Heat Pump Sizing?

michaels

Octopus suggested their biggest unit, 11.2kw but is that the right size

I currently have a modulating boiler with weather comp and a smart meter so I know that the boiler runs at a steady 8.2kw (gas input) to maintain the house temp at 20c when the outdoor temp is 0 degrees (flow temp low 50s C).  It uses a fair bit less overnight when we reduce the temp to 16C but I suspect we might want a steady 10kw if it got to say -5 and we wanted an indoor temp of 22C

On top of that obviously we also need to heat water - up to about 25kwh per day (based on our immersion usage) with about half overnight for morning showers and the other half at some point in the day for the evening showers.

So my question is, is a nominal 11kw unit going to be able to provide sufficient output when we need most energy when it is coldest and thus the COP is also at its lowest?

[I know the sizing based on the EPC and heat loss calcs but would have thought our lived experience is more relevant than the theoretical loss]

MP1995

So what does you gas boiler do when the temperature outside goes below 0 for a sustained period of time? Do you not let it use more power and slowly lose internal heat and tough it out or do you let it use more power?

shinytop

@michaels, 

First of all, make sure that your chosen ASHP can deliver your required output at the temperature it's going to be needed. Ie, can it deliver say 10kW @ -5 deg rather than at +5.  Some (not all) manufacturers are a bit misleading in this area.

Also, at ambient temps around zero, most current  ASHPs will be stopping and starting to defrost.  This (IME) can reduce output by 15-20% because the set flow temp is never achieved throughout the system (your radiators just don't get hot enough). Finally, even on a cold day, 25kWh of HW is going to use another big chunk of the overall heat output and the defrosting effect gets worse the higher the flow temp.

I think an 11.2kW ASHP might struggle on its own. 

I assume your gas boiler has a much bigger output that 11kW. I also assume your gas bills are quite high!

     

matt_drummer

If you are asking about an Octopus Daikin installation then the 11kW unit will not be enough.

Depending upon weather conditions and flow temperature that 11kW Daikin heat pump will spend 30% of it's time defrosting. You may see as little as 70% of its rated output when you need it most, maybe even a little less.

Then there is also the fact that the 9, 11, 14 and 16kW Daikin heat pumps are all virtually the same mechanically, the two larger outputs have a slightly larger evaporator. They are all 16kW heat pumps but the the three lower outputs are derated using the software to limited fan speed.

There is no point getting anything other than the 16kW unit if you choose Daikin.

MP1995

Good info there from @matt_drummer

smallblueplanet

matt_drummer said:

If you are asking about an Octopus Daikin installation then the 11kW unit will not be enough.

Depending upon weather conditions and flow temperature that 11kW Daikin heat pump will spend 30% of it's time defrosting. You may see as little as 70% of its rated output when you need it most, maybe even a little less.

Then there is also the fact that the 9, 11, 14 and 16kW Daikin heat pumps are all virtually the same mechanically, the two larger outputs have a slightly larger evaporator. They are all 16kW heat pumps but the the three lower outputs are derated using the software to limited fan speed.

There is no point getting anything other than the 16kW unit if you choose Daikin.

Would this hold true with the 4, 6, and 8kW Daikins?

matt_drummer

smallblueplanet said:

matt_drummer said:

If you are asking about an Octopus Daikin installation then the 11kW unit will not be enough.

Depending upon weather conditions and flow temperature that 11kW Daikin heat pump will spend 30% of it's time defrosting. You may see as little as 70% of its rated output when you need it most, maybe even a little less.

Then there is also the fact that the 9, 11, 14 and 16kW Daikin heat pumps are all virtually the same mechanically, the two larger outputs have a slightly larger evaporator. They are all 16kW heat pumps but the the three lower outputs are derated using the software to limited fan speed.

There is no point getting anything other than the 16kW unit if you choose Daikin.

Would this hold true with the 4, 6, and 8kW Daikins?

In my experience, yes.

One thing that gives it away is that the heat pumps in both ranges heat DHW at the same power.

That is, the 4kW Daikin generates 7 to 8KW of output when doing DHW, the same as the 6kW and 8kW units.

It can only do this as the compressor and pump are the same size.

The 4kW and 6kW are just software restricted in space heating.

Unless anybody knows different I wouldn't get anything other than the larger units from either range.

smallblueplanet

matt_drummer said:

smallblueplanet said:

matt_drummer said:

If you are asking about an Octopus Daikin installation then the 11kW unit will not be enough.

Depending upon weather conditions and flow temperature that 11kW Daikin heat pump will spend 30% of it's time defrosting. You may see as little as 70% of its rated output when you need it most, maybe even a little less.

Then there is also the fact that the 9, 11, 14 and 16kW Daikin heat pumps are all virtually the same mechanically, the two larger outputs have a slightly larger evaporator. They are all 16kW heat pumps but the the three lower outputs are derated using the software to limited fan speed.

There is no point getting anything other than the 16kW unit if you choose Daikin.

Would this hold true with the 4, 6, and 8kW Daikins?

In my experience, yes.

One thing that gives it away is that the heat pumps in both ranges heat DHW at the same power.

That is, the 4kW Daikin generates 7 to 8KW of output when doing DHW, the same as the 6kW and 8kW units.

It can only do this as the compressor and pump are the same size.

The 4kW and 6kW are just software restricted in space heating.

Unless anybody knows different I wouldn't get anything other than the larger units from either range.

What are the downsides to having a heat pump that is 'perhaps' either under or over specced? I presume under spec can't provide enough heat? Or is it in both cases to do with efficiency/scop of the pump?  

michaels

MP1995 said:

So what does you gas boiler do when the temperature outside goes below 0 for a sustained period of time? Do you not let it use more power and slowly lose internal heat and tough it out or do you let it use more power?

MP1995 said:

So what does you gas boiler do when the temperature outside goes below 0 for a sustained period of time? Do you not let it use more power and slowly lose internal heat and tough it out or do you let it use more power?

Thing is it would need to be below freezing and day time for me to know and that hasn't happened recently. Perhaps I can look at previous years weather records and gas usage but have not had a smart meter for that long.

Boiler max output is 19kw which we only see when heating hot water.

Don't think Octopus offer bigger than the 11kw unit due to their onset size fits all installation policy

saveallmymoney

smallblueplanet said:

What are the downsides to having a heat pump that is 'perhaps' either under or over specced? I presume under spec can't provide enough heat? Or is it in both cases to do with efficiency/scop of the pump?  

Undersized is actually likely to be more efficient at higher outside temperatures, as it won't cycle, but in very cold weather additional heat source might be needed.

Oversized will cycle and lose efficiency, although the way it heats the house will be more familiar to people who have a typical fossil fuel boiler setup so i can understand most installers going that direction.

matt_drummer

michaels said:

MP1995 said:

So what does you gas boiler do when the temperature outside goes below 0 for a sustained period of time? Do you not let it use more power and slowly lose internal heat and tough it out or do you let it use more power?

MP1995 said:

So what does you gas boiler do when the temperature outside goes below 0 for a sustained period of time? Do you not let it use more power and slowly lose internal heat and tough it out or do you let it use more power?

Thing is it would need to be below freezing and day time for me to know and that hasn't happened recently. Perhaps I can look at previous years weather records and gas usage but have not had a smart meter for that long.

Boiler max output is 19kw which we only see when heating hot water.

Don't think Octopus offer bigger than the 11kw unit due to their onset size fits all installation policy

I am pretty sure that Octopus would fit the 16kW, in fact I know they have because installers have told me so.

They may not offer it if they think it is too big, but that is the bit they don't understand about the heat pumps they fit.

A heat pump that is too big is a problem but not because of the maximum output, it is to do with how low they will go.

Common sense would tell you that if you need say 10kW of heat when it is -3c outside then when it is +12c you will need a lot less.

So you would think that all heat pumps would have a range of modulation in their output, and they do, some more than others. It is all to do with how low the compressor can go, a smaller compressor with a smaller maximum output should modulate to a lower output than a bigger compressor.

Focussing on the Daikin 9kW to 16kW range

The 16kW has a range of output between around 4kW and 16kW, that means it modulates down to about 25% of it's maximum output which is pretty good.

But, the whole range have the same compressor which means that all four heat pumps have a minimum output of about 4kW and the lower maximum outputs are achieved by clipping the top end of the compressors operation.

That means the 9kW goes from 4kW to 9Kw, the 11kW goes from 4kW to 11kW and the 14kW goes from 4kW to 14kW

They all have the same minimum output, there is no point in having anything other than the 16kW at your expected heat loss.

The problems come when they put the 9kW in a house with a heat loss of 5kW,

The issue goes away with larger heat losses.

The point is that you could get an 11kW and find that it is not quite enough and you wish you had the 14 or 16.

The only reason for getting the 11kW over the 14kW or 16kW is that it would modulate lower than the other two, but it doesn't as it is effectively the same heat pump.