Are ASHP the way to go?

Reed_Richards

matt_drummer said:

Unless somebody knows better, I think you have to have a special tank for ASHP's.

Didn't we just cover that and you said that the tank was part of the Octopus package? 

The reason you need a "special tank" is that you are trying to heat it with water that is not that much hotter than your desired temperature for your DHW.  For the same reason that you may need radiators with a larger surface area, you need a coil inside the tank with a larger-than-usual surface area, which means it has to be longer than usual.  Copper is quite expensive so it's not very likely your current tank has a suitably long coil.  Without this, it would take a long time to heat your hot water and while the heat pump is doing that it's not heating the house.  

Qyburn

Just quickly referring back to the figures cited earlier, from the data plate, do i read correctly that both are based on heating water to 35 Degrees with an outside temperature of 2 vs 7 Degrees?  What do people reckon performance would be at typical Winter outdoor temperatures of say -5, or more extreme but regularly experienced -10 or colder?

Or if heating to 40 Degrees as often seems to be implied.

Mstty

Qyburn said:

Just quickly referring back to the figures cited earlier, from the data plate, do i read correctly that both are based on heating water to 35 Degrees with an outside temperature of 2 vs 7 Degrees?  What do people reckon performance would be at typical Winter outdoor temperatures of say -5, or more extreme but regularly experienced -10 or colder?

Or if heating to 40 Degrees as often seems to be implied.

Just to interject 

@Reed_Richards may well have historical data for you from his house and setup over the years and past harsh winters.

Personally we have only known two mild winters so far with our ASHP. We did get a few days at circa -5oC to -9oC but not what I would call sustained. Sadly our ASHP was a one off build install so no COP monitoring which is frustrating to say the least. All I can say is for the M2 of floor space compared to our previous similarly matched high EPC B house at a similar age the yearly COP must be reaching 3.25 or higher.

We don't chose a particular flow temperature but let the custom weather compensation curve control the house from October til mid April.

matelodave

Ideally you need to see proper performance curves that extend the temperature ranges to see what happens when the unit is working harder.

It used to be a rough rule of thumb that the energy consumption increased by around 2.5% for every degree outside the 7/35 (7 degrees outside, 35 degrees flow temp = deltaT of 28 degrees) which implies that running it at 40 degrees with an outside temp of 7 (deltaT = 33 degrees) will increase the consumption by 10% and running at -5 to 40 degrees (delta T = 45 dgrees) could increase its consumption by some 42.5%

I dont know how true that is nowadays - I did manage to get COP figures for my 13 year old 11kw Daikin which I'll try and find, but have a shufti here - unfortunately its American so all in degrees F but does give you some idea of how the COP varies with the temperature - https://learnmetrics.com/heat-pump-efficiency-vs-temperature-graph/

Heres what I can find for an Ecodan based on deltaT - (ie temperature uplift) which implies a CoP of around 3.6 at a nominal deltaT of around 30 degrees dropping to a CoP of 2.5 with a deltaT of 45 degrees.



Just be aware that COP is measured at a specific delta T whereas SCOP is based on an average over the year and so may be more representative of real world performance over time, however, its looks a bit hairy when the unit is chomping away through 50-60kwh in a day when its well below zero outside.

Reed_Richards

If you have patience you can read my thread here:  https://forums.moneysavingexpert.com/discussion/6240076/i-bought-a-heat-pump/p1 but it has reached 43 pages now!

I heat my DHW tank to 50 C.  I heat my house with radiators.  These have a target flow water temperature of 50 C if the outside temperature is -4 C or below.  At higher outside temperatures Weather Compensation reduces the flow water temperature to suit.  My heat pump was installed in December 2020 and we have rarely if ever experienced temperatures below - 5 C here since then.  In principle if it performs exactly as specified then every degree below - 4 C outside would drop the inside temperature a degree below 21 C so if it was -10 C outside it would only get to 15 C inside!  In actuality I'm pretty sure I have more headroom than this but I have a wood burner that I could use as necessary.  The reason for this is that the heat pump was specified to provide enough heat 99.6% of the time, as per MCS guidelines.  And the reason for that is that heat pumps typically have a more limited range of modulation than a gas boiler and if you make them too powerful they end up cycling a lot in milder conditions.

I don't know what SCOP my heat pump achieves but comparing it's electricity usage to the oil usage from the oil boiler I had before makes me believe the SCOP is close to 3.   

Edit:  Just to clarify, where I live the temperature was calculated to be -3.8 C or warmer for 99.6% of the time.  An MCS installer would match the size of your heat pump to whatever temperature that 99.6% figure is for you so if you lived somewhere colder you might end up with a bigger heat pump.     

Qyburn

Thanks, I'll read through that other thread as well. One thing that strikes me is that Home Energy Scotland in their promotion of ASHPs makes no suggestion that they aren't quite expected to provide enough heat all year round, or the advisability of some supplemental system to top it up. In my case I wouldn't want to be without a stove anyway, but the mantra is to get rid of everythig except electric heating.

Is that 99.6% supposed to be annual or over a longer period?

For reference we're in NE Scotland. I've no idea what that 99.6% temperature would be here, but our coldest day in December had a maximum midday temperature of -7.6, and minimum of -13.4. 

matelodave

Mine was set up 13 years ago to the specs that were prevailing at the time and it was assumed that it would achieve satisfactory perfomance down to around -3 degrees where it would then kick in a 6kw backup heater. I've disabled the heater and we are still comfortable down to around -8.

Like @Reed_Richards we've also got a log burner which can keep us going when it does drop below comfort levels.

If you actually check out the performance info the rated output (mine is 11kw) starts dropping away and there's what's called the equilibrium point (Balance Point in Americanese) the heatpump output is less than what is required to adequately heat the house.

Daikin used to have a heatpump simulator/configurator programme which allowed you to input your parameters and relate them to the local weather station (I'm near RAF Marham) and it would recommend a suitable sized unit and give anticipated  consumption figures and estimate back-up heater use as well.

As far as I can remember my system was sized with an anticipated use of the backup backup heater time of around 4% per annum.

The MCS and other specs have been updated and revised several times since my system was installed. The specs and calculations involded are much more stngent.

I've not tried recalculating it all based on current requirements but I suspect that it's now miles away from what would be acceptable nowadays. I did get the RHI but it was based on a deemed COP of 2.5 in lieu of having a heatmeter and a proper monitoring system.

Have a look here - https://idronics.caleffi.com/article/sizing-considerations-air-water-heat-pumps-balance-point

Reed_Richards

Gas and oil boilers are frequently oversized with respect to actual heat output versus required heat output.  In the case of a combi boiler this is necessary to heat the hot water for the taps fast enough.  In general it's done because it makes little difference to the cost of the boiler, it avoids the need to do a detailed heat loss calculation for the building and it gives you insurance against a spell of really cold weather.  In the case of a heat pump, pushing up the power output can raise the cost of the heat pump.  Potentially it could increase the size of the heat pump beyond the size which is a "permitted development" for planning permission.  It could also possibly increase the running cost.      

matt_drummer

Reed_Richards said:

Gas and oil boilers are frequently oversized with respect to actual heat output versus required heat output.  In the case of a combi boiler this is necessary to heat the hot water for the taps fast enough.  In general it's done because it makes little difference to the cost of the boiler, it avoids the need to do a detailed heat loss calculation for the building and it gives you insurance against a spell of really cold weather.  In the case of a heat pump, pushing up the power output can raise the cost of the heat pump.  Potentially it could increase the size of the heat pump beyond the size which is a "permitted development" for planning permission.  It could also possibly increase the running cost.      

An ASHP that is too large will cycle too much if my understanding is correct?

ASHP run most efficiently if they run continuously, they won't do that if they are too big.

My gas boiler is 28 Kwh (vented system boiler), the heat loss of my house is 6.80 Kwh

Reed_Richards

Any heat pump or boiler will tend to cycle when it is milder outside.  The larger the minimum power of that heat source, the greater the range of outside temperatures where cycling will occur.  The modulation range of a heat pump tends to be less than that of a gas boiler, although gas boilers tend to be so oversized that a heat pump might still be capable of a lower power output.

I don't think cycling is a bad thing for a heat pump per se but very short cycles might well be.  It's possible to prevent short cycling in the control algorithm by restricting the number of cycles per hour but I don't know if all heat pumps do this. 

Compare a heat pump that is running continuously at a given output water temperature to one that is cycling (slowly).  For the cycling heat pump, some of the time the water temperature will be lower so the heat pump will operate more efficiently.  Some of the time the water temperature will be higher giving less efficient operation.  Some of the time the heat pump will be off before the next cycle starts.  I think the fact that the cycle includes off times means that during the on times the average water temperature will be higher than that of a heat pump running continuously, giving rise to less efficient operation.