@mjr what is your flow temp set at when you were seeing your radiators at and 28 and 38c.
Yep that was me with the temperature gun. Will go around the house to measure. I guess if the weather compensation is on the temp flow rates could be variable too.
@derek-m where would the mixing of water happen? I will measure the temperature from the heat pump pipes and check. I can see from the monitoring the water flow keeps going up and down.
@derek-m where would the mixing of water happen? I will measure the temperature from the heat pump pipes and check. I can see from the monitoring the water flow keeps going up and down.
Your system is switching on and off, not good. Either set your system for weather compensation control or lower the leaving water temperature to 40C and repeat the test.
@soniks it's exactly as per @derek-m comment. Something is switching your ASHP on and off 2 or 3 times an hour. Because of that your flow temperature is all over the place, jumping up and down almost 10degC. The switching is likely to be caused by an external control device like a master thermostat or something.
It could be the Grant unit controller doing the switching, which is also has a built in room thermostat, it will override any external thermostat wired in to it. So set the Grant controller thermostat up to 35degC (pressing the up and down arrow buttons on the RH side) to guarantee it never ends up controlling and overriding any other control.
Grant has a habit of fitting HIVE/NEST and simple on/off room thermostats to their systems that will switch the ASHP on and off several times an hour, exactly as yours seems to be, and it's a very bad installation method. The ASHP will never run efficiently controlled like that.
You need to crack that first. If you have a room/main thermostat other than the Grant controller, set it up to a temperature higher than you want the rooms to reach, 25degC. Then use weather compensated flow temperature to control the heat into each room, TRVs and other controls can be set to avoid rooms overheating, by setting them a little higher than what you want the comfort temperature to be. So, if you normally set a TRV to 3, set it to 4. But you want your ASHP to be running more or less continuously not switching on and off. That's absolutely fundamental to efficient operation, decent CoP, and in your case, getting heat reliably into the system pipework.
@derek-m OMG I can't believe the Grant Engineers must have not set the weather compensation off! I've just changed that to be on now that I've found the instructions to do that. Is there any guidance on what the max and min temp settings should be set for the weather compensation?
@soniks use the default Grant settings as a start. Have you found out why your ASHP is cycling on and off and what is causing it? That's a separate issue to whether WC is enabled or not. (My WC was not enabled by Grant, there's a pattern emerging here....)
@mjr what is your flow temp set at when you were seeing your radiators at and 28 and 38c.
Yep that was me with the temperature gun. Will go around the house to measure. I guess if the weather compensation is on the temp flow rates could be variable too.
It looks like that was the target the weather compensation was outputting.
I'm so annoyed at the installer and Grant engineers for having the weather compensation turned off. Having read a bit more of the installation manual weather compensation being on is the default and a requirement for the RHI Payment not sure why it would have been changed. The Grant engineers have been in a couple of times and I have shown them the graphs indicating the unit cycling and they basically said it's because the heat is not getting away faster enough and related to the flow issue.
However, in the short period since turning the weather compensation on I have not seen the unit cycle once so that is good news and I can already see my electricity meter at half the consumption it was before. I think this is going to make a big difference to my bills and also rooms feel warmer already as the heating is not switching on and off and those rads that weren't getting hot feel a little warmer. The real test will be when the temperature drops to see how well the system holds up. I'm going to leave it with the current settings and then will look to reduce the max setting to 45c which is the default. Should help a little further on efficiency.
Here's the chart which looks much better now and will report back as it runs over the next couple of days. Thanks @derek-m for making the point about the weather compensation, I should have checked myself rather than relying on being told by the installers/Grant engineers.
@soniks this is good news. I'm going to contact Grant regarding why my weather compensation was not enabled, as you point out, it's clearly stated as a requirement in the commissioning procedure. It's also essential for energy efficient operation. I did SMS the Grant engineer who commissioned mine, who seemed a very knowledgeable chap, and he didn't respond to my question as to why WC was left disabled. He responded to all other queries. Call me a conspiracy theorist, but I wonder if Grant's engineers have been intentionally not enabling WC for some reason.
With WC in my house set at 42degC max flow temperature @ 0degC I'm currently consuming about 18kWh a day heating to 18degC based on running for 18hrs a day. Heating off 10pm to 4pm as I prefer the house cooler overnight. That includes a daily 250litre hot water tank boost to 50degC which will consume 2.5-3kWh. At 7degC outside air the Grant unit is consuming less than 1kWh typically when heating and providing a heating CoP of between 3.5 - 4.0 based on indicated flow rate and supply/return temperatures. Daily all-in electricity consumption in the mid 20s kWh, which for an all-electric house is sitting under £10/day in winter. We do have a log burner on evenings for temperature boost in the lounge, and solar PV, but it's not generating much at this time of year.
Call me a conspiracy theorist, but I wonder if Grant's engineers have been intentionally not enabling WC for some reason.
One oft-suggested reason for that sort of action is to avoid customer calls complaining that the radiators aren't getting hot on milder evenings (which is correct because they probably shouldn't be then).
@allyfish thanks for the instructions I found them in one of your other posts when I changed the settings earlier today.
I think when it's on fixed set point the HP tries to get to temperature as fast as possible which is obviously not efficient. It hits the target temp and then switches off only to restart again later and do the same thing. I suspect this was possibly what was happening with my system when the weather compensation was turned off. Definitely a big and expensive lesson to learn.
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