Octopus Heat Pumps

Nick_Dr1

They tried that with us too. Wanted to put us onto 3 phase (at vast expense to me) just because of the heat pump. I think they were supplied with the heat pump output rather than the electrical draw.
Any way after a lot of arguing with the DNO they relented and everything was ok.
Note there is no current way of load limiting a heatpump, unless it is built into the heatpump. The one's Octopus supply do not have this feature (hardly any do).
If you want to fit a load limiter it has to be on the EV charger and this is a fairly common thing to do.

There is no test where everything is turned on to check -it is purely done by a set of arbitrary rules and an assessment of how much slack is left in the cable in the street - they won't guarantee that just because you have an 80 A fuse that they will supply 80 amps. Our original DNO survey was done by video! They are the final arbiters at the end though. No transparency and if they stick to their guns there will not be much you can do.

murmeltier

So we had the Heat Geek in for a heat loss survey. Results came out lower at 8.3kW vs Octopus at 10kW.

I haven't seen the full details yet of the Heat Geek proposal but from what I've seen so far two rooms have big differences, possibly due to the amount of insulation assumed. In general, Octopus comes out higher for every room. 

Heat Geek propose installing a Vaillant aeroTherm plus 7kW vs Octopus Daikin 10.6kW. These are big differences in output I think.

Not sure what to do next. Initial install cost, while obviously a consideration, is not the most important factor for me.

Would people here agree with the Vaillant 7kW if heat loss is indeed 8.3kW?

Will ask Octopus whether they would consider installing a smaller unit but if they insist on the 10.6kW Daikin I think I will pull out.

The rest of my household think I'm crazy - "big is good" ...

Netexporter

Oversizing isn't generally good for efficiency. The Arotherm seems to dominate the top end Heatpump Monitor efficiency charts and has the bonus of using propane (R290) as its refrigerant, rather than the commoner R32, the latter of which is more harmful to the environment if it leaks.

Do you have any back-up heating to help out when the weather is extremely cold?

Reed_Richards

Make a like-for-like SCOP comparison, how do the two heat pumps compare at the same leaving water temperature?

Heat loss surveys are only ever provide estimates.  And the nominal output of a heat pump isn't always the same as the actual output.  If your house really does need 8.3 kW constantly to keep it warm enough in cold weather then you need to allow for the time the heat pump spends heating your hot water and the time it spends defrosting itself and specify a heat pump that gives you a bigger output than 8.3 kW.  Maybe the nominally 7 kW Vaillant can do that?  Maybe it can't?  Although your household isn't correct that "big is good" when considering running costs, they won't be happy if small doesn't keep you warm enough in cold weather.

Another issue is what minimum power output each heat pump is capable of?  As a general rule the smaller the better.  So if the Vaillant is better in that respect it's likely to be more economical to run in warmer weather.

        

murmeltier

In terms of SCOP, Vaillant is slightly better than Daikin.

According to the datasheets I've found, the 7kW Vaillant can output 8.4kW at -3 outside with 45 flow. I can't find a directly corresponding number for the Daikin.

I couldn't find anything for minimum power for Vaillant, but from looking at heatpumpmonitor it looks like it might be able to go down to 600W. The Daikin can only go down to 900W from what I understand from matt_drummer.

Another data point from our actual energy use over the last year - we seem to need roughly 2.5kWh gas to keep the property at 17 degrees when -1 degrees outside (boiler running at roughly 46 flow). Can this be translated into a meaningful number? I have a feeling that whatever it is, it is lower than the 8.3kW heatloss.

Reed_Richards

murmeltier said:

Another data point from our actual energy use over the last year - we seem to need roughly 2.5kWh gas to keep the property at 17 degrees when -1 degrees outside (boiler running at roughly 46 flow). Can this be translated into a meaningful number? I have a feeling that whatever it is, it is lower than the 8.3kW heatloss.

17 degrees is quite a bit lower than most people would want.  If your household is okay with this, did you make it clear to Octopus and your Heat Geek?  Typically the heat loss calculations assume you want 21 degrees.

When you say you needed 2.5 kWh of gas do you mean 2.5 kWh per hour, or 2.5 kW (with the heating on all the time)?  The implication of this is that your heat loss was about 90% of 2.5 kW, assuming your boiler was achieving 90% efficiency (possibly it would be better with 46 degrees flow).  The heat loss could be more than this if your house has thick stone walls or a thick concrete slab underneath as these can store heat so your house would not cool so quickly and you would need a few weeks at -1 outside to get a more accurate number.      

murmeltier

Octopus didn't want to know about the lower temperatures, they were adamant it had to be 21 degrees - they're quite rigid in their approach.

I meant gas usage of 2.5kW per hour to keep the house at set temperature (~17 degrees) while outside conditions unchanged (~0 degrees). I've observed this usage on multiple occassions this winter. Obviously very crude estimate, so even if true figure is higher, I can't see it ever getting to 10kW.

Had a discussion with a family member in Switzerland who got a heat pump last year. Obviously different climate profile (likely lower than ours), but some characteristics of their property are similar (age, insulation, floor size). They got a 5.6kW Viessmann, and seem happy with it.

As an aside and much to my amazement no heat loss calculation was done. Instead the Swiss installer took their annual gas usage, divided it by 3 and used that to size & install the heat pump. Total cost ca £35k after government grant ... 

koru

murmeltier said:

I couldn't find anything for minimum power for Vaillant, but from looking at heatpumpmonitor it looks like it might be able to go down to 600W. The Daikin can only go down to 900W from what I understand from matt_drummer.

As I understand it, most heat pumps can modulate down to about 25% of their maximum, so the Vaillant should get down to less than 2kW. The Daikin is actually a 16kW device with software throttling, so should get down to 4kW, which is more than double the Vaillant.

Gerry1

murmeltier said:

I meant gas usage of 2.5kW per hour to keep the house at set temperature (~17 degrees) while outside conditions unchanged (~0 degrees). I've observed this usage on multiple occassions this winter. Obviously very crude estimate, so even if true figure is higher, I can't see it ever getting to 10kW.

Do you really think that switching off the central heating and plugging in just one 3kW fan heater would keep the whole house warm when it's freezing outside?

koru

murmeltier said:

As an aside and much to my amazement no heat loss calculation was done. Instead the Swiss installer took their annual gas usage, divided it by 3 and used that to size & install the heat pump. Total cost ca £35k after government grant ... 

You mean divide by 3000? Interestingly, this is pretty much the rough estimate method suggested by Michael de Podesta, as summarised in the fabulous essay on heat loss here: https://energy-stats.uk/what-size-heat-pump/