Heat Pump Sizing?

matt_drummer

michaels said:

Whatever my motivations I want to go into it with my eyes open.  I don't want to be told the scop will be 3+ and then actually it it is 2.5 (or vice-versa for that matter).

Octopus want to charge me 6k less than others but will the achieved SCOP (or lack of warmth) mean that it costs more over the long term?

And yes even with gas, there is uncertainty over the gas vs electricity price ratio with for example suggestions that the non-energy supply costs that are currently loaded onto electricity rather than gas should be moved to gas.

IMHO it is not such a new technology that we should be left to guess what the running costs will be, using heat loss, radiator deltaT, degree days and heat pump characteristics it should be possible to be fairly accurate on energy demand (in kwh even if not cost).

All the MCS installers will tell you all of this information when they quote, they have to.

I think you have quotes from lots of different installers?

Why not choose the one that gives the best outcome at a cost you are happy to pay?

I am sure that with Octopus you are not left guessing, they must have told you what to expect?

NedS

You need 3 things for an efficient ASHP system:

1. An appropriately sized heat pump

2. Large enough radiators (or UFH) to emit the required heat at as low flow temps as possible

3. Pipework large enough to carry the amount of heat you need to emit.

Everything starts with knowing your total heat loss figure (to appropriately size the heat pump to the property), and then individual room heat losses to appropriately size each radiator to the room at design temps. Then knowing how much heat output is required in each room, you can ensure your pipework is sufficiently large that you can transmit that much heat at the required flow rates.

If you get those 3 things right, you are 90% plus of the way there to a very efficient system.

You know roughly what sized heat pump you need. Next you can either decide on a target flow temp (say 40C at your coldest ambient temp) and calculate the radiator sizes you need to output the required heat in each room at that flow temp, or you can go the other way and look at the largest radiators you can accommodate and then calculate the flow temps that would be required for those radiators to emit the required amount of heat. Either way, you now have your flow temp at design temps, and you can begin to estimate what your COP may be. But it is what it is, and the only way it's going to improve or be any different is if you run with lower flow temps which requires larger radiators. 

For my own installation, I am unlikely to ever need flow temps above 40C for heating unless we see extremely cold temps. For 95% of the time, I'm likely to be running in the 33-38C range, as my heat pump really doesn't like running much below 34-35C, and 38C will provide sufficient heat down to around 0C outside.

Reed_Richards

matt_drummer said:

As the difference between indoor and outdoor temperatures close it becomes harder for radiators to give up heat.

This isn't technically correct (although that's mostly by-the-by; I agree with @matt_drummer in every other respect).  The rate at which a radiator gives up heat is determined by delta T, the difference between the room temperature and the average radiator temperature; it has nothing to do with the outside temperature.  However the rate at which the radiator raises the room temperature has everything to do with the difference between the outside temperature and the room temperature.  I think this rate of heat loss from your house increases linearly with the temperature difference between inside and outside; it's certainly roughly linear if not exactly so. 

michaels said:

Not sure that makes sense to me, the water temp probably needs to be at least 34 to output enough heat to keep the house at 20c, sure you can run long cycles but that would be at the expense of fluctuating internal temperatures.

No, because when it's 10 C outside your house is losing heat relatively slowly so you can operate your heat pump with longer cycles than you would need at 0 C whilst maintaining a relatively stable room temperature.  But by the time it gets down to 0 outside then you should be able to balance the heat output from the heat pump with the heat loss from the house well enough that it doesn't cycle at all.  You are only bound to get cycling when the minimum heat output from your heat pump exceeds the rate of heat loss from your house.   

matt_drummer

Reed_Richards said:

matt_drummer said:

As the difference between indoor and outdoor temperatures close it becomes harder for radiators to give up heat.

This isn't technically correct (although that's mostly by-the-by; I agree with @matt_drummer in every other respect).  The rate at which a radiator gives up heat is determined by delta T, the difference between the room temperature and the average radiator temperature; it has nothing to do with the outside temperature.  However the rate at which the radiator raises the room temperature has everything to do with the difference between the outside temperature and the room temperature.  I think this rate of heat loss from your house increases linearly with the temperature difference between inside and outside; it's certainly roughly linear if not exactly so. 

michaels said:

Not sure that makes sense to me, the water temp probably needs to be at least 34 to output enough heat to keep the house at 20c, sure you can run long cycles but that would be at the expense of fluctuating internal temperatures.

No, because when it's 10 C outside your house is losing heat relatively slowly so you can operate your heat pump with longer cycles than you would need at 0 C whilst maintaining a relatively stable room temperature.  But by the time it gets down to 0 outside then you should be able to balance the heat output from the heat pump with the heat loss from the house well enough that it doesn't cycle at all.  You are only bound to get cycling when the minimum heat output from your heat pump exceeds the rate of heat loss from your house.   

Yep, you are right. radiator output is only relative to dT between room temperature and mean radiator flow temperature. I should have expressed that correctly.

But the reality is, that when it gets warmer, unless you have sufficient emitter capacity and/or a heat pump that modulates low enough then your heat pump will need to cycle. Not many people have enough modulation/emitter to run continuously at 10c outside or over.

Choosing a heat pump that quickly gets into its stride in terms of efficiency is important here when you need it to cycle. That was my big issue with my 9kW Daikin, by the time it had got going I didn't need any more heat. I was always operating in the most inefficient part of its heating cycle. It used lots of electricity and the efficiency was mediocre at best.

Heat pump owners generally get to a point where the heat pump won't modulate low enough and the emitters can't cope with the heat. The heat pump has to cycle.

The biggest complaint is that this happens too easily and is usually caused by a poor choice of heat pump (not knowing what it really is) and emitters that are too small.

You are correct about heat loss being linear.

I understand what you mean but I think you should say longer intervals between heating cycles at 10c compared to 0c rather than longer cycles, some people may be confused.

Reed_Richards

Yes, agreed.  Matt and I see eye-to-eye on this; it's just a matter of explaining ourselves in the clearest way we can.  

michaels

I feel we may be letting the almost perfect be the enemy of the good.

Sure the lower the flow temp the better but there must also be a tradeoff with radiator pricing.
Similarly heat pump choice, some may modulate more than others or run more efficiently on shorter cycles if they don't modulate enough but if they cost considerably more then perhaps it is not worth it?

For me I don't want the most efficient system regardless of cost but the best value for money. Given that I can use a battery and run off night rate electricity I am pretty sure heat pump Vs gas would have a payback of less than 10 years but the question is, what system would give the best cost Vs return

Reed_Richards

@michaels, I imagine Octopus have given you the best quote, or will do when you get that far (I've lost track).  The quote will tell you what leaving water temperature they are aiming for at that magic outside temperature that is exceeded 99.6% of the time.  But there is no single right answer, it depends on what radiators they will fit.  And if your quote is anything like my quote from 2020 you will be told exactly what radiator will go in each room.

Pick some major radiator company like Stelrad https://www.stelrad.com/radiators/standard-steel-radiators/ or Ultraheat https://www.ultraheat.co.uk/product-range?product_cat=Range&fwp_range_type=radiator&fwp_design=panel and you can look up the price of each radiator (without any bulk discount your suppliers might get),  Look at the space in your home that this radiator will occupy and think about whether you have room for a bigger one, or a fatter one if you don't mind the depth of a K3 radiator.  Make a little spreadsheet for yourself showing the cost of the radiators you have been offered and the cost of the radiators you would like.  Each radiator will need to match the heat output requirement of the room when run at the temperature of your choice (assume 5 C drop across the radiator).  In some rooms you may find that you need to add an extra radiator (if feasible) when a bigger one is not possible.

My installer was very accommodating and we went through 4 iterations of radiator choices before arriving at the final one.  I have no idea how Octopus would respond but looking at the cost difference for the radiators would give you some important numbers that you are craving.      

matt_drummer

michaels said:

I feel we may be letting the almost perfect be the enemy of the good.

Sure the lower the flow temp the better but there must also be a tradeoff with radiator pricing.
Similarly heat pump choice, some may modulate more than others or run more efficiently on shorter cycles if they don't modulate enough but if they cost considerably more then perhaps it is not worth it?

For me I don't want the most efficient system regardless of cost but the best value for money. Given that I can use a battery and run off night rate electricity I am pretty sure heat pump Vs gas would have a payback of less than 10 years but the question is, what system would give the best cost Vs return

You never mentioned batteries before.

That's another spanner in the works.

I have lots of solar panels and a large amount of battery storage.

I want my house to run off batteries for as much time as possible when there is not enough solar power.

It is a big factor in my choice of how to run my heat pump.

If you look at the profile of a heating cycle you will see that most heat pumps use a lot of electricity at the beginning of a heating cycle, maybe not so much if short cycling, but if running for say 30 minutes and then off for an hour.

On a typical day, say 7c outside, the heat pump will daw 2kW and then gradually reduce down to about 400W. This takes 30 minutes usually.

Pulling 2kW will often exceed what my inverter can supply as other electrical items are in use at the same time and that means I pay more for my electricity overall.

Also my heat pump will have drawn about 750W in those 30 minutes, so that means that during the whole day my heat pump is averaging 500Wh compared to 400Wh if I just leave it running. It was more like double this with the 9kW Daikin.

I use 25% more electricity at a higher average price because less of it comes from my batteries.



So it sounds like you don't know want, the system that is the best value for money when all costs are taken into consideration.

That's excellent news.

What is that system, do you know yet?

Or are you looking for somebody else to tell you?

Reed_Richards

matt_drummer said:

If you look at the profile of a heating cycle you will see that most heat pumps use a lot of electricity at the beginning of a heating cycle, maybe not so much if short cycling, but if running for say 30 minutes and then off for an hour.

I'm glad you said "most" because my LG heat pump doesn't follow this rule.  It starts off at low power and steps up as it thinks necessary.  This makes it less responsive because when it needs to run at a higher input power it takes a while to figure that out.  But it mitigates against short cycling and is probably good for economy of operation.

Spies

I have gas data from when we had that period of sustained minus temperatures, is this a good indication of heat loss?