Hi Ally and Derek,

Thanks for the advice whilst I was at home to assist me to get the best out of my Ashp. I've left it to the OH now so she can get equally as frustrated with it as myself.

All seems to work well in plus temperatures at a flow rate of 35c. And on WC.

Hit minus temperatures and we are seeing 80 plus kws whole house usage and even 105 one day. We have a very open plan house high ceiling etc at 310m2 so that's probably why we see the usage.

This usage when cold is where I struggle to understand when I hear of others that there sysytem just seems to tick along at half the usage. Our 17kw just sounds like a jet engine ready for take off when it's minus temps.

I obviously want it to work but I just don't see it. We installed it to heat the house and not needing 2nd heating sources to help out. We have a Stuv log burner but that just heats one room. 

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@roamingbull hi there, as a comparison, I have around 155m2 of heated space, and the 10kW Aerona. I was hitting 50kWh per day last weekend in the exceptionally cold snap. I turn the unit off overnight, as we like the house cooler. The really cold days I kept it running 24/7. Then the unit pretty much runs flat out in high output in between continuous defrost cycles. Target flow temperature according to weather curve was seldom reached, it fell short by 3-4degC usually, and the house was colder than I wanted at 18degC rather than 20-21degC as a result. It's not optimal - the defrost seems to adversely affect the Grant Aerona more than other makes, with more frequent defrost cycles. We would have fitted a 13kW unit, which would have given us a bit of capacity headroom, our heat loss is around 10kW at -4degC, so factor in maybe 25% capacity lost through defrosting cycles, effectively 7.5kW from the unit. Log burner to the rescue! Our house supply was 80A and not rated for the 13kW Aerona, which then was not connect and notify. (It is now).

To install the 13kW unit was going to cost me £800 to get Northern Powergrid to replace their fuse, +3 month lead time, and cause massive disruption internally as my 25mm2 incoming feeders from external fusebox to internal consumer unit would need to be changed out for 35mm2 for 100A rating. That would have involved lifting up tiled floors in 3 downstairs rooms, so it was a non-starter.

So I accept in winter I need to light the log burner or have some other supplementary heating such as oil filled rads to make up the deficit that very occasionally shows itself in really cold weather. That's probably for 4 weeks a year out of 52. Rest of the time the unit chugs away modulating right down to 20% output running very efficiently, keeping the house sitting at 20-22degC and providing me a tank of hot water each day at 55degC. So on balance I'm happy with the ASHP, but we all tend to forget the laws of physics - in very cold weather COP drops off and electricity consumption rockets. All ASHPs do that, and it's gets scary for a few weeks watching the smart meter, but we also forget that the units spend 90% of their year modulating down to provide cost efficient heating.

The Grant Aerona has some issues with frosting and defrost, and it's SCOP is distinctly average, (few real world installations get anywhere near Grant's published efficiency figures) but it is a very reliable and well built workhorse, I've never heard of anyone on here who has had a major fault with one, save one suspect faulty outdoor air temperature sensor.

I'd echo what @allyfish said. I too have a 10kW Grant, and about 150m2 to heat. During the cold snap I was consuming 50-60kWh per day, and this didn't quite keep the house warm. It's the defrost cycles that kill performance, and I can see it happening; between 0 and -3 seems to be the worst (below -3 to about -7 it picks up slightly; below -7 I've yet to see!) and hitting target LWT doesn't happen; between 0 and 3 it will slowly get there; above 3C and things are fine. Better insulation, and a secondary heat supply is what I'll hopefully have by next winter, and I do think there might be some mileage in using the built-in 3kW heater on the LLH to help during defrost, but that needs a little wiring up to look into. Maybe January.....

Posted by: @roamingbull

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This usage when cold is where I struggle to understand when I hear of others that there sysytem just seems to tick along at half the usage. Our 17kw just sounds like a jet engine ready for take off when it's minus temps.

It confuses me as well but then it doesn't take much. Are others really not seeing increased power consumption in a cold snap? How can they *not* have that if the defrost cycle is standard for ASHPs?

Same as Ally and Mike, we had a spike on Saturday of just under 30kWh for the day (pump, not total house), but now it's back to less than 10kWh.

Hey Ally and Mike, Maybe I was expecting too much out of the system. 

I just didn't realise how much it would struggle at -3c. Maybe I should have adjusted 2102 from 35c earlier knowing it was becoming cold but I thought with being on WC it would adjust to suit. 

I read on Do set backs save energy topic!

During cold spell last week, i first started with setback, however the usage turned into 5/6kw per hour and house wasnt at ambient temp until late in the evening, i switched to running 24/7, much more comfortable and cheaper at approx. 1kw per hour after system reached set lwt. Today for instance, its mild so i ran from 1pm to 9pm, approx 15kwh used. If i was running today 24/7, it would have been bit more comfortable but would have been near24kwh. For me, it depends on the weather, when cold below 6deg OAT its beneficial to run 24/7 but when mild above 8, setback keeps us comfortable and we use less kwh. The fabric of the house im sure isn't was warm as it would be running 24/7 but i dont notice the fabric temp, i notice IAT and at the end of the day that's all that matters.

Also found fixed flow @29 even warmed hosue to 22 in minus weather but 27 wouldnt warm house to 22 in mild weather so think i have found sweet spot regards flow so no wc needed.

Would the above be any better?
 

Grants commissioned our system to a set Flow temperature and as you know told us to leave alone. Tbh this is our 3rd winter and 1st winter on WC but we are seeing similar usage as the first year set up from Grants.

The 2nd year I thought right I'm going to try and use you as I would a boilered syetem and it was a 1/3 less in price for the year. So I'm now thinking what is the best option. Atm I know.

Elite Heat Geek sent us in the direction of WC and prior to the cold snap everything seemed to be rolling along great and confidence was building nicely. Wake up on the morning of -3c and go downstairs and it was crikey what's going on here. 

I have read folk using ashp as a kind of hybrid running alongside a  boiler but again to me what is the point. Same with supplementing the house with the log burner. 8k for a heating system to struggle when the going gets tough doesn't really cut it. 

And like you guys state we don't see minus temps very often but still should heat the house nicely. The usage I can handle to a point, but not being able to heat the house is something I struggle with.  

Hi, some info on Grant website on user level parameter adjustment. Not had a look in detail, Grant are slowing realising that the installer manual is useless of owners and installers alike - fragmented, too many cross references, and very hard to fathom. Hopefully resources on their website will help out perplexed owners. https://www.grantuk.com/support/product-support/air-source-heat-pumps/

Posted by: @mikefl

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@mab I think most have changed their low end to 0 from -4, I have. I guess once you're at zero, you'll need 'high LWT'.

I'm getting the figures from the Aerona unit, via its RS485 interface (basically, a serial interface), which allows all of the parameters to be read, and updated (where possible), following info I found on this page. I've wired up some network cable to the outside unit, and the other end of have this, which is connected to a Raspberry Pi sitting in an upstairs cupboard. Then I just run a bit of code every minute or so, to record at least the 10 values available easily(!) from the controller, and write them to a database, for analysis and graphing as and when.

The IT is fairly straight-forward. Figuring out what's happening...

@mikefl (Belated) thanks for this - i went off down the rabbit hole of exploring your links, then the weather turned mild and anyway and i haven't got back here since.

@mikefl hey, are you able to give the grant heat pump a flow temperature via your modbus connection? Or only read data? I've found a way to have a set back flow temperature on the grant on demand, but would be better if i could specify a flow rate at all times remotely

@mikefl hey, are you able to give the grant heat pump a flow temperature via your modbus connection? Or only read data? I've found a way to have a set back flow temperature on the grant on demand, but would be better if i could specify a flow rate at all times remotely

@heat_help Hi. You seem to be saying "flow rate" at some points and "flow temperature" at others? As you probably know, the flow rate on a Grant is fixed (or more correctly, mechanically set by the installer/user), so is not controllable. I'll assume you mean, 'Outgoing Water Temperature' (parameter '01 09') in Grant's terminology, or LWT (Leaving Water Temperature) as it's generally know as.

In which case, you can set this value over the RS485 interface, using the modbus protocol. You'd need to first set '21 00' to 0 to disable Weather Compensation (this is Coil register 2); and then put your desired LWT in parameter '21 01' ("Maximum outgoing water temperature in Heating mode") multiplied by 10, in steps of 0.5C only e.g. for an LWT of 37.5 set Holding register 2 to a value of 375. 

If you are planning on doing this, and I am aiming to also, to create a form of intelligent adaptation based not only on Outdoor Air Temperature (OAT) but also Internal Air Temperature (IAT), then I think it's wise to ensure sensible measures are in place to ensure:

• there aren't several sudden jumps in the target LWT over a short time period (which I feel wouldn't be good for the HP, a bit like turning a gas boiler on and off all the time);
• that the minimum/maximum stay within the specified range of the WC parameters;
• that in a failure of scenario, the HP falls back to its own WC setting (i.e. '21 00' = 1, with appropriate values in '21 02'...'21 05')

@mikefl - hello! I believe Homely's Grant/Chofu controller will have a true set-back function where the weather compensation curve LWT can be offset by a user-configurable number of degrees for overnight. I don't think the Grant smart controller due to launch soon has this capability, (trying to get clarification), so we're still left with no simple means of setting back Grant ASHPs overnight other than bouncing them off a room or zone thermostat on/off, which is incredibly inefficient and very expensive - COP drops through the floor. The Grant smart controller with WIFI thermostat and control App replaces the Chofu interface controller.

The Grant smart controller has LWT and RWT sensors plus flow rate sensors, so those values combined with the Aerona's own power consumption monitoring will be able to display real time COP. Also a DHW flow sensor interface which may be useful to schedule DHW boost. Why external temperature sensors for LWT and RWT are required I'm not sure, as the Aerona has sensors already for that.

@allyfish I haven't heard about the Grant smart controller. Do you have a link you can share?