H2 versus H4 System Boundary and treatment of Secondary Pump electricity consumption as 'Heat'

Hardware Heatpump
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Following last week’s EcoHome Lab webinar which some forum members attended (“In-situ heat pump performance in the UK” with Colin Meek) I’ve been thinking I should implement the changes to report my heat pump’s performance against the H4 System Boundary rather than H2 as I do currently.

I’ve got three circulation pumps with accompanying zone valves feeding different circuits on the emitter side of a Low-Loss Header which are powered separately from my main heat pump unit and so excluded from the sub-meter on the supply to the main unit. I’ve already added a second sub-meter measuring the electricity consumption of those pumps and valves which shows a total of 60W when they’re all ‘on’:

  • The pumps are all the same (Grundfos ALPHA2L 15-50 130) so that’s 20W per pump & zone valve combination, which doesn’t seem bad
  • The total of 60W is not insignificant though: 60W is 1.44 kWh per day, 43.2 kWh per month.

My plan is to amend the script I use to upload the readings to emoncms.org (for HeatpumpMonitor) so as to report the sum of the two electricity sub-meters:

  • The one for my main (Ground Source) Heat Pump unit, which covers the compressor, the heat pump controller and the primary circulation pump (plus the ‘brine’ pump)
  • The new one for the supplementary pumps and zone valves

So far so good; my question is whether it is legitimate to also include the second meter’s electricity consumption in the heat output figure (i.e. add it to the heat meter reading too).

The electricity consumed by each pump is partially going into the pumped fluid (and so appearing as heat via the emitters) and partially being dissipated via the pump’s housing. In both cases this is extra heating inside the house’s insulated envelope (albeit at CoP 1.0) but it’s ‘downstream’ of the heat meter so it’s not being recorded elsewhere.

I’ve reviewed the EST Heat Pump Field Trial paper from 2013 (PDF link) which contains a good summary of the SEPEMO-defined System Boundaries such as H2 and H4 but doesn’t appear to address my specific question.

I also found this DECC RHPP Detailed Analysis Report from the UCL Energy Institute in 2016 (PDF link) which has a specific Appendix B.4 addressing “Circulation pump power consumption” which includes the following:

…throughout this report it has been assumed that 100% of the electricity consumption of the pump is transferred as heat to the heating circuit…

My inclination is to add the supplementary consumption readings to the heat meter readings, since they do represent heat going into the house, but I don’t want to compromise the ability to compare my system with others.

For reference, my house is a nearly-Passivhaus and doesn’t need much heating. On average, during the heating season (October - March) the heat pump is using 190 kWh of electricity per month (as currently reported; H2 system boundary) and metering 705 kWh of heat output per month, so the SPF (H2) is 3.7, across CH and DHW.

The monthly 43 kWh from the supplementary pumps therefore represents 18.5% of the combined electricity consumption - which seems (to me at least) like a surprisingly large proportion, but only because the heating demand is relatively modest. From a quick calculation I reckon Including versus Excluding the heating effect of the pumps’ power consumption is worth about 0.2 of SPF.

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This did come up in another thread, which I can’t find now. It’s an interesting idea and I can see the reasoning for it, don’t believe this to be standard practice. The same could be done for standby power, which does feel like cheating.

It might be fine if everyone did it, otherwise it skews the comparison between systems.

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Thanks Tim. The treatment of standby power (about 45W for my NIBE controller) was going to be a follow-on question :wink:; I agree that’s more controversial. All of the ‘active’ components of my GSHP system are inside the house so all of the consumption ends up as heat one way or another, which is not always the case for an ASHP.

In terms of building physics, my preference is to include the pump power as ‘heat’ but I agree that it’s more important to be consistent, for comparison purposes, which is why I’m keen to get input from the wider community and (hopefully) establish a consensus position.

My hunch is that in most installations this is too small a proportion of the overall consumption to be worth worrying about, but that’s not true in my case.

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Interesting question, I don’t think this should be done because:

  1. It would be an unfair comparison because this is not the standard done for other systems.
  2. I would think that a significant amount of the heat will be dissipated from the pump motor body water than enter the flow of water.
  3. If the energy is put into the water the heat meter should someone measure since this, since the return temperature will be slightly higher than it would be without the additional heat being added
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I wonder if we are confusing accuracy with precision here?
Lots of CH pumps offer only about 30-35% hydraulic efficiency (i.e. the heat that actually makes it into the circulating fluid so may end up being “useful”).
The rest just escapes as heat from wherever the pump is located, which may or may not be “useful”.
And since we may be only talking of a few dozen watts anyway, is it worth getting so picky? :wink:

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I think Glyn is onto something here.

The argument is that the whole amount of energy consumed is heat into the house as the pump(s) are within the house.

That is probably true.

But some or maybe even most of the heat goes into the water and is therefore measured by the heat pump. Who knows with these particular pumps in this particular house?

Adding it or at least some of it again can’t be right.

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I wonder what the hydraulic efficiency of modern high efficiency pumps such as the Wilo-Pico or Yonos are? Is lost heat also transferred via the pump body into the water as well? My guess from feeling the heat on an efficient pump is that most of that heat must be transferred to the water…

The position of the pump on the primaries is also important, if it is between the heat meter and the heat pump , or in the heat pump itself, the heat will be measured by the heat meter. E.g a 20W with 100% heat transferred and 20 L/min would increase DT between flow and return temperature sensors by 0.014K.

If it is after the heat meter it’s heat gain will not be measured as it will not modify the DT across the heat meter. The return water might be fractionally higher but the DT and therefore measured heat should be unaffected.

The same has to be true for the secondary pumps and so I can see your point @dMb the heat contribution should be added to the heat side of COP calculation as well.

We’d have to have the option to track these separately in emoncms and we do need to add in an immersion heater feed anyway to do this for immersion heaters…

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If the secondary pump(s) are before the heat meter and most of the heat transferred to the water, then isn’t the heat already being included (as it raises the incoming water temperature before the heat meter)?

If that is the case, then all that is missing is the electricity consumption.

Wouldn’t adding the electricity consumed to the heat produced be double counting?

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Thought Id draw a diagram, couple of thoughts:

  1. Heat meter could read less than zero heat if heat pump compressor is off in this scenario and outside primaries are not well insulated etc as the slightly warmer return water would loose heat and flow temp could be lower than return temp (when compressor is off).

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  1. If the outside primaries are perfectly insulated the warmer water should go all the way around the outside part of the circuit and be the same temperature when it reaches the flow sensor and so would measure 0 on the heat meter - the heat would then be dissipated in the house somewhere…
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Thanks to all who have responded so far.

I don’t really want to lock this question into the specifics of my installation, but for clarity here’s an annotated schematic:

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Note this is a Ground Source system so the only thing not inside the house’s insulated envelope is the ‘ground loop’ pipework - the compressor and all the pumps and controls are inside - so any heat they give off is “useful”, to use Sarah’s terminology.

Good point Glyn and I’d not considered that: so there is a sense in which the heat transferred to the water by the Secondary pumps is influencing the Heat Meter (indirectly) - although if it’s increasing the Return Temperature that’s tending to reduce deltaT as seen by the Heat Meter, isn’t it?

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The secondary pumps are on the flow by the looks of it so they increase flow temperature?

But in any case, isn’t it all just mixed in to the low loss header?

That is what the heating heat meter is measuring, the secondary pumps are irrelevant in heat produced terms except for any heat going into the house from the heat lost out of the casing?

If lots or even most of the heat goes into the water flowing around the llh and the emitters, then doesn’t the heat meter on the heat pump side of the heat meter pick up all the heat delivered?

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Good point. and so in your case if you imagine the scenario where the pumps are all running but the compressor is off.

  1. If you move the return sensor to after the primary circulation pump, there is now only a little internal pipe between the flow and return sensor and so regardless of what the secondary pumps are doing the heat meter will read zero.

  2. With the return temperature sensor in it’s actual position before the primary circulation pump you should now only measure the DT associated with the heat gain from the primary pump.

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The case Im trying to make perhaps unsuccessfully is that the heat meter reading will not detect any heat from the secondary pumps. While the return water temp might be slightly higher that gets passed directly through to the flow temperature… (and the best way to understand that is to imagine the case when the compressor is off and scenario 1 above with the return temperature sensor after the primary circulation pump)

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Sorry Trystan,

I think I am unsuccessful in saying what I think :slight_smile:

Maybe unsuccessful in understanding too!

If the heat meter and temperature sensors were as close as possible to the heat pump then all the heat delivered to the house through water flowing would be measured, including any heating of the water from the secondary pumps.

The only other heat going to the house would be any radiated(?) heat from equipment.

As we don’t know what the radiated(?) heat from the secondary pumps is, I can’t see that is is correct to include any of it, certainly not all of it

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I think the penny has just dropped in my head now! Certainly as far as a system without the llh.

Sorry for being a bit slow @TrystanLea

I’m still not sure how the llh affects things though.

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Thanks @matt-drummer ! I also find it quite hard to wrap my head around what is happening in these dynamic hydraulic circuits, as you might have noticed with the other thread on long primaries :exploding_head: it really isnt that easy sometimes to see what’s going on as there’s so many different bits contributing in different ways…

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These kinds of questions really call for simulations. As I already said in the thread on the long primary pipes, Ill hopefully get on it once I’m back from my vacation.

Just my two cents for the moment: The energy stored in the moving water will, via friction, be turned into heat. Any power deposition into the flowing water after the heat meter would actually lead to a lower measured power output by the heat meter. This makes sense intuitively: Any power deposition into the circuit after the heat meter would increase the return temperature and hence decrease dT. So it’s quite a tough challenge to account for those pump losses. Even if there is a heat exchanger or secondary circuit separated from the primary, the pump losses will have the same effect. Anything added after the heat meter will be subtracted from the measured power.

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I don’t think so - the dT is energy put in by the heat pump, so extra heat put into the return will also raise the flow temp by the same amount. Measured energy is unchanged.

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Very true. But a higher flow temperature would also lead to higher power emission from radiators, which would then lead to a larger drop on the return temperatures. This is a bit of a head scratcher. :sweat_smile:

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It’s not easy is it!

I can’t convince myself either way now,

But I still come back to the fact that all the flowing water is measured by the heat meter.

I think I agree with @Timbones

The heat pump is benefitting from a return temperature boosted by the addition of heat from the secondary pumps and therefore works less to get the flow temperature back up. That is how the extra heat is accounted for.

I probably haven’t explained it very well but I think that is what Tim is saying?