Posted by: @heatgeek

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However, the WC facility is linear which is fine for UFH. It appears that there are no radiator compatible curves. There is a mid-point setting so you can produce a simple triangle. Nevertheless, it will do me fine as it meets the 25% criterion and all-UFH is the only way to go if you are rational. As @jamespa rightly said in my view, if you want air to air heating, use fan coils, not radiators.

Im dont recall saying " if you want air to air heating, use fan coils, not radiators." but if I did then apologies!

The deviation of the 'ideal' WC curve for radiators from a linear best fit isnt so bad (a bit more than 1C mid point if I recall) provided your heat pump operates at constant water flow rate.  If it modulates the water flow rate to keep DT constant, then the deviation is a bit worse.  A simple triangle would, I would guess, make it acceptable.

Posted by: @heatgeek

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This new Ecodan validates your argument about following WC closely for maximum efficiency. You can now set your design flow and radiator size at 15C and turn down the top end accordingly.

Surely that's true only if the max output of the heat pump matches the house demand fairly accurately.  If it doesn't (either because the heat pump is oversized or because there isn't a heat pump available with this extended modulation range that is 'right sized') then still you are limited by the modulation range of the heat pump.

We need more like this ecodan and more accurate heat pump sizing!  It does seem like an interesting development though.

Posted by: @sunandair

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Posted by: @jamespa

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Posted by: @sunandair

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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.  

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.

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.  

 

james you appear to be sighting an example which I have never said. 

I have never asserted this statement so I don’t see why you are saying it. Above you stated

“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”

My actual comments are the opposite of this statement.

The solution I have described is to increase the designed radiator capacity ie the size of the radiators and not the flow temperature which your statement above implies. This size calculation is based on low flow operation and the minimum output requirements of the Heat Pump for continuous operation.

If the weather curve is set correctly then the house envelope will never reach the set temperature and therefore never shut down.

The radiator capacity at low flow temperatures are the tools of the heat pump giving it the free unhindered access to low modulated operation. If the radiator capacity is too small then the fewer radiators have to run hotter which is less efficient………..and causes more cycling……

 

 

 

hi @james

the importance of radiator sizing justifies revisiting this post so a bit more clarification is needed to  make the benefits understood.

So, The intention of increasing a radiator size in my post above is not to add more heat to the home. The increased size as defined in the table I posted allows the HP to operate at a modulated state at say 35c or lower depending on the Weather Compensation Curve. Therefore if the outdoor ambient temperature dictates it, the heat pump would only operate at the flow temperature dictated by the WC Curve. Ergo the heat pump output would match the heat loss of the house. This is not an absolute exact statement but more about establishing a principle that you can understand.

There are also other effects if the Rad sizes are smaller than the low operating output demanded by the WC CURVE. for example if the radiators are smaller than the 35c output requirement the HP Will start to see the DT narrow as the hp still tries to follow the WC Curve this will lead to the HP flow temperature rise 2 or 3 degrees as the hp tries to extend the cycle time until it cannot sustain operation. So there are other reasons for getting capacity into the radiators to allow the on-cycle to be longer.  

There are many designers who say you cannot have too many radiators. Even on the recent YouTube blog video by @editor.

They are after all just a facility for transferring heat, they cannot provide too much heat in themselves. It is down to the flow temperature being too high if the system calls for too much heat. 

For our more technically-savvy members here, and prompted by @robs
I've done a bit of background research on Compound Compressor Heat Pumps.

There's an interesting academic research paper on Cold Climate Heat Pumps Using Tandem Compressors
which analyses four main candidates for the topology:

Notes:

a. The paper is from the USA, and assumes that the main use of heat pumps is for cooling.

b. some of their technical terminology doesn't match what we'd write (a 5-ton heat pump, for example)

c. they often refer to interior blowers/coolers rather than radiators or UFH

I think Rob Berridge states on this episode that you can’t have “too much” radiator capacity on a HP installetion

(if you want the quote try

12 minutes 47 seconds into the video)

Posted by: @sunandair

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Posted by: @sunandair

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The solution I have described is to increase the designed radiator capacity ie the size of the radiators and not the flow temperature which your statement above implies. This size calculation is based on low flow operation and the minimum output requirements of the Heat Pump for continuous operation.

...

The radiator capacity at low flow temperatures are the tools of the heat pump giving it the free unhindered access to low modulated operation. If the radiator capacity is too small then the fewer radiators have to run hotter which is less efficient………..and causes more cycling……

 

Posted by @jamespa

Im sorry but I am still not understanding how this helps with cycling.  The output required from the radiators remains fixed by the house, and unless you can do something which changes the minimum output of the pump (such as reducing radiator size and increasing flow temperature - but at the expense of COP) you are snookered.

 

Sorry James the radiator output is governed by temperature of the water inside them and the lower the flow temperature the slower and sustainable the room temperature will rise.

And ideally this temperature gain would be in line with the heat loss through the fabric of the building. So if  a flow temperature of 35c is needed through 12sq mtrs of R22 radiators to replenish the heat loss through the fabric of the building then you would need 12 sq mtrs of radiators.

the cycling is reduced or eliminated by only heating at the target flow temperature dictated by an accurate WC Curve. 

Its true that the house thermostat would shut it down if the room temperature is exceeded but should only be an intervention situation. 

 It is an idealised situation to reach this perfectly but a target low flow can be achieved. 

Regarding changing the minimum output of the HP… that is a mechanical limit of the HP itself as you’ve seen on the graphs. The purpose of the increased radiator size is to meet the minimum output requirements.

Posted by: @sunandair

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hi @james

the importance of radiator sizing justifies revisiting this post so a bit more clarification is needed to  make the benefits understood.

So, The intention of increasing a radiator size in my post above is not to add more heat to the home. The increased size as defined in the table I posted allows the HP to operate at a modulated state at say 35c or lower depending on the Weather Compensation Curve. Therefore if the outdoor ambient temperature dictates it, the heat pump would only operate at the flow temperature dictated by the WC Curve. Ergo the heat pump output would match the heat loss of the house. This is not an absolute exact statement but more about establishing a principle that you can understand.

There are also other effects if the Rad sizes are smaller than the low operating output demanded by the WC CURVE. for example if the radiators are smaller than the 35c output requirement the HP Will start to see the DT narrow as the hp still tries to follow the WC Curve this will lead to the HP flow temperature rise 2 or 3 degrees as the hp tries to extend the cycle time until it cannot sustain operation. So there are other reasons for getting capacity into the radiators to allow the on-cycle to be longer.  

There are many designers who say you cannot have too many radiators. Even on the recent YouTube blog video by @editor.

They are after all just a facility for transferring heat, they cannot provide too much heat in themselves. It is down to the flow temperature being too high if the system calls for too much heat. 

 

I completely understand and agree with most of that.  The only point I would clarify is your statement 'So there are other reasons for getting capacity into the radiators to allow the on-cycle to be longer.'   It is certainly true that more water capacity in the system will reduce the cycling frequency - ie increase the 'on' time.  It wont change the duty cycle though, so the 'off' time will also increase.  A volumiser has the same effect (on system capacity and cycling) and may be a useful design option if radiator size is limited by aesthetics or sheer space, as it often is.

However I understood that you were claiming that it was possible to 'design out' cycling, in particular by taking into account the radiator performance at higher OATs than the 'design' OAT.  I still maintain that isnt the case. for the reasons I have set out above.  I dont believe that your argument above goes against this and nor does it provide a reason to consider radiator size at higher OATs, assuming you have already optimised* for the 'design OAT' and picked a heat pump that is not oversized.

*by 'optimised' I mean designed for the lowest FT reasonably possible given the physical and aesthetic limits on radiator size.

Posted by: @sunandair

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Sorry James the radiator output is governed by temperature of the water inside them and the lower the flow temperature the slower and sustainable the room temperature will rise.

Of course I know that. 

I am making the implicit assumption that the objective is that this equals the loss from the house.  If it doesn't then the house will either cool down or heat up, whereas the whole point of a heating control system is that it stays at a target temperature.  I am making the further implicit assumption that we, or rather the heat pump controller, adjusts something (the flow temperature) so that this is the case.  Hence why I say that ultimately the demand is governed by the house.

Posted by: @sunandair

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And ideally this temperature gain would be in line with the heat loss through the fabric of the building. So if  a flow temperature of 35c is needed through 12sq mtrs of R22 radiators to replenish the heat loss through the fabric of the building then you would need 12 sq mtrs of radiator

I also know that see above!

Posted by: @sunandair

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the cycling is reduced or eliminated by only heating at the target flow temperature dictated by an accurate WC Curve. 

Its true that the house thermostat would shut it down if the room temperature is exceeded but should only be an intervention situation. 

The first statement is true in circumstance where cycling is due to a thermostat switching the heat pump on and off (which, as you say, should only be an intervention situation) but it is not true when cycling is caused by the heat pump being unable to turn down to the level of the demand, ie the heat loss from the house.    

I was (and am) assuming we are discussing the latter because I believe we agree that control by thermostat is the wrong way to go.

I am beginning to wonder if you are trying to explain something I already understand, but because I already understand it I am reading too much into what you are saying and wrongly concluding that you are saying something that I believe isn't true.

The bottom line however is that I still cant see why there is a requirement to (or any benefit from) size radiators for higher OATs if you have already properly sized them (by which I mean designed for a FT as low as possible consistent with all the other constraints) for low OATs.  

Hi @jamespa

I’ve been travelling for a few days and felt I needed to address some unanswered messages from you.

I think and hope we just had some crossed lines of communication and perhaps different ways of saying similar things.

It looks like it’s all sorted now. Hope you think likewise.

Posted by: @sunandair

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Hi @jamespa

I’ve been travelling for a few days and felt I needed to address some unanswered messages from you.

I think and hope we just had some crossed lines of communication and perhaps different ways of saying similar things.

It looks like it’s all sorted now. Hope you think likewise.

I hope and think so.

this stuff is quite complex to describe!