Heat Pump Sizing?

michaels

Anyone looked at degree days? Heating & Cooling Degree Days – Free Worldwide Data Calculation

A model might suggest that at -3 the heat pump flow will be 47C and the manufactures COP about 2.3 whereas on a 10C day the flow would be 35 and the COP more like 5.5.  Degree days as they assign a single number to each day are far from perfect and that is before you consider that the coldest temps are typically at night when there is potentially a room temperature setback but nonetheless degree days might be a useful proxy for overall how much heat generated at 2.3 COP is likely compared to heat generated at cop 5.5.  Link this with daily actual gas usage and perhaps a picture can emerge.

Although this of course doesn't answer the question of what the real COP is at -3 when the are are also defrost cycles nor at +10C when there may be short cycling inefficiencies. 

Reed_Richards

michaels said:

Although this of course doesn't answer the question of what the real COP is at -3 when the are are also defrost cycles nor at +10C when there may be short cycling inefficiencies. 

Actually at - 3 C you are reaching the point where the air is so dry that defrosting is not necessary. 

If you think the leaving water temperature will be 35 C at 10 C outside and all your pipes pass through the heated volume of the house then they will cool to your house temperature when the heat pump is off for long enough.  So potentially you are cycling the water in your heating system between 21 C and 35 C.  That would not be a short cycle unless your heat pump is way too powerful or you don't have the minimum volume of water specified.  Now you could have a heat pump that heats the water to 35 C, turns off, lets the water cool to 33 C and then comes back on again and those cycles would be short.  But that is a failure of the control algorithm of your heat pump controller, nothing fundamental.          

michaels

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.

matt_drummer

Reed is really talking about the return temperature.

All heat pumps need the returning water to be cooler than it leaves at.

When the returning water temperature is the same as that leaving the heat pump has to stop as no heat is being delivered.

Some heat pumps or controllers will look for a very small hysteresis before starting the heating cycle again.

That is short cycling.

Reed is talking about setting the heat pump for longer off periods between heating cycles to allow the water to cool more.

matt_drummer

michaels said:

Anyone looked at degree days? Heating & Cooling Degree Days – Free Worldwide Data Calculation

A model might suggest that at -3 the heat pump flow will be 47C and the manufactures COP about 2.3 whereas on a 10C day the flow would be 35 and the COP more like 5.5.  Degree days as they assign a single number to each day are far from perfect and that is before you consider that the coldest temps are typically at night when there is potentially a room temperature setback but nonetheless degree days might be a useful proxy for overall how much heat generated at 2.3 COP is likely compared to heat generated at cop 5.5.  Link this with daily actual gas usage and perhaps a picture can emerge.

Although this of course doesn't answer the question of what the real COP is at -3 when the are are also defrost cycles nor at +10C when there may be short cycling inefficiencies. 

It is unlikely that you will be able to run at 35c at 10c outside for very long at the radiator sizes you propose.

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

It won't take very long for the returning water temperature to rise to the point where the heat pump needs to stop.

That is fine and will work.

But you won't get continuous running and it is unlikely that you'll get a COP of 5.5 running like that.

It all depends upon which heat pump you choose and the size of radiators but it could be way less than that.

Heat pumps are easy to deal with when it is cold outside, it is when it is warmer that it becomes difficult and where the choice of heat pump and emitters really counts.

Over 5c outside and heat pumps can become very troublesome.

Nobody ever has much trouble when it is cold as long as they are set up for 40c or less flow temperature.

All the trouble starts when you hit 5c and warmer, it's where most of the complaints come from, mostly related to oversized heat pumps, undersized emitters and a lack of modulation.

michaels

matt_drummer said:

michaels said:

Anyone looked at degree days? Heating & Cooling Degree Days – Free Worldwide Data Calculation

A model might suggest that at -3 the heat pump flow will be 47C and the manufactures COP about 2.3 whereas on a 10C day the flow would be 35 and the COP more like 5.5.  Degree days as they assign a single number to each day are far from perfect and that is before you consider that the coldest temps are typically at night when there is potentially a room temperature setback but nonetheless degree days might be a useful proxy for overall how much heat generated at 2.3 COP is likely compared to heat generated at cop 5.5.  Link this with daily actual gas usage and perhaps a picture can emerge.

Although this of course doesn't answer the question of what the real COP is at -3 when the are are also defrost cycles nor at +10C when there may be short cycling inefficiencies. 

It is unlikely that you will be able to run at 35c at 10c outside for very long at the radiator sizes you propose.

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

It won't take very long for the returning water temperature to rise to the point where the heat pump needs to stop.

That is fine and will work.

But you won't get continuous running and it is unlikely that you'll get a COP of 5.5 running like that.

It all depends upon which heat pump you choose and the size of radiators but it could be way less than that.

Heat pumps are easy to deal with when it is cold outside, it is when it is warmer that it becomes difficult and where the choice of heat pump and emitters really counts.

Over 5c outside and heat pumps can become very troublesome.

Nobody ever has much trouble when it is cold as long as they are set up for 40c or less flow temperature.

All the trouble starts when you hit 5c and warmer, it's where most of the complaints come from, mostly related to oversized heat pumps, undersized emitters and a lack of modulation.

Which is exactly my point - in theory I can have a pretty good estimate of the total heat kwh demand at each outdoor temp (and thus HP theoretical efficiency level) but simply applying the theoretical efficiency will not give me a true picture of the actual input kwh due to the issues of defrosting and short cycling due to insufficient modulation.  (I know the heatpumpmonitor website in theory might give me some more insight but it is not the easiest question to answer)

matt_drummer

If the install is good enough you will get the theoretical efficiency or better.

If the installation isn't designed to prevent defrosting and short cycling then you'll probably get lower efficiency.

A good idea of how much less could be gained by a knowledgeable person visiting your house to carry out an assessment and advise you accordingly.

If you know which heat pump you want (?), your heat loss and your total proposed radiator capacity it would be easy to make an educated assessment of the flow temperatures you will require and what the COP might be at various outdoor temperatures.


It sounds like you are now trying to work out whether a heat pump will cost less to run than a gas boiler in your house?

michaels

matt_drummer said:

If the install is good enough you will get the theoretical efficiency or better.

If the installation isn't designed to prevent defrosting and short cycling then you'll probably get lower efficiency.

A good idea of how much less could be gained by a knowledgeable person visiting your house to carry out an assessment and advise you accordingly.

If you know which heat pump you want (?), your heat loss and your total proposed radiator capacity it would be easy to make an educated assessment of the flow temperatures you will require and what the COP might be at various outdoor temperatures.


It sounds like you are now trying to work out whether a heat pump will cost less to run than a gas boiler in your house?

There are a lot of 'moving pieces':

1) I can choose to spend zero, 1500 or 3k on rads (excluding fitting) to end up with a flow temp at -3 of 52, 45 or 40 (my best estimates based on a room by room heat loss calc from Octopus)

2) I can choose to spend between 4k and 7.5k on a heat pump, probably the more I spend the better the modulation and the higher the efficiency

3) There is definitely a trade off between total spend and overall efficiency but then there is a question of payback period - is it worth spending 3k on rads rather than 1.5k to get a scop that is 0.5 better (assuming I can even calculate this scop impact accurately)

4) But perhaps there is also a trade off between rad spend and the heat pump spend - max out the rad spend and I might then manage with a cheaper heat pump - perhaps rather than spending nothing on rads and 7.5k on a HP I could spend 3k on rads and 4k on the heat pump.

5) Throughout I need to consider that the heat requirement is of order 18k kwh and the hot water requirement 8k kwh (which will have its own heat pump specific SCOP)

Annual Heating Electricity Costs
Rad Spend     £0k (52C)     £1.5k (45C)      £3k (40C)   Hot Water

£4k Heat Pump    £1000      £800                £600          £400 
£5.5k HP              £900        £700                £500          £350
£7k HP                 £750        £600                £450          £300

Made up numbers but this is what I would like to understand - for example between the max spend and min spend options I might save £650 energy costs a year at an investment cost of 6k of spend and there are lots of other possible options - even before I compare against staying with gas. But without being able to populate the table I find it hard to make a sensible decision.

matt_drummer

Wow.

I wouldn't consider any of it an investment, you're just installing a heating system.

You need new heating and at some point in the future you won't have a gas boiler.

The grant won't be available forever. Buy a gas boiler today and miss out on the grant later?

You'll get more out of bigger radiators than you will a costlier heat pump.

The heat pumps are all more or less the same, the difference in SCOPs will be small comparing like with like. Maybe the more expensive ones will last longer?

All that matters is what the heat pump is at its heart, is it a derated bigger heat pump? That's all you need to know.

You can't work anything out like you are trying to do, you don't even know what gas and electricity will cost over the next ten years.

Lots of people get comfort from the fact they are no longer burning anything to heat their home, the cost is a secondary factor within reason.

Replacing gas and oil with heat pumps is primarily for environmental reasons as opposed to being a sound financial investment.

Whatever you calculate today could be invalid at any point in the future.


If the money matters as much as the detail in your calculations would suggest, I would just get a gas boiler.

I find the attitude to cost slightly confusing in view of how much hot water you get through. I know that's your business and your choice but the two varying attitudes don't seem to fit? You are worrying over what is quite a small difference in costs over time but you are prepared to spend so much heating water?

I think you are guaranteed to be happy with a gas boiler and the level of `investment'

I think there is a strong chance you will be disappointed with a heat pump.

michaels

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).