How quickly do your radiators get up to temperature with a heat pump (ASHP)?

Hi @mrmojorisin04 

I think it’ll depend on the heat demand from the house. Heat pumps try to maintain a difference between the flow and return of 7-8C on our radiator based systems so if you are getting a bigger drop because lots of heat is being emitted in to the house it’ll take longer to get up to working temperature. 

55C is very warm for a heat pump, unless it’s the high temperature variety, but I think your pump is the same as mine. Today mines running around 40-42C, but I’ve changed my heating schedule to 20C between 6am -10pm, 18C overnight. House stay warms and the heat pump isn’t working so hard. COP has improved too. 
my weather curve is set to 48C at -3C and 25C at 18C. 

Agreed, my target LWT is 42 deg, it only heats to 54 deg when doing the DHW cycle. 

Thanks for the input @hydrosam 
My heating schedule is similar: 20.5° between 6am and 10:30pm and 18° overnight.

I’ve just switched weather dependancy on and am using the default. However, I was previously using 50° fixed LWT so it seemed to make sense that it would be higher on very cold days. The pump is capable of 55°, indeed that’s what the installers left it at.

My concern is the amount of time that it is running. At 7am this morning the room temp was 19.2°. 2.5 hours later it is 19.7°. That’s still pretty comfortable but at this rate the pump will be running constantly all morning and still only get to around 21° and the cost of that is pretty steep.

At the same time, my DHW, which is on reheat is stuck at 20°, presumably because it’s waiting for the house to get up to temp first.

I don't think the heat loss in the house is excessive. Generally with a target of 20.5°, on days where the outside temp is 5°+, the pump heats the house to 22° by around 9am and then it kicks in again when the room temp has dropped to 19°, which is normally early evening.

With weather dependency the idea is that the pump runs slow and steady. It probably took my house a day or two to get up to a nice 20-21c but now it’s just running on tick-over with a leaving water temperature currently at 39C. As we have similar sized houses (both 4 bed) it might be useful to compare the energy the heat pump used and produced, though insulation/heat loss levels do play a big part too. You might need your compensation curve a few degrees higher but not running at 50C.

I used 28kwh elec and the pump produced 86kwh of heat. On similar days back in November I used way more 40kwh+ but the house is more comfortable now. 

DHW takes priority over heating. Reheat only mode is not an appropriate setting for our systems despite what our installers may think.  A schedule will be much more efficient in terms of daily energy usage and not detriment space heating when you need it most. 

Thanks for the input @hydrosam 

The way you describe it, your system system starts and stops all the time to maintain a steady 20-21°. Mine doesn't do that, it runs for quite a while to get to 21-22° and then does nothing for a long time until it drops to 19° ( 16-17° overnight) at which point it starts again.

The issue I have had on the other thread does seem relevant - the engineer says that without the secondary cylinder to prevent frosting of the pump, it is using energy from the pump to defrost which means less efficient heating.

I've had to run on reheat up to now as my tank doesn't keep the water hot enough to run a schedule, but it seems like as well as the frost protection issue, my pressure valve is faulty and is letting hot water out for no good reason. Hopefully that’ll be the explanation for the loss of heat and I can look at running on a schedule when it is replaced.

Hi @hydrosam

Can I just check - does your heating come on pretty frequently during the day or is it once or twice like mine (i.e. only when the house temp has dropped by >3°, which takes quite a few hours normally)?

@mrmojorisin04 i have mine running permanently on a low and steady setting, it’s leaving water temperature today is around 29C. I found it used too much energy to heat the house back up from a cool temperature, because heat pumps can only heat up a house relatively slowly and they need to run hard to get there.  

It’s a different operating philosophy to traditional gas/oil boilers which is overheat, switch off, cool down, overheat again. 

@hydrosam Many thanks, but I’m still not sure what that means in terms of how often your pump spins up to warm your rads and how long it then runs for to get the house back up to temp.

We have similar schedules/values for our room temperature (mine is 20.5° between 6am and 10:30pm and a little less overnight). On colder days, if I let it, then my pump would be spinning from 6am to midday to heat the house from 19° to 22°. One time when I did let it, the cost was £5 by midday. This was with the LWT set to 49°. I can imagine that if I had it lower, then it would basically be running all the time without ever reaching the desired temp and I can’t see that would be more efficient. Or maybe it would?

Once it's up to temp, there’s nothing that I deliberately do to stop it running. It's just the hysteresis that means it stops running for a while until the room temp drops enough for it to kick in again. This usually means that it spends 5/6 hours doing nothing (similar to a gas system).

I guess basically what I am asking is if your pump is constantly spinning and if so, do you find that your bills are reasonable when doing so (e.g. <£5 per day even on cold days). Or maybe you have set your hysteresis to less than 3° so that it starts and stops more frequently?

Hi @mrmojorisin04 I’ll try and explain a different way.

I have set up my system so that its running as long as possible through the day by adjusting the weather dependent curve to thresholds that keep the house temperature just below that which would ‘trip’ the thermostat to turn off the pump.  So to answer your question of how many times a day does my pump spin up, it does this once at 6am and continues running through till 10pm, when it drops to a lower running temperature overnight but it is still running continuously through the night (at the moment whilst it’s cold). (There is a little bit more to the running continuously point because the pump cycles on/off a bit but that is another subject relating to defrost cycles, etc).

To do this you need to adjust the weather dependent curve and the schedule. It is a bit of trial and error before getting it right which will be why it’s much easier for an installer to follow the traditional approach of heating a house with just a thermostat and a hot water temperature flowing through the radiators. If you don’t have a technical background like me, or don’t have the time, or just aren’t interesting in adjusting the system settings, then it would require more intervention from the original installer. With the Mitsubishi systems (that I/we should have got  and signed T&C’s for) these settings can be altered remotely by the installer but the functionality of the Daikin systems isn’t as good in this department.

Once I started adjusting the weather dependent curve it took my house a couple of days to settle up to a nice comfortable temperature because it needs the thermal mass in the house to catch up but once there the house remains comfortable and uses less energy to heat the house.

Savings weren’t huge for me because I’d already lowered the values in my weather dependent curve a bit but might be more significant for you if your system is running with a leaving water temperature of ~50C.

To compare my energy usage yesterday to a similar day last month.

2/12/21 (thermostat controlled) - Elec used by pump 23.3kwh, heat produced 61.6kwh, COP 2.64 Internal house temp failed to get to desired except late evening. On my tariff that’s £4.54 per day.

11/12/21 (weather dependent controlled) - Elec used by pump 19kwh, heat produced 596kwh, COP 3.14 Internal house temp 20C all day. On my tariff that’s £3.70 per day.

My worst elec usage before changes was:

28/11/21 - 42kwh used, 102kwh produced, COP 2.4

After changes:

08/01/22 - 38kwh used, 108 produced with COP 2.84.

I don’t have external temperature data to compare the two days but they were both close to 0C if I remember correctly for a lot of the day, during the second example the house was warm, back in November the house was cold for a lot of the day. 

If you want to start adjust the weather dependent settings I can try and help but I don’t know how that sits within the terms of our trial, and I’ve now owned up publicly to adjusting the weather dependent settings on my system which might get me… 

And remember I’m not a trained heating engineer just a layman who thinks he’s figured out how the big technologically advanced (but not as advanced as it should be) lump of kit installed in his house works.

That’s perfect, thanks @hydrosam 

It’s really interesting that you have got your system running almost continuously by effectively never reaching the thermostat setting. I wonder if reducing the hysterisis to say 0.5° (if that were possible) would achieve something similar?

I don't think our actual costs are that far apart as the figures that I quoted are for the whole house, not just the heating. My strategy so far has been to get the house warm during the warmest part of the day and limit the pump use during the colder periods. So I manually turn down the thermostat in the morning when it has got to around 20° rather than letting it run for hours to get to 22° and then during the afternoon I let it go to 22°, which generally keeps us warm enough during the night (supplemented with a log burner!).

I do still think/hope that once I have the additional cylinder installed that will take care of defrosting the pump, then it will run more efficiently during the cold snaps as it will utilise the energy in heating the water rather than defrosting the pump.