@jamespa - thanks as ever for your analysis.

Posted by: @jamespa

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Interestingly the difference in energy supplied by the heat pump (ie heat to house) is large (29kWh, =27%).  

This is indeed interesting. Take the back of a fag packet assessment of the saving, 6/24 hours (off for 6 out of 24 hours) and the savings is 25%. This is for energy out ie delivered to the house. Yet as you correctly say the energy in saving is of the order 4%. What this tells us is that the COP must have changed, quite substantially. And indeed it has, the COP for the setback period is 3.9, for the no setback period it 5.1, a substantial difference, all the more so given the OAT was pretty much the same over both periods. If it isn't the OAT affecting the COP, then what else might it be? Let us look at various factors and numbers side by side:

                                  Setback                    No setback                     Comment

When                          Feb 24                      Feb 25

Mean OAT (°C)            10.2                          9.9                                Much the same

Mean IAT (°C)             18.7                          20.2                              As expected, should reduce energy in a bit over setback period

Energy in (kWh)          20.38                        21.22                             4% less in setback

Energy out (kWh)        78.73                       108.29                            27% less in setback 

COP                            3.9                           5.1                                 24% less in setback

Set LWT (°C)               39-41                       37-38                             Slight higher in setback, could lower efficiency (COP)

WCC                           56 @  -4, 34 @ 15     52 @ -2, 31 @ 15            WCC set ~ 3-4 degrees higher in setback, could lower efficiency (COP)

System state               'throttled'                  'free flow'                       Refers to lock shield valves restricting secondary flow

Taken all together, the bottom line is our old enemy, we are not comparing like with like. The setback period was characterised by a throttled system with a higher WCC and as a result Set LWT (and actual, though I haven't calculated it, only eyeballed it), all of which are going to lower efficiency (COP), and have the effect of increasing energy in, very possibly wiping out any saving. To make a valid comparison, we would need to control for those changed parameters, a difficult task to say the least. Overall, it will be easier to wait for further data to come in over a period when the key parameters remain the same, and then make a comparison.

In fairness I would expect the COP to change, because you are working the heat pump harder in setback (which is the whole basis of the argument that setback may cost money).  What is bizarre is that the continuous running case 'requires' 27% more energy to be delivered to the house.  The questions are

1. is this real?

2. if it is, where is it going?

As you can see from the spreadsheet I cant account for it.  It must, however (if it exists) be going somewhere!

The actual difference in electricity use was only 4% and as you say there are other moving parts and we aren't, it transpires, comparing like with like.  Given all of this we cant take much away from this at present and need to wait further data!

Posted by: @jamespa

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In fairness I would expect the COP to change

So would I, I am just underlining the differences between the two running states, and asking whether things are moving in the expected direction.

Posted by: @jamespa

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1. is this real?

A question I ask myself all the time! First of all, the continuous running period had a slightly lower (0.3°C) mean OAT, a and a definitely higher mean IAT, by 1.5°C. Both of these difference will need (consume) more energy to the house. If the 10% rule of thumb applies (10% extra energy up or down for each degree IAT up or down then that might suggest the continuous period will need perhaps 15% more energy per se. But that still leaves 10% going down a rabbit hole somewhere.

The energy out data comes from the minute readings, where I record Midea reported primary flow rate and LWT/RWT delta t and then multiply together with the circulating fluid specific heat to get the power and then over the hour the kWh of energy delivered. The flow rate seems to be reasonably accurate, judged against a separate analogue flow meter in the circuit. The LWT/RWT delta t also seems consistent with readings I have taken from the pipework entering and leaving the plate heat exchanger. There is nothing obviously out of whack.

Then we also have the Midea total lifetime energy out (the integer values, but rounding errors will lessen over longer periods), which can also be used to calculate energy out per hour (reading now - reading an hour ago). This is in column H in the spreadsheet, and it includes all energy out, ie DHW as well as space hearing. The numbers for the two periods are:

continuous period: 112kWh for both space and DHW heating (vs 108.3kWh calculated for space heating only)        

setback period: 85kWh for both space and DHW heating (vs 78.7kWh calculated for space heating only)

and they seem in pretty close agreement, given the continuous period used 3.6kWh for DHW giving a net space heating use of 108.4kWh, and the setback period used 7.2kWh for DHW (the DHW was a lot colder that day), giving a net space heating use of 77.8kWh. Both are within 1kWh of the calculated values. It is of course possible that two wrong values, ultimately derived from the same incorrect raw data, could appear to confirm each other, but... As things are, it seems they are plausible values, what is missing is the explanation of where the extra 10% (after making allowance for the lower OAT/higher IAT) went. Perhaps heat pumps have invisible flues, up which heat can mysteriously disappear!

Another possibility is the setback period, which happened during the 'throttled' ('pinched PHE') period, somehow throttled the energy out to make the total less than it would otherwise have been. @bobtskutter may have something to say on this.

As you say, and I said earlier, we need more data from a continuous period where fundamental system changes didn't happen.

Hello, sorry I've been quiet for a while.

In CathodeRays case the plate heat exchanger was acting as restriction in the flow of energy from the heat pump into the room.  The energy balance of the house would still work, but the temperatures would be different.

Posted by: @cathoderay

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Perhaps heat pumps have invisible flues, up which heat can mysteriously disappear!

You will loose energy through the outside pipework, so there is some "lost energy".  A 10% error doesn't sound too bad to me considering how much input data there is and how it's been measured.  Small errors on each measurement would compound up through the calculation.  You could do an uncertainty analysis of your calculation.

A lot of threads talk about Leaving Water Temperature because that's what makes the heat emitters hot.  However, it's the Return Water Temperature to the heat pump that determines the heat pump compressor power requirements. 

Remember the heat pump works by compressing gas, which makes it hotter.  That GAS then condenses in the heat pump plate exchanger and gives up it's energy to the flowing water.  You can only condense the gas if the return water temperature is cold enough, if it's not cold enough the compressor must increase discharge pressure.  The higher discharge pressure will increase the dew point of the gas and allow it to condense at a higher temperature, but the input electrical energy to the compressor motor will go up.

How could we account for RWT in the electrical power calculations?  I have a suspicion that set back saves energy because the RWT is colder for longer.  There was a thread on here with someone saying their UFH was most efficient if they cycled the heat pump.  I bet that was because the RWT was kept low.

Bob

Posted by: @bobtskutter

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sorry I've been quiet for a while

No need to apologise, we are all free to come and go as we please, but good to see you back!

The heat loss from the outside pipes is an interesting idea and the insulation isn't perfect but the runs are short but more to the point would any heat loss there be proportional to the conditions, and so unlikely to explain why similar conditions have similar energy in, but different energy out values?

The missing 10% may well just be random error, and possible outliers. We are after all comparing only two 24 hour periods when albeit the were similar. More comparisons when we have more data may help, by way of some some regression to the mean.

I think @jamespa may also have pointed out the real driver of the system is the RWT, which sort of makes sense to non-experts like me, since it is how the heat pump gets its feedback about what's going on the building. I do record the RWT so it is certainly available for use in any calculations. More generally, I think maybe we should talk a bit more about the RWT, so we don't forget its importance.