what model HP is it the new R290 or the PUZ-WM60? (They have different minimum outputs)
Do you know the output rating of your radiators? Typically quoted as DT50 capacity.
how many radiators.
here is a graph which is for the PUZ -WM60. The bottom line on both graphs shows the minimum output in KWs. It shows 35c flow temp operation and 40c flow temp. If the output is forced below this line (such as low heat exchange or low circulation volume etc) the HP will stop operating for a while. And restart. You will see that at the higher flow temperature (ie colder outdoor temp) the heat pump can operate down to a lower minimum output.
OK, removing all the smart TRVs, opening the wheels fully, opening all the lockshields, and making sure the Evohome controller stays on all the time has made no difference to the "cycling" rate - it just made a few minor blips on the temperature graph.
It did make the rads furthest from the heatpump run a lot cooler, while the 2 closest ones are nice and warm.
I think the water is now taking the "easy route", so the system volume hasn't really increased?
I'll adjust the lockshields down on the 2 closest radiators, and see what happens.
I think the water is now taking the "easy route", so the system volume hasn't really increased?
As you plan I would balance the radiators using the lockshields then measure again, but you may well be right about the easy route.
If this doesnt fix it then I would be suspicious of the role of the LLH. They do make things difficult and, in most properly run domestic systems, serve no useful purpose!
what model HP is it the new R290 or the PUZ-WM60? (They have different minimum outputs)
Do you know the output rating of your radiators? Typically quoted as DT50 capacity.
how many radiators.
here is a graph which is for the PUZ -WM60. The bottom line on both graphs shows the minimum output in KWs. It shows 35c flow temp operation and 40c flow temp. If the output is forced below this line (such as low heat exchange or low circulation volume etc) the HP will stop operating for a while. And restart. You will see that at the higher flow temperature (ie colder outdoor temp) the heat pump can operate down to a lower minimum output.
the R290 model has a lower minimum output.
It's the PUZ-WM60, R32 model.
I have 10 radiators.
I did add up all the radiators, corrected for deltaT 25, and it was very close to the design calcs of 5.5kW heatloss.
Do those graphs, and my actual experience, mean that tweaking the WC curve upwards, to increase the LWT, would reduce cycling?
@clockworks Higher flow temps will get less cycling but at the cost of efficiency and you will find then it will run for a couple of hours and then reach the thermostat upper limit and shut off, either that or you will get very hot depending on your thermostat setting
Here is mine running for the last hour at 32C, its 7C outside, you can see its crept up to 33C and it can't keep the temp at 32C if it rises to 33.5C it will turn off, cool down and turn on again its doing this once an hour which is acceptable to me.
what model HP is it the new R290 or the PUZ-WM60? (They have different minimum outputs)
Do you know the output rating of your radiators? Typically quoted as DT50 capacity.
how many radiators.
here is a graph which is for the PUZ -WM60. The bottom line on both graphs shows the minimum output in KWs. It shows 35c flow temp operation and 40c flow temp. If the output is forced below this line (such as low heat exchange or low circulation volume etc) the HP will stop operating for a while. And restart. You will see that at the higher flow temperature (ie colder outdoor temp) the heat pump can operate down to a lower minimum output.
the R290 model has a lower minimum output.
It's the PUZ-WM60, R32 model.
I have 10 radiators.
I did add up all the radiators, corrected for deltaT 25, and it was very close to the design calcs of 5.5kW heatloss.
Do those graphs, and my actual experience, mean that tweaking the WC curve upwards, to increase the LWT, would reduce cycling?
In principle, most heat pumps are over sized at some point. This usually occurs at the lowest demand and lowest outdoor temperature when we still need heating. The pragmatic solution, * if there is no other problem, is to find the flow temperature which keeps the HP operating without dipping into the shutdown range. Or at least the temperature that lowers the cycling to an acceptable level. Ie 2 cycles might be a good maximum to target. As @gary is prescribing, 38c flow might take your HP output above its rapid cycling. You could find out by adding +degC to your controller screen.
What is your WC curve numbers?
your data on DeltaT 25 might need clarifying FYI. A DT25 would work out at a flow temp of 50c which isn’t what might be thought to be the output of a 25c flow temp. I’ll clarify if needed.
Yes, I meant dT25 as flow temp of 45, room temp of 20 degrees. I appreciate that I've been trying to run at a lower flow temp than that, it was just to make sure I'd have enough headroom for the coldest normal day here - rarely goes below zero for a whole day.
Since I've been trying to run at less than a dT of 20, maybe as low as dT15, I think the most the rads can "lose" is a fair bit less than 3kW (0.41 correction factor for dT25, 0.21 correction factor for dT15. Both based on the manufacturer specs at dT50). Doesn't look like that's possible with this heat pump, hence the 20 minute cycles.
My starting point heat curve was 50 flow at zero OAT, and 34 flow at 15 OAT.
This is a little bit lower than the curve set up by the installer as "a good starting point for this area".
For the past week I've been dropping the offset for the curve on the controller steadily, and minus 5 was keeping the downstairs at an acceptable temperature, but struggling a bit upstairs.
It feels like minus 3 would give me decent temps in all the rooms, so that would be a curve of 47 at zero, 31 at 15.
I think the original heatloss calcs were a fair but off though, maybe a lot less air changes?
On the coldest day we've had, zero plus or minus 1 for 24 hours, it consumed 29kWH over the full 24hr period. Basic maths, that suggests the true heatloss at zero degrees is around 3.9kWH averaged over the day. System was specced at just under 6kW.
Maybe it's just too big, and I'll have to put up with the cycling and relatively low running costs, or turn up the temperature, pay more, open some windows, and get less cycling?
I don't understand why it seems able to modulate the power to a lower level on the way up the cycle for a few minutes (600 watts), but keeps going at 1000watts until it shuts off completely?
I've put the curve back up to the starting level, will post another graph when it settles down
I’ve just noticed @gary and I sent crossing messages at same time. I don’t want to overload you with too much information.
So I’ll leave you in Gary’s capable hands.
However I would suggest you make only small changes at a time.
Regarding the radiator DT question, This helps to work out your lowest radiator capacity when operating at 30 or 35c Flow temps. it’s good that you appreciate the correction factors to arrive at the true output values and that if this is too low this can cause cycling. Just remember to deduct the room temperature (20c) and also the 2,5c mid temp of the radiators.
You’re right, the graphs I posted show a minimum output of around 3kw at 35c flow temp so if your rads can offload 3 kw at their minimum flow temp (which might be only 30 or 35c) that should help reduce cycling. I think Gary suggested 38c in a post above which might ensure your system is above shutdown level. I hope this is useful.
This post was modified 2 months ago 2 times by SUNandAIR
I put all the lockshields back to where they were, and refitted all the TRVs. I had the weather curve set 3 degrees higher to see what happened when it ran overnight.
I've been running on a heating schedule to make the most of the Intelligent Go cheap period. Heating goes off at 20:00, coming back on at 23:30, and doing the hot water at midnight.
It got a fair bit warmer outside overnight, then dropped again.
I think the problem here was that most of the rooms reached their set temperature quite early in the morning. Combination of me setting the weather curve 3 degrees higher, and the outside air temp going up?
With your increased curve and increased OAT (outside ambient temp) it was still trying to run around 38C.
Between 1.40 and 3.40 you have the same cycling as before every 20 mins, but at 3.40 you get a relatively steady period for 20 mins what happened there? Same for between 5.00-06.00 relatively steady continuous performance, to me that suggests the TRV's are turning on an off and you are getting more available water volume in the system that is enabling the heat pump to modulate.
I would still take the TRV's off and exclude them from the equation and get a days running without them and see what happens.
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