I am nine months into a refurb of a 1920's house with a big extension. I have spent everything I have and more trying to make the house work well for the family and be affordable in retirement.
In the original house we have added high quality double glazing throughout including replacing external doors so pretty airtight as chimneys blocked. The existing walls are terracotta hollow bricks with no cavity so we added 70mm external celotex. Ceilings have 130mm celotex, no insulation in old floor which is mostly wooden parquet flooring on concrete. Original house maybe 140m2. Extension is maybe 90m2 and has 130mm celotex in ceiling and 150mm mineral wool with lightweight engineering blocks.
The old oil central heating and pipework & rads have been replaced with 17kw Grant ASHP, new pipework, new rads. 300L DHW tank is in loft and horizontal. Extension has UFH split into two zones. There is a hall thermostat for the rads and each room has a TRV.
System was turned on late November but is still pending commissioning ... held up pending refurb of upstairs bathroom when building work wont require any more rads being moved.
I have some concerns
a) cost of running the system .... I can see we use upto extra 40kw during day and 20kw at night ... Dec bill will be over £700 which given the money I have spent is very disappointing .... I have googled lots, but this tuning business is complicated.
b) not heating the house well when sub zero outside ...
with the TM1 set to 40 degrees at -4 .... the system failed to heat increase the heat of the house from 18 degrees in the morning to the requested 19.5 degrees ... and was basically on all day.
c) not heating the DHW over night when sub zero
I have the Grant Appendix A - S Plan installation ... where DHW supply is direct from the ASHP not thru the low loss header
This uses EP001 control box to provide DHW priority by closing the master valve pushing all the hot water up into the loft and cylinder instead of through the LLH. But at this time both the UFH and Rad circuits switched on their valves which switched on their pumps which are from the LLH secondary circuit. Well I say secondary .... its really one big loop with two sub loops for rads and UFH, the return is shared. This seems inefficient to me and I wonder is this the reason the ASHP couldnt bring the temperature to 55 when DHW was requested ?
I have the DMH set to come on for 90mins from 3:30 to 5am, then immersion for hour from 5am till 6am .... but this morning no hot water ... just warm .... how long should a 3kw immersion take to heat a 300L tank ?
When weather was 10 degrees, during day I could see flow temp over 50 degrees and the DHW tank was full after 90mins ... the thermostat is set to 52 degrees and I could see it turning off the DHW demand when reached.
Earlier this week the electrician changed the RAD pump to switch on when the radiator 2 port valve came on ..... instead of the master valve. So previously when DHW priority came on and the master valve closed, the RAD pump stopped and therefore didnt bring the return temp down .....
Question is should it be possible to get the UFH and RAD pumps to only switch on when master valve is open AND the corresponding valve is open ? ie when the master valve switches off it turns off the UFH / RAD pumps.
Sorry for rambling ... its been a stressful couple of months tweaking params and trying to make the danged thing work efficiently.
Any advice greatly appreciated ... anything I should ask the plumber when he gets back from the Grant commissioning course to commission the system (it’s his first one) ?
Hi @Denevil, just reading this, I'm probably a month or so further on my Grant ASHP system journey, 10kW Aerona3 and 150m2 EPC C house. It's a learning curve for sure.
17kW ASHP. Who sized it & on what basis? Has a thorough heat loss calcs been prepared? Check the unit is right sized, not too small, not too big.
The unit could have been in defrost every 45-50 minutes this week with temperatures at or below freezing. ASHPs can lose output quite drastically when that happens. They also guzzle power as the compressor inverter never gets a chance to modulate down, it's always playing catch up and working hard to reach climatic curve flow temperature. Sometimes the target flow temperature takes an age to reach going up in a saw-tooth pattern. The bigger the ASHP evaporator coil, the more energy has to be taken from the house wet circuit to defrost it. If the ASHP unit is oversized that can be very costly and inefficient.
The best time of day to generate your hot water from an ASHP is from mid-day to early afternoon. The air temperature is warmest, with higher enthalpy, and less chance in winter of a defrost cycle in the middle of the hot water generation. Chances are your ASHP never reached HW set point flow temperature in the time you scheduled it to generate hot water, it might have gone into one or two defrost cycles during the 90 minutes. The worst time to generate hot water is the wee small hours of the morning when the air temperature is lowest. Program your DHW to charge the cylinder to coincide with warmest outdoor air temperature for best CoP and lowest energy consumption. When you are generating hot water, everything on the secondary side should be closed off, zone valves and pumps, hot water has priority. Schedule heating to come on straight after to dump the residual energy from the primary circuit into your secondary circuit and space heating.
Hot water cylinder in the loft. Is the loft converted & the roof itself insulated? Or is the cylinder sitting at just above freezing point? The standing losses from the cylinder and any uninsulated pipework or valves could be high in this weather if the tank is in a very cold location. Much higher than manufacturer's data, which usually are based on water at 60degC and surrounding temperature 20degC.
The Grant template installations are overly complicated, yours has 3 pumps, one on the primary side in the ASHP, two on the secondary side. What's the flow rate on the primary side during heating and during hot water generation? What's the ASHP internal pump flow rate dip switch set to? With the LLH, you need at least two pumps. You would benefit from a flow gauge/setter valve on the return of the secondary circuit, because you can balance the LLH that way, and check primary and secondary flows are closely matched, to avoid water passing through the header in either direction. More flow on the secondary side than the primary will rob the secondary of heat capacity, as it will pull return CH flow back through the header. Without a second flow valve, there's no way other than measuring temperatures in and out of the LLH to balance it. Basic oversight by Grant, but the LLH is questionable anyway. For smaller circuits, the built in pump in the ASHP could provide adequate flow.
TRVs - set them all to at least one number higher than you want the room temperature to be, purely to prevent overheating rooms. Main thermostat, should call for heat all the time, set it to a level that, again, just prevents overheating. Use climatic compensation to regulate temperature, which is a trial and error process to get things balanced and get the parameters set.
The best time of day to generate your hot water from an ASHP is from mid-day to early afternoon.
I have been wondering about this myself. My night rate is almost 3x cheaper than my day rate (at the moment) and my cop day vs night likely to be less than 2x. So ignoring additional wear and tear for the night run, I think topping up at night makes sense for me (I set it to top up hot water during the last 30 mins before the night rate ends).
a) cost of running the system .... I can see we use upto extra 40kw during day and 20kw at night ... Dec bill will be over £700 which given the money I have spent is very disappointing .... I have googled lots, but this tuning business is complicated.
I am facing the same challenge, how to optimise for efficiency. As per my posts on another thread, it seems,
#1 - efficiency all comes down to one single thing, flow temperature [immutable laws of science]. The lower the flow temperature the higher your efficiency.
#2 - How warm you can get your house to comes down to basic engineering maths - can you match the heat leaving the emitters dotted around your house and in the floor to the heat leaving the envelope of your building?
Unfortunately you now need to do both #1 and #2 at the same time, and that is where the "fun" begins. High flow temp & large dt, low flow, small radiators, nice warm house = bad COP lots of $$$. High flow, low dt, low flow temp = good COP, need UFH or very large radiators, or radiators with fans or passiv house ...... not an easy balancing act. Of course the outside weather is vital as well. Live in Cornwall good COP, live in Scotland, not so good COP.
Sorry I could not help your particular points, but I am sure the really experienced people on this forum could add something much more useful than the above.
Weather compensation made a big difference to my setup although more recently with the lower temperatures I've seen it cycling again which you want to avoid. There is a specific setting you need to check 2100 I think on the installer setup which needs to be enabled for weather compensation to work. The installer (ex grant engineer) and also the grant engineers who came to check the heat pump didn't set this properly. I would double check yourself.
I have the DMH set to come on for 90mins from 3:30 to 5am, then immersion for hour from 5am till 6am .... but this morning no hot water ... just warm .... how long should a 3kw immersion take to heat a 300L tank ?
When weather was 10 degrees, during day I could see flow temp over 50 degrees and the DHW tank was full after 90mins ... the thermostat is set to 52 degrees and I could see it turning off the DHW demand when reached.
You should calculate 1.161 watt hours per deg C. So assuming you are going from 45-52 degC for all 300L using a 3kW element (no stratification) that is 2.43 kWh so maybe 48 mins. As you have a horizontal tank, stratification should be limited. If you element is limed up, then it could take longer.
Here is my hot water top up - running morning of Jan 16, was -2 outside using heat pump. Tank is also 300L. Temperature of tank drops a bit as it is an indirect vented, and I put an external heat exchanger on, so there is some de-stratification happening at the start of the cycle. The temp change was from 46.5 to 51.5 degC (0.000277778*4180)*300*5 = 1.7kWh in 30 mins (so equivalent of 3.4kW immersion heater) Given my heat pump is rated at 16kW, I would expect more, still investigating.
Hi @AllyFish, thanks for the detailed reply, ... I had no idea of the defrost details ....
answers follow
"17kW ASHP. Who sized it & on what basis? "
Plumber working with Grant and Stone who sized and recommended unit. Simon at Grant did the calcs and has been here and checked the plumbers implementation of the standard plan in Appendix A.
"The best time of day to generate your hot water from an ASHP is from mid-day to early afternoon. "
My logic was that in afternoon the tariff is 48p/kwh at night its 18p/kwh .... it seemed effective at night when it wasnt freezing ... now its simply not heating the water .... its tepid .....
"Hot water cylinder in the loft. Is the loft converted & the roof itself insulated? "
No the cylinder is new and is factory insulated with 50mm celotex equiv.
There is still a fair amount of unlagged pipework including in loft ... thats on plumber todo list.
"What's the flow rate on the primary side during heating and during hot water generation? What's the ASHP internal pump flow rate dip switch set to? You would benefit from a flow gauge/setter valve on the return of the secondary circuit, because you can balance the LLH that way, and check primary and secondary flows are closely matched, to avoid water passing through the header in either direction. "
Only thing I can see is flow rate on the gauge in primary return to ASHP and its around 32
"What's the ASHP internal pump flow rate dip switch set to"
I believe it was set to max
"TRVs - set them all to at least one number higher than you want the room temperature to be"
They are all on max and the weather compensation is enabled on the ASHP
I had increased TM1 to 45 but I could see the flow temp going up to 45 then down to 30 .... then backup .... I wasnt given a monitoring option so its just from random checking at the remote control. Took 15 hours to raise temp of kitchen to 19 which fell to 18 overnight even tho thermostat was 19.5. ...... and thats with DHW turned off.
This is becoming somewhat depressing ... last week all seemed good, house was warm and I had hot water in the morning which stayed hot all day ...
The TM1 was 45 degrees ... my understanding is this is the max flow temp at -4 degrees for the weather compensation ... I reduced to 40 to try and see if would help reduce bill ... it was fine until temp outside hit -4 at which point I am thinking 40 is not enough.
We have new rads sized based on T+30 ie 50 degrees, UFH with natural stone in extension.
The TM1 was 45 degrees ... my understanding is this is the max flow temp at -4 degrees for the weather compensation ... I reduced to 40 to try and see if would help reduce bill ... it was fine until temp outside hit -4 at which point I am thinking 40 is not enough.
We have new rads sized based on T+30 ie 50 degrees, UFH with natural stone in extension.
Without looking at the Grant manual, I assume that TM1 is the cold end setting for the weather compensation (WC). Whilst lowering this setting may save a little money, it will undoubtedly lead to lower indoor temperatures.
I assume that your system is set to operate in WC mode. The settings of WC need to be adjusted to match the heat loss of your home, and the heating capacity of your heat emitters, to provide the desired indoor temperature. I normally suggest setting the cold end of the WC curve to a LWT of 50C at an outside temperature of -5C, the warm end setting would be 25C LWT at 20C outside temperature. Then turn any TRV's up fully and set any thermostats to 1C or 2C above the desired indoor temperature, and allow the temperatures to stabilise.
If the indoor temperature is now higher than that desired, change the WC curve cold end for a LWT of 50C at -10C outside temperature.
If the indoor temperature is now lower than that desired, change the WC curve cold end for a LWT of 50C at 0C outside temperature.
Do you have a buffer tank or low loss header in your system? These can have an adverse effect of system operation and efficiency.
