fit a heat exchanger and secondary pump (with all the risks that carries of mis-plumbing) so only the outdoor loop is protected.
As you say a/f valves offer primarily local protection, but assuming you are only trying to protect the outside unit and anything in the immediate vicinity that's whats wanted.
I don't know of any definitive tests nor indeed have I seen reports of frost damage. The risk is largely a function of the reliability of your mains power.
This post was modified 1 year ago 2 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
I'm going to run the raw data that you kindly supplied, but I need to set the initial conditions for your system. Could you please remind me of your calculated heat loss at what OAT, and the total heating capacity of your heat emitters.
Also an assessment of the thermal capacity/thermal mass of your home would be useful.
There are arguments both ways on this and each has its following. Glycol has a higher viscosity and a lower heat capacity than water so may slightly degrade system performance. Also some don't want their system entirely filled with glycol so fit a heat exchanger and secondary pump (with all the risks that carries of mis-plumbing) so only the outdoor loop is protected.
As you say a/f valves offer primarily local protection, but assuming you are only trying to protect the outside unit and anything in the immediate vicinity that's whats wanted.
I don't know of any definitive tests nor indeed have I seen reports of frost damage. The risk is largely a function of the reliability of your mains power.
My calculated energy out uses a lower specific heat capacity than pure water because my system has glycol in it, meaning I have assumed the performance is degraded a bit (by about 4% as I recall). I also have a heat exchanger (these have generated a lot of discussion in the past) and the reason i still have one is it limits the amount of pipework with glycol in it. Food grade glycol is not cheap, and it needs to be food grade because the fluid also heats the DHW, and if the coil in the tank developed a leak... As it happened, my heat exchanged was mis-plumbed originally, parallel rather than contra-flow, but that was very soon corrected. You only have to flip the pump through 180 degrees, no need to re-plumb.
I think the anti-frost valves might in practice offer only very local protection. Maybe there is a little corner in the heat pump body that gets even colder, or the valves by chance of where they are, behind the heat pump, are a little warmer than the other side of the heat pump, which by chance is subject to an icy blast.
Someone else made the point a while back that there seem to be precious few actual reports of frost damaging heat pumps, and it may well be more of a theoretical risk than an actual one. But I live in an area where we do get a fair number of power cuts, lots of overhead lines, and as I said earlier, a very small amount of frost can do an awful lot of damage, not to mention being without heating when you most want it. Normally I don't pay much heed to remote theoretical risks, but the risk of frost damage spooks me!
Midea 14kW (for now...) ASHP heating both building and DHW
I'm going to run the raw data that you kindly supplied, but I need to set the initial conditions for your system. Could you please remind me of your calculated heat loss at what OAT, and the total heating capacity of your heat emitters.
Also an assessment of the thermal capacity/thermal mass of your home would be useful.
Derek,
I'll find the numbers but it may be a day or two as I'm a bit tied up with family stuff and not at home much atm.
I'm going to run the raw data that you kindly supplied, but I need to set the initial conditions for your system. Could you please remind me of your calculated heat loss at what OAT, and the total heating capacity of your heat emitters.
Also an assessment of the thermal capacity/thermal mass of your home would be useful.
Derek,
I'll find the numbers but it may be a day or two as I'm a bit tied up with family stuff and not at home much atm.
Thanks Kev, no rush. Could you also please add the RWT values so that I can ascertain when the compressor is running.
The last 48 hours data, comparing a cold night with a less cold night, showing the impact of marginal capacity and defrost cycles when OAT goes below and stays below 5 degrees. Note how in the cold period the LWT/RWT fail to reach the Set LWT, and the much higher energy use during that period:
Midea 14kW (for now...) ASHP heating both building and DHW
The last 48 hours data, comparing a cold night with a less cold night, showing the impact of marginal capacity and defrost cycles when OAT goes below and stays below 5 degrees. Note how in the cold period the LWT/RWT fail to reach the Set LWT, and the much higher energy use during that period:
Excellent data once more.
It would be interesting to see how the OAT value on the chart compares with OAT readings from local weather stations.
The last 48 hours data, comparing a cold night with a less cold night, showing the impact of marginal capacity and defrost cycles when OAT goes below and stays below 5 degrees. Note how in the cold period the LWT/RWT fail to reach the Set LWT, and the much higher energy use during that period:
The difference is clear. A couple of points. I'm surprised your ASHP never runs constantly. Surely at 8 deg or so it should be ticking along at about half power rather than cycling (albeit not very frequently). Also (and I think I said this before), your flow temps drop very low, well below IAT when it's cold. Where are the sensors? Are they somewhere that's colder than where the IAT is measured?
It would be interesting to see how the OAT value on the chart compares with OAT readings from local weather stations.
Thanks. I think the Met Office weather stations are mostly fairly similar, despite the fact one is on the coast, and the other inland. The data is in HA and I will see if I can pull it out and plot it against the Midea reported OAT.
I'm surprised your ASHP never runs constantly. Surely at 8 deg or so it should be ticking along at about half power rather than cycling (albeit not very frequently). Also (and I think I said this before), your flow temps drop very low, well below IAT when it's cold. Where are the sensors? Are they somewhere that's colder than where the IAT is measured?
I am also surprised. Every now and then it settles down eg around noon yesterday, but it definitely prefers to cycle even when it could run steady. I've never been able to detect a pattern as to why it does this, and have sort of accepted it, given it isn't short cycling.
The LWT/RWT sensors must be in the heat pump enclosure outside, there is nothing outside the box that could be them. There is a sort of logic to measuring them that way. What I am not sure about is whether the primary circulating pump operates at all during the setback. On paper, it shouldn't, the heat pump is meant to be fully off, just as it would be on a warm summer's day, when the IAT is above the room stat set point. I think what we are looking at is an exponential decline of temps inside the heat pump as they approach the OAT.
Midea 14kW (for now...) ASHP heating both building and DHW
@derek-m I'm trying to get decent data out of my Grant Aerona over modbus too. Sadly, it also provides low resolution data; key values such as LWT and OAT are only provided to a resolution of 1°C, as (signed) integer values.
(Sorry .. I'm replying out of sequence ... this relates to something from 3 days ago!)
As an aside my Grant enters defrost cycle every 42 minutes, which it's been doing almost constantly for the last 3 sub-zero days.
I think that you will find that the cycling is most likely due to the 'cold well' effect, which appears to be shown by the raw data.
When the heat pump starts the LWT increases, and the heat pump commences pushing out cold air. Some of this cold air is being drawn into the intake, where the OAT sensor detects this lower air temperature and increases the required LWT setting, which in turn causes the heat pump to work harder and push out even colder air. This process continues until the LWT reaches the upper limit and the heat pump stops.
The OAT sensor reading starts to increase and the LWT reduces to the lower limit at which the heat pump restarts. This cycle will repeat until the actual OAT rises to the point where the cooling of the OAT sensor is less pronounced. Obviously at lower true OAT values this problem becomes even worse, hence the frequent defrost cycles.
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