Yes, it is yours. In my mind, it was the perfect comparator, because as you say they are very similar and location meant weather wasn’t a differentiating factor either.
I’d previously looked at the Heat Demand tool for yours and it looks to be perfectly sized (there seems to be very few others like that), and I wonder if that’s where the problem is, in that there’s just no headroom when it comes to defrosts.
Oversized units, even a little bit, don’t appear to suffer in the same way.
Thanks for putting my mind at rest, I really appreciate it.
On a slightly different note, I’d expressed my concerns to my installer last week and they’ve spoken to Vaillant, who are now coming to do a refrigerant/health check on Saturday. I’m not expecting them to find anything now, but the more people that say it’s OK the better IMO.
Thought I would add a postscript to this Topic on defrost hell. In the ASHP sizing for Freezing conditions Topic I calculated the impact of humidity on heat pump maximum output to explain why I could not get anything like the 10.6kw out of my Daikin heat pump. This explanation I think works for any make of heat pump unless it has a massively oversized evaporator. I just found a handy ‘Defrost Hell’ prediction tool that should, at least, let you know why your HP goes to Hell on some days and not others.
Odiham | Weather History & Climate | Meteostat
This is my local weather station and from this simple dew point and temp chart you can predict that Dewpoint Hell happened when all of the following were true;
a) Temp was 3c or below and
b) there was less than 1c difference between temp and dewpoint
c) The controller was calling for maximum heat from the heat pump
I do not have any real data below -2c but suspect that by -4c you can play the ‘get out of Hell’ card as there is much less water vapour in the air to start with.
Do you have VF1/S1 disconnected? Can’t remember if I asked over on FB or not. This can cause a bit of a death spiral with defrost if it’s connected and can cause a similair picture to this where it over-compensates coming out of defrost causing compressor to increase to 100% provoking another defrost sooner.
Yes, a temperature sensor plugged into connector VF1 of heat pump controller (or S1 of wiring center).
The other thing to look at is: is ashp targetting a flow temp higher than it needs to for heat curve? If it is this also points to this sensor being connected and causing havoc.
I’ve had a look at the controller, there’s nothing in VF1.
I don’t have a wiring centre.
I’ve had a Vaillant engineer visit today (via my installer), who gave everything a thorough check. He replaced a “compressor outlet sensor” as it was reading high when everything else was correct. Don’t know if that’s relevant.
Regarding target temps, would I be right in thinking the only way I can see if this is an issue would be to look on the interface live monitor when the HP is coming out of a defrost? Actual flow temps (as seen in the app) seem to match the WC graphs/tables just fine.
Yes, look at live monitor and see if target temp is as expected for heat cuve or higher. If you have no VF1 sensor though, this is unlikely to be the issue. Let us know if/how the sensor replacement affects things…
I sympathise with your concerns about the impacts of frequent de-icing.
I have an aroTherm Plus 13.2kw and yesterday, there were 3 de-icing events:
00:12 Air Temperature 1.3C, RH 92%, wind speed 3 km/h
01:51 Air Temperature 1.6C, RH 92%, wind speed 0km/h
15:19 Air Temperature 6.2C, RH 76%, wind speed 73km/h
The first two events are expected for the RH and temperature but the 15:19 event is odd.
The wind was from the north and my heat pump faces north. Would a strong wind affect the air flow in the heat pump and could this be related to the unusual de-icing event?
It does sound somewhat reasonable. If the wind is strong and opposed to the normal air flow direction of the heat pump fans, the evaporator could freeze up because there’s just not sufficient fresh (warm) air coming in.
We had 2 rather unexpected defrosts yesterday at 07:00 and 14:45, both at about 6deg OT and 4deg difference to the dew point.
Previously, I’m not aware of us having a defrost at anything above 3deg.
We’re quite exposed to the wind, so I can only put it down to the constant 40kph wind chill across the back of the evaporator.
We had a couple of unexpected defrosts yesterday. I caught one of them just as it was finishing and at the point it switched from defrost to heating, the target temp went from 12deg to 28deg (correct for the WC and OT at the time). The flow temp ramped up within a few mins and then overshot to 28.5deg before gradually coming back down to 28deg.
On coming out of defrost, the power spiked at approx 1500W, before dropping off again, but I would estimate it was a good half an hour before returning to the expected level of around 600W.
I don’t know whether changing the sensor has made any difference, as I haven’t got detailed info from previously to compare it with. I guess I’ll have to wait until it’s colder and we’re having frequent defrost to find out.
That sounds like expected behaviour if the overshoot is minimal like that, and not the VF1 issue multiple others have been impacted with.
As to when defrosts should be expected, how long they should last, and how long system should take to recover from them, this isn’t something i’ve seen much data on personally. Be interesting to see if changing that sensor makes any real difference though.
The outside temp showing on the main controller was stuck at 15, then a few days later I got a ‘clean sensor’ message. I thought because we’d had a very dull couple of weeks and it faced north, it had run out of charge, however even after putting under good light for a few hours and checking the battery was good, still no good. No flashing LED on attempting re-pair. Vaillant came out within a week and replaced under warranty.
I thought I’d report back on my situation, following all the useful advice and reassurance I received.
I’ve become a lot more relaxed about the frequency of defrosts given that the property is just about maintaining temperature. From Tueday this week, our average outdoor temperature has been -1.5deg and as of 1pm today, it’s reached the dizzy height of -0.7deg.
If I had OEM kit, my traces would look almost exactly the same as those of @tommyt
I’ve also calculated our actual heatloss from the data I have available and it comes in at 6.6kW@-3deg, so right on the money for a 7kW Vaillant.
One useful thing I have discovered is that I can make use of the noise reduction function to stretch out the time between defrosts just a little. It also prevents the large power surge coming out of a defrost cycle.
Vaillant state you can adjust NR between 40-60%, although this doesn’t actually equate to the same power reduction, 40% NR seems to give 50% power reduction.
Handily however, my unit allows me to run with 30% NR (which is not 30%), which give a max power draw of 1700W. This discovery has allowed me to run “normal” power during my cheap electric period, when it can defrost as much as it likes but store some heat in the building, then with NR on the rest of the time to hold the temperature steady. This has been surprisingly successful.
One other thing that’s happened since Xmas is we were getting some resonating through the building at certain compressor frequencies, typically when the compressor was drawing multiples of 700W. I’ve since had my installer back and they’ve fitted those semi-flexible hoses to successfully isolate the heatpump from the building. Additionally, they took all the casing off the heatpump and added foam insulation tape and cable ties to various internals to stop them vibrating/buzzing against each other.
Anyway, hope everyone is nice and warm and thanks for the help.
