Newbie here, so might be asking obvious questions…
We recently had an aerotherm plus 5 installed in our house. We are based in Scotland.
All the valves on our radiators are full on, and we do not have a buffer tank. The heating is on all day.
Our curve is 0.6, we are running on expanded mode, max flow at 40 Deg.
Set back temp is 16deg from 10pm to 8am, then daytime at 17.5, low I know but we are comfy at that temp.
The highest I have seen for our heating flow temp is around 32deg, normally around 25-30.
So even though our flow temp is always low, according to the sensoComfort our cop seems to be averaging around 3 to 3.5.
Looking at the graphs etc, I would have thought that we would be well above 4 at these flow temps?
So before I complain to our installer, would I be right in expecting better?
Energy wise, so far from the 1st to the 24th Sept we have used 63kWh just heating which TBH is great, much better than our old boiler. In that time range the average temp has been 9deg.
At 9°C and with less than 35°C flow temp you are totally correct to expect a better COP - far in excess of 4.
It is well documented (in this forum and elsewhere) that the built-in temperature sensors in Vaillant heatpumps can be very inaccurate, which can lead to huge discrepancies in displayed vs actual COP. Before I fixed mine, I got an overestimated COP by 40%. Underestimation is equally a possibility.
One way to determine whether that’s the case is to go to the installer menu on your indoor unit (not the room thermostat but the big unit with all the pipes going in that is indoors, access code is typically 17) and let the system show you flow and return temperatures. You have to wait for a time when it’s relatively mild outside so your heatpump needs to cycle, i.e. turn the compressor on and off intermittently. While it is off, the circulation pump is still running. During this period, flow and return temperatures should be equal. Note down this offset. If flow is lower than return temperature your COP is underestimated.
The other possibility I’m seeing is that your heat pump is currently cycling as it is still relatively mild outside and the calculated COP is lower than the datasheet COP because the circulation pump uses energy between active cycles where no heat is generated by the heat pump, which lowers COP. Especially during the milder times of the year you will see larger discrepancies between datasheet and actual COP precisely because of that reason. As a random datapoint, my own COP last night was 5.2 when just looking at the compressor on time and 4.4 when taking the circulation pump electrical use into account between the cycles.
Overall I’d say you’re right to question the numbers but it might very well be that the heat pump is performing ok. If you show us more data, like from the flow/return temperature experiment I explained above, we can help more.
I suspect one factor in the low COP you are seeing is the fact you are running on Expanded room compensation.
I have found when running in this mode the proportional part of the (PID) controller drives the flow temperature too high initially. This high temperature means that the desired room temperature is rapidly achieved and the thermostat (Sensocomfort) shuts the system down before the heat pump can modulate down sufficiently.
Example below where I changed room temp control from Expanded to Active
You can see the frequency increase at pretty much steady outside temperature where I made the change.
Associated with the mode change you can see the flow temperature reduces and the COP goes with it. The COP in the left hand half is 3.85 and in the right hand half is 4.48.
Been doing as you asked for the last couple of days (See attached)
So yeah the flow and return are different by around 0.4 when the compressor is inactive, is this a meaningful amount?
Although we seem to be sipping the electricity, we have seen that it takes more than 4 hours to get from 16 deg to 17.5 when it is cloudy. We benefit a lot from solar gain, so today for instance (26/10) wasn’t particularly warm but it was sunny, so our temp overshot to 18.9))
Now that I know to look for it, the compressor also seems to be inactive when I though we were meant to be heating. So this is cycling?, why would it be doing this?
Whilst the compressor is active the flow rate is 860l/h.
Our heat pump is along the side of our house, the other side to the pipework which is in the garage. So distance from heat pump to primaries is a matter of centimetres.
The pipework in the garage is 22cm I believe. The diameter of the pipes leading to the radiators is 18cm.
We have a floating floor with only around 80cm ish clearance I am guessing. The access hatch is restricted so difficult to see what is going on pipework wise. I did notice this splitter thing, whatever it is. It seems to split a 22cm pipe into lots of microbore pipes. This confused me as all the rads are 18cm?
The installer looked at the existing pipework from the old boiler, and the size of the pipes at the rads. Becuase of access issues he didn’t check the pipes under the suspended floor. So no idea about the index circuit i’m afraid.
A 0.4°C offset is quite big. If you also log your flow and return temperature while the compressor is on, we can estimate what this in percent error in your COP. It could easily be a 10-20% error, and the direction of the offset (flow is lower than return) is consistent with your gut feeling of having a too-low COP.
To your question why the heat pump is cycling: Heat pumps (and also gas boilers) have a minimum power output that they cannot go below. If your heating demand is lower than this minimum, the heat pump has no choice but to intermittently turn off in order to not overheat the house. Say your current heat demand is only half the minimum output of the heatpump, then it will be off approximately 50% of the time. This is very common, even more so in mild weather and with low indoor temperatures like yours.
Think I am following what you say. The following might be scrambled thinking on my part as still getting to grips in this area.
We moved to expanded as under the old active the temperature of the house was sometimes too high, i.e. it ignored the sensoComfort (which initially confused us a lot).
We definitely do not see the desired room temperature rapidly achieved, it takes more than 4 hours to go up 1.5 degrees without any solar gain through the windows. (we didn’t upgrade our rads as part of the install.)
Flow temp wise I also haven’t seen it go above around 32deg whatever setting.
So I guess we could experiment with putting it to active, but then move the heat curve down from 0.6 to 0.4, but that would presumably mean that it would take even longer to get from 16degs to 17.5?. Then if it is a sunny day and the temp jumps because of radiated heat through the windows, then we could carry on heating when the room temp overshoots?
Okay that makes sense to me (the cycling/miniumum power). So could we potentially actually use less electricity if we avoided continual cycling by raising up our desired house temp?
Am I also right in saying that because it is a minimum power output issue, lowering the heat curve wouldn’t be of any benefit to reduce cycling during this mild period in the weather?
Righto I’ll gather some more metrics when the compressor is on tomorrow and report back.
So would I contact Vaillant and ask them to do a house visit to re-calibrate the temp sensors?
No, you won’t save any electricity this way. Instead of being on e.g. half the time and consuming electricity, the heatpump would be on all the time and consume energy.
Slightly off piste but do you need to drop the house temperature overnight?
It can be a false economy as the energy you save overnight, you end up adding back in the morning, to get the house back up to temperature. The two will pretty much balance each other out.
(I do allow my house temperature to drop but this is just so I can add it all back between 04:00 and 05:30 to maximise my off peak electric usage. See chart sent earlier.)
If it’s to get a bedroom down to lower temperature you could balance the radiator flows to reduce the bedroom radiator flow.
Thinking about this a bit more; I think one of the issues with your low COP may be the fact that the heat pump is in standby most of the time (because of the low desired temperature). It will be sat in standby either because it has reached the desired room temperature or it has shut down on integral control.
COP is heat produced divided by power consumed.
Your heat requirement at 17”C is fairly low.
Also while shutdown the heat pump is using a small amount of power to keep the electrics active and to run the circ pump (if shutdown on integral control). Though the amount of power in standby is small it is over a long time period and therefore adds significantly to the power consumed.
Below is an example of this where I went on holiday and my desired temperature was 15”C.
The thing to note here is that the COP while the system was running was 5.33 but averaged across 4 days it was 4.15.
My personal take on COP is that it is very much secondary to power usage/cost.
From as much as I can understand of your system, the issue I would be looking to be satisfied of, is that you have enough heat emitter surface to achieve desired temperature when it gets properly cold. I am puzzled by why it’s taking 4 hours to get back from 16 to 17”C.
(Just for extra follow up; post 21 on this thread gives a good explanation of integral control and post 336 and 443 show it graphically.)
Hi Andre,
Just been looking through your notes in regards to your flow temperature issues.
Did you contact Vaillant about this before you sourced the replacements? If so what did they say?
Exactly that, the bedroom temp.
I have been trying to manage this by turning the valve down at a certain time in the evening, then back to 6 after waking up.
Pardon my ignorance, but is this what you mean by balancing the rad, or is that by adjusting the valve on the other end of the rad?
Hi Jim,
Yeah as part of our heat survey we were advised to update our rads, but we decided to try one winter as is and see how things perform with the 0.6 heat curve, then revisit next year.
I guess we will find out soon once the autumn/winter starts to properly bite.
I had no idea what the ‘usual’ time was for a heat pump to get to desired temp, apart of course from knowing that it was slower than a gas boiler due to the lower flow temps.
I guess too many variables, but is there a consensus average ‘norm’ time to raise the temp 1 deg?
I don’t think there’s a norm, but it is expected to be slow. If you increase your desired temperature by 1°C, the flow temperature increase is also very low - also around 1°C. Your house then slowly heats up towards its new desired temperature. Actually, on a gas boiler it would be the same, given you’re using the same heat curve. If you want faster transitions between the two temperature settings, you could program your thermostat to aim for say 20°C for an hour and then set it back to 17. This way you get a faster rise at the cost of efficiency.
For many people COP is the factor telling them whether they are in fact saving money with a heat pump compared to a gas boiler. This is also why I am mostly looking only at the compressor-on COP and ignore the standby electricity. A gas boiler also uses electricity in standby, has pumps running (maybe at a slightly reduced throughput, but still not a huge difference for my house) etc. My heating-only COP of ~5 since February (during compressor on) tells me that as long as gas was more expensive than ~6 cent/kWh, my running costs were lower with a heat pump compared to a gas boiler. On top of that come the savings of no monthly costs for just being connected to the gas.
Balancing radiators is about making sure there is adequate flow to each radiator to provide the necessary heat. This is usually done by adjusting the lockshield valves on the radiator.
To reduce the bedroom temperature you shut in the lockshield on that radiator.
Alternatively you can get TRVs with automatic flow control incorporated. This video shows how these work. I have these on my system and they have worked fine for the last couple of years. You just take the room heat demand and dial in the flow required. I don’t use the TRV head as I want to run with an open flow system.
All the above however assumes you have enough surface to dissipate the heat.
As you say there are too many variables to say what typical is.
For my system (with an overpowered heat pump and new radiators) it takes about 1-1.5hrs to recover the 1°C I set back every evening (see post 535).
When I came back from the holiday in the example shown in post 543 the graph below shows the temperature recovery timescale.
Hi
I take your point but I suspect the questioner is just using their Vaillant monitoring to determine COP and doesn’t have an OpenEnergy monitoring system. They won’t be able to isolate their “when running” COP.
On the broader point COP is certainly important and a good benchmark but I will happily sacrifice some COP if I can reduce my energy cost. For example I run my DHW heat up in the middle of the night and do the set back and reheat in the way you describe in post 548. In this way I can use an off peak tariff.
Maybe the Yorkshire in me;I like to save some brass!
