@mtn Just thinking back to when we had this installed our heat loss was about 12-13kw, because we were really on the border of having the smaller 13kw ASHP. However, given the system drops it output in slightly more severe weather, we were recommended to go with the 17kw system to give us a bit more headspace.

@MTN Just found the email from our original quote;

The heat loss shows that your demand is too much for a 13kw unit, and you will need to have the 17kw model. This puts out 14.9kw at -3 degrees outdoor temp and the 13kw unit will only give 11kw. Your heat demand at -3 is 12.7kw

@bn06amp my secondary pump is the Grundfos UPS3:130 which gives me 1.6m3/h (26l/min) at about 5m head set to maximum speed/curve. The primary pump in the Grant Aerona3 10kW unit I have is a lower flow rate than the 13 & 17kW units. That's set to minimum flow as it's a very short pipe run in 28mm to and from my low loss header and the outdoor unit.

The 13kW and 17kW Grant units share the same 3 speed/curve pump.

It's a little bit of trial and error with both ASHP and hot water tank charging. How long you need to run a 3kW immersion for depends on how high your thermostat is set to and how much water you use, add to that standing losses from the cylinder, and secondary return piping losses when running. My guess is around 2hrs should cover a daily top up. Depends on how long you like to shower for! The cylinder stat or immersion stat will open when demand is satisfied.

I'm hoping to be under 40kwh or £15 electricity today heating about 130m2 to a fairly modest 18degC and topping up the water morning and night to a similarly modest 45degC. I have 155m2 in total but a couple of unheated rooms. I lose heat into those rooms from adjacent heated rooms as the house internal stud walls & doors are not insulated, but that's still cheaper than heating them when I'm not using them. I have benefitted from 9kWh of solar PV today mind. My house EPC is C. I plan to improve the loft insulation in the Spring, It's about 150mm at the moment and I want to double that. It won't make a massive difference but it's a fairly easy and cheap DIY job.

@bn06amp

It takes approximately 3.5kWh of energy to raise the temperature of 300l by 10C.

System seems to *hopefully* be settling down a little bit now. Thanks to everyone for all of your advice, especially @derek-m and @Allyfish it really has been extremely useful.

The house is now about up to temperature (arguably a little bit too warm) and I'm gradually reducing our curve to try and find the sweet spot (down to about 46 @-2 at the top of the curve at the moment). I'm also now purely using the immersion to heat water overnight at the 7.5p Octopus Intelligent rate. Whilst I appreciate it is warmer so the ASHP won't be working as hard, I've checked the usage on the pump a couple of times today and it has been 40% and then 60% lower than when I first posted on here the other day.

I've still not got absolute confidence in my installation and I contacted a Heat Geek certified engineer to come and have a look, he noted a few issues as well:

• The old UFH control should have been removed as it's effectively getting in the way and is more suited to the old boiler system.
• Remove auto bypass under LLH, they don’t suit heat pumps. The flow rate is that high and pump pressure so high that they just open too much which ruins flow. (I don't really understand this so would be good to get any views on this)
• The primary flow rate is correct however the secondary side isn’t going fast enough so a good amount of flow from the heat pump is going straight into the LLH, down and returning to the heat pump. The heat pump is getting a higher return temperature than it should be. Whatever flow rate enters the LLH must leave at the other side. (I think this may be something that Derek or Allyfish were asking me to check the other day as well!).

Would be great to get any views on the above and whether these are issues that will be massively impacting efficiency, or should be made just to refine and improve things.

Thanks again all!

@bn06amp that's all encouraging. The Grant pre-plumbed cylinder has an auto bypass, and Grant show it on their recommended installs. They are designed as a protection against a pump working on a closed or nearly closed circuit, as can happen when all rads have TRVs and shut down. A lot are adjustable, the Grant one between 0.1 and 0.5 bar. It has a red plastic conical top you can turn to adjust. 0.5 bar is about 5m head, which is near the top range of most domestic CH circulating pumps. It's at the lower end of the Grant built-in pump however. If you can find it, ensure it's at the highest setting by turning fully clockwise.

Increasing secondary flow might need a higher capacity pump on that side of the circuit. Not a disaster, but first ensure any other valves and flow restricting devices are fully open or removed first. What make and model pump is on there now? You might be able to get a direct drop-in higher capacity pump.

@allyfish Thanks Ally, so is your suggestion that the auto bypass should remain? (as this is the standard installation for Grant).

I couldn't find anything with a red conical top? I took some more pictures in case you are able to point this out to me?! 🙂

@bn06amp I'm afraid I can't spot it either there! It depends on where it is on the circuit, you don't need a bypass on the primary (ASHP) side with a low loss header. But the controls on the secondary (house) side might be arranged such that the secondary pump piping needs a bypass. Unlikely though if everything is set to open and full flow.

Your secondary pump looks like a Grundfos, what model?

Secondary bypass is here, black top...

@hughf Thanks Hugh, is this the one I need to have turned fully clockwise @allyfish?

@allyfish I've just seen this as just posted elsewhere whether to apply low tariff mode.

As above complete novice to all this and still figuring stuff such as finding out how to adjust flows etc. And assuming that's the dial on the cylinder?

As you can gather commissioning handover missed so much!

@bretix the dial on the HW cylinder is just a cylinder stat, turn and set it to whatever hot water storage temperature you want, it will then revert to showing actual cylinder temperature after a few seconds.

By 'Low tariff mode' do you mean enabling weather compensation for flow temperature compensated for outdoor temperature? That's done at the Grant controller, in 'installer mode'.

Water flow adjustment on Grant is by use of a flow setter and regulator valve. The pump inside the Grant monobloc unit can be set to one of 3 speeds if the flow is too high on the primary side or, which is better than having the pump full speed working against a nearly closed flow setter and regulator valve. For systems without a low loss header, too high a flow is probably unlikely however. There are small dip switches on the PCB behind the power cable cover.