Optimising a heat pump

FreeBear

Reed_Richards said:

Reed_Richards said:

70sbudgie said:

I am a little confused by the comment

Heat pumps don't have the modulation range of a gas boiler

I thought that was the whole point of the weather compensation - to modulate the flow temperature. And I thought gas boilers rarely modulate, they switch on and off. 

What words do I look for to find out the modulation range of my proposed HP? It is an Ecodan 11.2kW. 

You're confusing gas boilers, which typically have a wide modulation range, with oil boilers, which are rarely capable of modulation and do indeed just switch on and off.  But the modulation I am talking about is the power that they draw.  The lower limit for any heat pump can be hard to discover.  In the spec sheet of your proposed Ecodan 11.2 kW it mentions a power draw of 1.88 kW giving an output of 6 kW under certain operating conditions but there is no indication of whether that is a minimum.   

The lower limit for any heat pump can be hard to discover.  In the spec sheet of your proposed Ecodan 11.2 kW it mentions a power draw of 1.88 kW giving an output of 6 kW under certain operating conditions but there is no indication of whether that is a minimum.   

I think you are looking at the basic spec sheet for a 6kW model. This one for the PUZ-WM112 quotes 3.73kW input for 11.2kW out.

Had to do some digging, but came across this little publication -> https://midsummerwholesale.co.uk/pdfs/Ecodan_ATW_Databook_Vol.6_.0_.pdf

Page 83 gives a range of outputs dependent on load & water temperature. For the PUZ-WM112, minimum is 2.8kW @ 35°C for a COP of 3.0 - First time I've come across data like this for any heat pump.

Reed_Richards

FreeBear said:

I think you are looking at the basic spec sheet for a 6kW model. This one for the PUZ-WM112 quotes 3.73kW input for 11.2kW out.

I think you are right; sorry.  I started out looking at the right model but must have followed a bum link.  

NedS

FreeBear said:

Reed_Richards said:

Reed_Richards said:

70sbudgie said:

I am a little confused by the comment

Heat pumps don't have the modulation range of a gas boiler

I thought that was the whole point of the weather compensation - to modulate the flow temperature. And I thought gas boilers rarely modulate, they switch on and off. 

What words do I look for to find out the modulation range of my proposed HP? It is an Ecodan 11.2kW. 

You're confusing gas boilers, which typically have a wide modulation range, with oil boilers, which are rarely capable of modulation and do indeed just switch on and off.  But the modulation I am talking about is the power that they draw.  The lower limit for any heat pump can be hard to discover.  In the spec sheet of your proposed Ecodan 11.2 kW it mentions a power draw of 1.88 kW giving an output of 6 kW under certain operating conditions but there is no indication of whether that is a minimum.   

The lower limit for any heat pump can be hard to discover.  In the spec sheet of your proposed Ecodan 11.2 kW it mentions a power draw of 1.88 kW giving an output of 6 kW under certain operating conditions but there is no indication of whether that is a minimum.   

I think you are looking at the basic spec sheet for a 6kW model. This one for the PUZ-WM112 quotes 3.73kW input for 11.2kW out.

Had to do some digging, but came across this little publication -> https://midsummerwholesale.co.uk/pdfs/Ecodan_ATW_Databook_Vol.6_.0_.pdf

Page 83 gives a range of outputs dependent on load & water temperature. For the PUZ-WM112, minimum is 2.8kW @ 35°C for a COP of 3.0 - First time I've come across data like this for any heat pump.

Sounds right - exactly 25% of the rated 11.2kW output, and at a COP of 3.0 will require electrical input of around 933W, so one could reasonably expect this unit to tick over at minimum drawing slightly under 1kW (very similar to my 12kW Samsung unit)

Edit: Actually, I think it's the data table on page 91 that you need to be looking at, which suggests a minimum output of 4.0kW (COP of 4.45) for a flow temp of 35C at 7-12C ambient temps (shame the 30C flow temp data is missing). The COP of 3.0 above is not realistic/too low for 35C@7C which tells me we are looking at the wrong table. Still, nice the minimum values are given as you say.

FreeBear

NedS said: Edit: Actually, I think it's the data table on page 91 that you need to be looking at, which suggests a minimum output of 4.0kW (COP of 4.45) for a flow temp of 35C at 7-12C ambient temps (shame the 30C flow temp data is missing). The COP of 3.0 above is not realistic/too low for 35C@7C which tells me we are looking at the wrong table. Still, nice the minimum values are given as you say.

The table on p.91 is definitely more informative. I only skimmed the document and stopped at the first page that gave min/max figures. Shame other manufacturers don't provide such detailed data..

NedS

FreeBear said:

NedS said: Edit: Actually, I think it's the data table on page 91 that you need to be looking at, which suggests a minimum output of 4.0kW (COP of 4.45) for a flow temp of 35C at 7-12C ambient temps (shame the 30C flow temp data is missing). The COP of 3.0 above is not realistic/too low for 35C@7C which tells me we are looking at the wrong table. Still, nice the minimum values are given as you say.

The table on p.91 is definitely more informative. I only skimmed the document and stopped at the first page that gave min/max figures. Shame other manufacturers don't provide such detailed data..

The first company that quoted me used Midea ASHPs, and when I looked at their documentation, they quote minimum values too, but as you say it is far from commonplace.

General rule of thumb of 25% is about right. What I don't know is if that Ecodan is genuinely an 11.2kW unit, or if it's actually the same hardware unit as their 14kW model that has been software limited to 11.2kW - I suspect it is as commonly manufacturers will only have two (or maybe 3) hardware units in their range (high and low powered), and will then software limit each down to add more 'models' to their range to fill in the gaps in the range. 4kW minimum output still seems high for an 11kW unit, so I strongly suspect it's actually a rebadged and limited "14kW" unit (which from the datasheet is actually capable of 16kW output). So if it is really a limited 16kW unit, then 25% giving 4kW output at minimum is bang on where I'd expect it to be.

Outside of that, there really isn't any hidden magic here. Heat pumps can modulate and run lower, but the performance/efficiency falls off a cliff, hence why the manufacturers limit them to 25%. That's kind of an industry standard, and you wouldn't want to be running them at 10% as you'd get less heat out per unit of power in. It's just down to the physics of the compressor, and all heat pumps are fundamentally of the same design. The important thing to understand is what it's 25% of - as in the case of this Ecodan, it's more likely 25% of 16kW, not 25% of the 11.2kW sticker value so it appears this Ecodan can only modulate to 35% of it's software limited sticker value output capacity. It's exactly the same for my 12kW Samsung gen6 unit - it's actually a 16kW unit that's limited to 12kW max, and can modulate down to around 4kW minimum (or ~3.6kW at a push if I give up a bit of efficiency/COP).

QrizB

FreeBear said:

NedS said: 4. Pay attention to flow rates, they need to be significantly higher than for gas boilers (typically 20-30L/min). Large pipe sizes help and also help increase volume (point 3).

Are you sure on the flow rate ?

I've been running my gas boiler at low temperatures (at one point, down to 35°C) and flow rate is just 4l/m.

You'll know this, and I'm pretty sure NedS will to, but for a given delta-t your flow rate is directly proportional to the heat delivered.

One litre per second at 1C delta-t is ~4.2kW.

So 4 litres per minute (1/15th of a litre per second) at 5C delta-t is 5/15ths of 4.2kW, 1.4kW.

Or, in NedS's case, 30lpm (1/2 lps) at 5C delta-t would be 5/2 of 4.2kW, 10.5kW.

NedS

QrizB said:

FreeBear said:

NedS said: 4. Pay attention to flow rates, they need to be significantly higher than for gas boilers (typically 20-30L/min). Large pipe sizes help and also help increase volume (point 3).

Are you sure on the flow rate ?

I've been running my gas boiler at low temperatures (at one point, down to 35°C) and flow rate is just 4l/m.

You'll know this, and I'm pretty sure NedS will to, but for a given delta-t your flow rate is directly proportional to the heat delivered.

One litre per second at 1C delta-t is ~4.2kW.

So 4 litres per minute (1/15th of a litre per second) at 5C delta-t is 5/15ths of 4.2kW, 1.4kW.

Or, in NedS's case, 30lpm (1/2 lps) at 5C delta-t would be 5/2 of 4.2kW, 10.5kW.

Indeed

I like John Cantor's "What flow rate do I need" calculator (below) so I don't have to remember or work this stuff out:

https://heatpumps.co.uk/calculators/

Reed_Richards

QrizB said:

You'll know this, and I'm pretty sure NedS will to, but for a given delta-t your flow rate is directly proportional to the heat delivered.

One litre per second at 1C delta-t is ~4.2kW.

So 4 litres per minute (1/15th of a litre per second) at 5C delta-t is 5/15ths of 4.2kW, 1.4kW.

Or, in NedS's case, 30lpm (1/2 lps) at 5C delta-t would be 5/2 of 4.2kW, 10.5kW.

For the sake of absolute clarity, delta-t in this context is the heat drop across the radiator, the difference between the temperatures of the flow-in and return-out pipes.  That's what you mean, isn't it @QrizB?  Because delta-t is more commonly used to refer to the difference between the average radiator temperature (halfway between in and out) and the room temperature.

My other comment is in the contest of a radiator that whilst this may be scientifically correct, you can't actually change the flow rate and maintain a constant delta-t. 

QrizB

Yes RR, the temperature difference across whatever you are measuring. In my example, I was thinking of the input and output temperatures of a heat pump but the same would apply to the flow and return temperatures from a radiator or HW tank. Or even eg. an instantaneous electric shower.

70sbudgie

For me, that is what makes getting my head around HP so challenging - there are multiple points where a delta T exists. It can be used to describe how much energy is available to input into the system (1 outside air temperature Vs out flow temp), how much energy is required to be taken out of the system (2 room temp Vs emitter in temp), how much energy a particular emitter can emit (3 in and out flow temperatures) and then how much energy needs to be put into the system (4 flow and return temperatures). 

The heat loss survey provides information to calculate 3, so with some assumptions on 1 and 2, the HP can be sized so that 4 is appropriate.

The SCOP is a measure of how all these fit together. And if you change any of the assumptions, (like a lower target room temperature, or a warmer / cooler winter) then everything else changes. 

As I am still at the design stage, I am trying to work out how increasing the emitter outputs and operating at a lower target room temperature (I like the house at 18°C, rather than the 21°C the installer has used) will affect the estimated electricity consumption. (I need to persuade the other half that a HP now is a worthwhile investment, rather than waiting for the gas boiler to die)