I don’t have OEM so rely on the Vaillant’s own data, fed into Home assistant for logging and see a similar effect for my 5kW unit.
During the recent cold snap it was averaging about 5kW when heating, when Vaillant claim about 6kW. It rarely reached the target flow temp before a defrost kicked in. Hot water runs also meant it wasn’t heating the house for about 3 hours a day, further reducing effective space heating to just 100kWh in 24 hours, with 14kWh of water heating on top.
Our heat geek design was about 5.5kW which seems slightly over reality, we probably needed around 5kW to maintain 20C. So we were ok until a water heating run triggered and the house dropped half a degree, which the Vaillant couldn’t recover. The log burner came to the rescue.
I had been thinking about asking heat geek to replace with a 7kW unit, but hearing about noise issues gives me pause. We haven’t had any noise concerns with our 5kW unit.
@Abamuk In your case, I would look at using the DHW tank’s emmerson when it is very cold outside, so your heatpump just needs to cover the central heating load. We only get cold snaps like that for a few days every few years in most of UL.
Looks like you can typically expect 5 kW from the 5kW Arotherm, though it’s probably worth avoiding getting too close to that when sizing based on accurate heat loss.
On average the 5kW is closer to the datasheet than the 7kW - but only marginally so, an average under-capacity of 23% rather than 27%.
I use the vailant outdoor temperature sensor (via ebus) as input for heatpump app, however the “air inlet temperature” is often a couple of degrees lower than this.
Currently my 7kW is in the middle of DHW run and just did a defrost! Looking at the heatpump app current temperature is 6.8C and it shouldn’t really be doing a defrost, however if I look at the air inlet temperature it is at 4.8C.
Met Office: 6C
Vailant Controller External Temp: 6.8C
Air Inlet Temp: 4.8C
So not only is the a potential difference between actual and met office temperature, but (unless my sensors aren’t correctly calibrated) there can also be a significant difference between vaillants own sensors. Do others also see a difference between outdoor sensor an air inlet sensor or could it be something about where the air inlet sensor is positions or air movement over it, that means it reads differently?
It’s cooled a bit by the air flow, as you can see by the periodic drops when the compressor comes on. Here’s outdoor sensor, inlet temperature and my own weather station.
LHS looks like air inlet temp drops 0.5C below when compressor is on and is slightly higher between cycles (maybe during defrost?). Yet after 3pm on 16th you can see a constant difference of 1C, any idea why behaviour changes?
In my case air inlet temperature seems to drop beween 0.5-1C when the compressor is on, but it’s also around 1.2C lower when compressor is idle. I’m guessing one of these sensors is slightly out. My concern is that if the airInletTemp sensor is under-reporting for some reason then could I potentially be getting defrosts when not needed (like today @ 6C). Did anyone work out what triggers defrost?
I’d expect defrosts to be triggered by the heatpump detecting that the coils are iced up, rather than any external temperature sensor. An under-reporting sensor shouldn’t matter.
Thats one posbibility. The other possibility is it looks at difference between calculated dewpoint and air inlet temperature and decides defrost frequency based on this somehow.
I think detecting ice up is more likely, but this has got to be via a sensor, there is noone outside looking at the ASHP. I’m just wondering what sensor it is and what the threshold is.
It’s definitely tied to output power because It always looks like this for me. Heat power decays for same electrical input and at some point I get steady state behavior where there should be cycling. Then a defrost is triggered
A Daikin special engineer told me that the refrigerant temp in the evaporator triggers defrosts. This makes sense as with max compressor power the refrigerant out of the evap clearly gets colder as the evaporator loses efficiency ie it picks up less heat from the atmosphere so gets colder. You can see it here.
thanks for this, well if it’s true that it’s putting out an average of over 11kw while defrosting this often: then my heat loss is wrong by a mile, and i mean a mile, maybe 50% off
On the Panasonic’s for example, the defrost cycle is triggered on a sliding scale between outdoor temperature and evaporator temperature. So an error in either of these sensors will affect things.
We had some fog this morning in sub-zero conditions, so that was perfect for another test.
At an average -1.2 °C and 100% RH (visible fog), I got 4 defrosts within 2 hours. Heat output at the approximately 40-45°C flow temperature was 11-11.5 kW, which fits the datasheet between the values for 35°C and 55°C flow temperature (given as approximately 12 and 10 kW). Actual heat output with defrosts was 8.5 kW, so approximately 25% less.
We also had freezing fog this night, a nightmare, defrost every 30 min at usual demand
First time for me the TRIVAI kicked in
but I had to push the heat curve to help the boiler due to ext temp not being low enough… automation now created for that, sorry for the off topic
Thanks @Andre_K that’s a useful data point, nice to see the target flow temperature well above the flow temp achieved, helps give more confidence to the result. I’ve added this to the spreadsheet. Did you perform any other similar tests? Any more you can perform would be much appreciated!
@Ulric is there any chance you could increase the curve significantly to get that target temp right up just to see what the heat pump can do under your current conditions?
Not quite reached 1200, but 1100 seems much better.
