So is the designer/installer charged with the responsibility for achieving the most economic operation?
They are required to commission the system so as to use as little 'fuel' (energy) as reasonably possible.
They are also required to instruct the home-owner how to operate and maintain the system in this state, and provide site-specific documentation on each particular system.
These requirements have been stipulated in the past three iterations of Building Regs, Part-L, the relevant section of which currently reads as follows:
And here's the previous version:
Thanks for the official documentation info @transparent . I hadn’t read the most recent offering…. Is it me or does it really avoid tying anything down? And is it really that short a statement.
to my mind it only requires a few simple calcs to establish if the designed / amended radiator system is capable of operation at the lower flow temperatures of a heat pumps most efficient output. I believe most manufacturers provide minimum operating outputs for their HPs. So it would follow that every system design could be reasonably expected to quote the lowest anticipated flow temperature that the HP LIMITS and system could operate at.
Ive done these calcs for a range of low flow rates based on a notional published output for DT50 rating for 10kwh (as you say) on the back of a fag packet. I’ve used 10kwh so it can be scaled up or down to any size system.
or it’s slightly longer version…
I think it’s reasonable to say the hub gets its fair share of topics relating to output cycling.
This post was modified 4 weeks ago 2 times by SUNandAIR
I think it’s reasonable to say the hub gets its fair share of topics relating to output cycling.
Firstly I think you accidentally pasted a picture not the actual excel
Your comment is certainly correct.
Most heat pumps have a modulation ratio (ie ratio of min to max output) of 3 at best. Pretty much all heat pumps cycle for defrost if its both damp and the temperature is 4C or below.
If you plug a modulation ratio of 3 into the heat loss equations for typical UK conditions, you can easily conclude that the best you can hope for the 'no cycle' window is between 4 and 12C, assuming the heat pump is 'right sized'. Many, perhaps most, heat pumps are oversized and that squeezes the top end of the 'no cycling' window down. Its not that difficult to get to a point where it is cycling most of the time.
The fundamental problems here are (1) the very limited modulation ratio of current heat pumps and (2) the uncertainty in the heat loss calculations (and the tendency for them to be conservative). Taken together its inevitable that most heat pumps will cycle a significant proportion of the time, and a significant proportion of heat pumps will cycle most of the time. Until one of the fundamentals changes, we are stuck with this!
Unfortunately the MCS rules dont really do much to counter the over-sizing (in fact they come close to mandating it), and do not specify a requirement to calculate the onset of cycling. Even if they did, unless the conservatism in estimating heat loss is modified, the design decision doesn't change.
The question to which we dont know the answer of course is - how much does cycling really matter
This post was modified 4 weeks ago 2 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
Thanks for the official documentation info @transparent . I hadn’t read the most recent offering…. Is it me or does it really avoid tying anything down? And is it really that short a statement.
to my mind it only requires a few simple calcs to establish if the designed / amended radiator system is capable of operation at the lower flow temperatures of a heat pumps most efficient output. I believe most manufacturers provide minimum operating outputs for their HPs. So it would follow that every system design could be reasonably expected to quote the lowest anticipated flow temperature that the HP LIMITS and system could operate at.
Ive done these calcs for a range of low flow rates based on a notional published output for DT50 rating for 10kwh (as you say) on the back of a fag packet. I’ve used 10kwh so it can be scaled up or down to any size system.
Most heat pumps have a modulation ratio (ie ratio of min to max output) of 3 at best. Pretty much all heat pumps cycle for defrost if its both damp and the temperature is 4C or below.
Yes I think That’s quoted quite a lot if I’m getting your ratio of 3 correctly. The problem is, like all rule of thumb approximations is that that many quoted sizes of HP does not represent the highest output and that different manufacturers quote different values for their HP sizes . I don’t like complicating things more than they need to be so I will post actual minimum outputs of a brand to discuss further…
I have highlighted the output range between the OATs of 5c and 15c.
the 6kw ecodan has a high minimum output for its rated size.
whereas the 8.5kw ecodan has has almost the same minimum of around 3kw to 3.2kw.
meanwhile the 11.2kw ecodan has a minimum output of approx 4kwh.
So by not relying on rule of thumb approximations it can be seen what the minimum output would be to prevent cycling.
So now when the radiator capacity is measured against the HP Operating limits It is hopefully now possible to identify the correct total emitter capacity in order to avoid rapid cycling.
as you have stated…. How harmful is cycling anyway? But I think it will be different risk for different type of cycling…
eg
defrost cycling in freezing conditions after 50c flow
vs
35c 30minute cycling for 3 minute reset at 30c….
Or 45 minute continuous cycle followed by 2 hour rest then boost restart….
I would anticipate that each of these would have very different stress levels on the compressor.
And it perhaps wouldn’t be a good idea to say ALL cycling is harmless.
Anyway I hope you can see in my graphs that there is good reason to know how low the output capacity is of a radiator system when operating below 35c. But also to size radiator capacity based on the intended low temperature capacity and paying due regard to the HP minimum operating output.
From what I’ve read and experienced I do think radiator sizing is infinitely easier than sizing the HEAT PUMP.
@sunandair The Approved Documents which comprise the Building Regulations are not as 'officious' as they used to be. They are now in a form which the general public would be better able to understand.
That makes them easier for a homeowner to use if they decide to pursue a tradesman under the terms of the Consumer Rights Act (2015).
Instead of that involving an application to the Small Claims Division of the High Court, it is the Citizens' Advice Bureaux who handle the dispute. The process involves arbitration or mediation, according to the nature of the complaint.
The reduced level of legal threat against a 'supplier' means that they are less likely to employ a solicitor in an attempt to 'win' the case on legal technicalities.
The advice and expertise which others share on this forum would be acceptable as evidence in a case being taken to arbitration. But it could too easily be dismissed as lacking professional authority in the Small Claims Court.
Here's Section-9 of the same Part-L Approved Document, the Part which addresses energy matters.
It's pretty straightforward.
Anyone who has just had a heat-pump installed should be able to understand the sort of documentation which they should receive.
Being handed a pile of Manuals published by the heat-pump manufacturer is insufficient.
So now when the radiator capacity is measured against the HP Operating limits It is hopefully now possible to identify the correct total emitter capacity in order to avoid rapid cycling.
Apologies but I don't understand this. The demand is determined by the house which in turn determines the radiator sizing. You can't prevent cycling if the house demand is less than the minimum output. Nor can you change the duty cycle, all you can change is the frequency. This is largely determined by the hysterisis (deviation from target temperature) you are prepared to accept and system volume.
Most, possibly all, heat pumps (and boilers) enforce a minimum cycling time which is generally adjustable. Of course if you increase it from the factory default it will inevitably increase hysterisis, which may or may not matter.
So far as I can see evrything you do is ultimately constrained by the house!
This post was modified 4 weeks ago 3 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
So now when the radiator capacity is measured against the HP Operating limits It is hopefully now possible to identify the correct total emitter capacity in order to avoid rapid cycling.
Apologies but I don't understand this. The demand is determined by the house which in turn determines the radiator sizing. You can't prevent cycling if the house demand is less than the minimum output. Nor can you change the duty cycle, all you can change is the frequency. This is largely determined by the hysterisis (deviation from target temperature) you are prepared to accept and system volume.
Most, possibly all, heat pumps (and boilers) enforce a minimum cycling time which is generally adjustable. Of course if you increase it from the factory default it will inevitably increase hysterisis, which may or may not matter.
So far as I can see evrything you do is ultimately constrained by the house!
My first thoughts are that the process is more dynamic than your model suggests. Your idea that demand is determined by the house isn’t quite how I see it… it is crucially and thankfully determined by the Weather Compensation Curve (incl. the Auto Adapt variants) this means heating commences at the right flow temperature and doesn’t race off into oversupply of heat. if the Design temperature is say 55c at -3c OAT and that’s what the radiator system is sized for then this limits how low temperature the system can modulate down to, because the radiators offload capability at say 35c become the limiting factor.
So in other words designing for the 55c flow temperature is only doing half of the design. The radiators are the tools of the heat pump providing maximum flexibility of lower temperature operation allowing the fullest scope of modulation based on the most efficient output.
The tables I posted above show that the radiators must be bigger to provide a high enough output when operating at 35c and lower. To ignore this is to seriously compromise the operating process.
Ultimately it’s down to the WC Curve to keep the room temperature below the target and it can only achieve this if the appropriate modulated temperature is attained and preferably without undue cycling. But this all assumes the heat pump is sized correctly but that is an entirely different set of design issues.
@sunandair Regarding radiators, UFH and WC, system design can have some quirks. Say one designs for the following: i) -3C design temperature with a matched 10kW heat pump ii) UFH and radiators sharing the heat load equally iii) WC enabled at 48C mean water temperature for -3C OAT and 30C for 15C OAT iv) Steady open loop house temperature of 21C
The room temperature difference is then 24C at -3c and 6C at 15C, a spread of 18C which matches the WC spread. As UFH output is linear, it will follow a linear WC as above, so no problem. However an average 1.3N radiator will need to be ~1.7 times larger at 15C than at -3C to maintain correct output. If sized for 15C, it will output 1.7 times too much heat at -3C. The heat pump can modulate down to 40% of its output so it will start cycling at 11C OAT.
This all ignores the impact of defrost cycling on heat output.
My first thoughts are that the process is more dynamic than your model suggests. Your idea that demand is determined by the house isn’t quite how I see it… it is crucially and thankfully determined by the Weather Compensation Curve (incl. the Auto Adapt variants) this means heating commences at the right flow temperature and doesn’t race off into oversupply of heat. if the Design temperature is say 55c at -3c OAT and that’s what the radiator system is sized for then this limits how low temperature the system can modulate down to, because the radiators offload capability at say 35c become the limiting factor.
So in other words designing for the 55c flow temperature is only doing half of the design. The radiators are the tools of the heat pump providing maximum flexibility of lower temperature operation allowing the fullest scope of modulation based on the most efficient output.
The tables I posted above show that the radiators must be bigger to provide a high enough output when operating at 35c and lower. To ignore this is to seriously compromise the operating process.
Ultimately it’s down to the WC Curve to keep the room temperature below the target and it can only achieve this if the appropriate modulated temperature is attained and preferably without undue cycling. But this all assumes the heat pump is sized correctly but that is an entirely different set of design issues.
Ultimately demand is defined by the house because ultimately the heat pump (plus any sundry sources of heating) must supply exactly the amount of energy as that lost by the house to the outside world, if the house is to stay at the same temperature. Of course there can be short term deviations from equilibrium, but long term I think this equivalence is a given based on conservation of energy.
It is true that, if I set my flow temperature, through the WC curve, at a level higher than that required to ensure that this amount of energy is lost from the radiators to the house, then two things will happen
1. The house will heat to above the design temperature
2. The house loss will increase because the IAT-OAT difference has increased
As a result the heat pump will indeed cycle less, or more precisely will cycle with a larger duty cycle (the frequency may remain the same depending on what determines the duty cycle).
Obviously if one is happy to tolerate the two effects above one can reduce cycling, but Im not sure why one would do that.
That said we can 'layer on' some diurnal variation to this 'steady state' argument. Its generally warmer at day than at night, so for much of the season cycling is more likely to happen during the day than during the night. If you overheat during the day, reducing cycling, the heat pump will have to supply less energy at night, when its anyway less likely to cycle. Overall therefore one might get less cycling. This is a sort of 'reverse setback' with the aim of reducing cycling and I think would work. Whether it makes sense is however a moot point.
In summary I don't think you are correct, unless you are prepared to allow the house to rise above the design temperature (and hence losses to increase) for the sole purpose of reducing cycling. Perhaps if you step through a specific example (with numbers) of how and why the radiators need to be designed not just for low OAT but also for high OAT it might become clearer where the respective arguments differ. I don't understand this either (and the paper spreadsheet you posted didn't enlighten me) and the two are obviously linked, so stepping through an example might well help.
This post was modified 4 weeks ago 5 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
@sunandair Regarding radiators, UFH and WC, system design can have some quirks. Say one designs for the following: i) -3C design temperature with a matched 10kW heat pump ii) UFH and radiators sharing the heat load equally iii) WC enabled at 48C mean water temperature for -3C OAT and 30C for 15C OAT iv) Steady open loop house temperature of 21C
The room temperature difference is then 24C at -3c and 6C at 15C, a spread of 18C which matches the WC spread. As UFH output is linear, it will follow a linear WC as above, so no problem. However an average 1.3N radiator will need to be ~1.7 times larger at 15C than at -3C to maintain correct output. If sized for 15C, it will output 1.7 times too much heat at -3C. The heat pump can modulate down to 40% of its output so it will start cycling at 11C OAT.
This all ignores the impact of defrost cycling on heat output.
So what would you do?
This is an interesting one and shows the challenge of mixing radiators and UFH IMHO.
I suspect that, in at least some houses, it 'resolves itself' because heat rises and moves around the house, and its not uncommon to have UFH downstairs and radiators upstairs and want the bedrooms cooler.
However if it doesn't, trying to get the two balanced at all temperatures given that UFH has an exponent of 1 and rads an exponent of 1.3 is obviously 'interesting'. If you mix down for the UFH then I guess the mixing ratio could be modulated, if you dont mix down you are stuck I think and probably have to put either the UFH or the rads on a thermostat, which of course then creates further problems and violates your condition. Messy!
I think if I were designer or installer (which I am not) I might be tempted to quote a high price unless it was clear that the occupants were very tolerant. If they expect it 'just to work', as many do, then you will likely be getting lots of call backs, 'cos it ain't going to!
Is there a 'known' solution standard to this problem?
This post was modified 4 weeks ago 4 times by JamesPa
4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.
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