Heat Pump Sizing?

matt_drummer

DougMLancs said:

It also needs to get MCS sign off for the grant though and they won’t do that if it looks way oversized on paper. 

Yes, exactly, you are correct.

All logic says it is oversized if the heat loss is only, say., 11kW

The reality is though that the 11kW Daikin only delivers 11kW when you don't need it.

This is the problem with Octopus, they don't fully understand how these heat pumps behave in real life.

matt_drummer

koru said:

matt_drummer said:

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

I have seen someone else post about Octopus only being willing to go up to the 11kW Daikin. (Might be on a different forum, I can't remember.) 

Perhaps this is because they don't understand the point you make. Perhaps they are just put off by the higher cost of buying the bigger models.

Maybe, but Octopus installers I have met have confirmed they have fitted 14 and 16kW models.

They also confirmed they are the same internally.

All four in the range look identical on the outside too.

Octopus don't pay very much for these compared to the retail prices you see.

There is very little cost difference between the four models, and for Daikin, no difference at all as they are the same heat pump.

michaels

matt_drummer said:

DougMLancs said:

It also needs to get MCS sign off for the grant though and they won’t do that if it looks way oversized on paper. 

Yes, exactly, you are correct.

All logic says it is oversized if the heat loss is only, say., 11kW

The reality is though that the 11kW Daikin only delivers 11kW when you don't need it.

This is the problem with Octopus, they don't fully understand how these heat pumps behave in real life.

They were trying to under report our heat demand so that we were low enough to qualify for the 11.2kw which is what made me think that was the biggest they would go to.

Reed_Richards

michaels said:

Based on gas usage, it appears my gas boiler happily modulates down to 2kw and of course adjusts the flow temp down into the 20s C to reflect the outdoor temp and thus heat demand.  It would seem a step back to move to a heat pump that can't do this and has to pulse hot and cold rads to maintain an indoor temp when I thought the whole point of efficient heat pump operation was to have a constant flow and flow temp....

You're right; it is disappointing feature of heat pumps that they don't have the modulation range of a gas boiler.

The MCS specification is that your heat pump
 will provide enough heat for temperatures encountered 99.6% of the time so in principle you could need supplementary heating during a really cold spell.

Do be careful to draw a distinction between cycling and short cycling.  Cycling is inevitable when it's moderately warm outside but you want it a bit warmer inside, even with your gas boiler.  A typical oil boiler will spend its whole life cycling.  It's up to your control system to ensure that cycles are suitably long.

matt_drummer

michaels said:

matt_drummer said:

DougMLancs said:

It also needs to get MCS sign off for the grant though and they won’t do that if it looks way oversized on paper. 

Yes, exactly, you are correct.

All logic says it is oversized if the heat loss is only, say., 11kW

The reality is though that the 11kW Daikin only delivers 11kW when you don't need it.

This is the problem with Octopus, they don't fully understand how these heat pumps behave in real life.

They were trying to under report our heat demand so that we were low enough to qualify for the 11.2kw which is what made me think that was the biggest they would go to.

Who was trying to under report?

My experience and understanding of Octopus surveyors is that they know very little about heat pumps and just gather data to complete the heat loss calculation on their tablet.

What is this 11.2kW heat pump?

Daikin don't offer an 11.2kW

The 11kW is actually a 10.6kW at best and the 14kW is actually a 12kW

Have Octopus actually quoted you?

I may be mistaken but I thought you wanted it installed on a flat roof and Octopus won't do it, or is that somebody else?

MP1995

It might be a good time ro realise Octopus designed and installed ASHP are not going to reach the dizzy SCOP heights or a more expensive install unless the customer is clued up.

That said they can probably still achieve a SCOP of near 4 so with the right smart meter tariff breaking even with a gas system is possible.

We achieve a SCOP of 4.3 with our 10 year old ASHP and it's getting more efficient each year as we gradually dilute the anti freeze. As during cikd months it's 24/7 so no chance of freezing up.

michaels

So more questions on sizing - it seems that the peak output which is often in the model name is not that relevant to sizing as it is done at +7c or similar whereas what matters is peak output at -3/-4C

Then there is the question re 'oversizing' and what impact it might have on efficiency.  I assumed that a big heat pump modulated down to its lowest output would be pretty much as efficient as it is running at a higher output but I have just been told that this is not the case.

I still think I need a heat pump that can produce at least 9kw at -4 and possibly more given we need about 20kwh+ per day of hot water on top of our heating demand, not all heated overnight.

I am tempted by the Viessmann '16kw' unit that seems to do just over 12kw at -3 and add in the integrated 190l cylinder.  Currently we do just fine with a 180l cylinder at 60C supported by a 19kw boiler - would we have the same success using a similar size cylinder but supported by a 12kw (smallest max) heat pump?

NedS

You will need a heat pump with an output that matches your heat loss at the design criteria. For me, that was 7.4kW at -1.9C (it will obviously depend on the heat loss of your property and where is it located). Anything less and you will not be able to heat your property to the desired temp on the coldest days in winter.

The flip side is that if the heat pump is oversized, then it becomes less efficient to run in the milder spring and autumn months (and you see far more days like this than days at -4C)

The holy grail is to run your heat pump constantly at as low a flow temp (LWT) as possible - that is the most efficient way to run, with the output exactly matching the heat loss of the property (on a Weather Compensation curve), maintaining a steady design temp.

My ECO4 scheme insisted on a 12kW Samsung. The minimum it can regulate down to is around 4-4.5kW output, drawing around 0.9-1kW input, so in spring/autumn it's 4.5kW of heat or nothing (on or off). As my house probably only required maybe 2.5kW of heat when it's 15C outside, I cannot run my heat pump constantly at a low flow temp (say 30C), and instead have to run it for a few hours at a higher flow temp (33-34C is the lowest I can achieve), heat the house up, turn it off for a few hours, let the house cool down, rinse and repeat - which is not very efficient and does not allow us to maintain a steady constant comfortable temperature in spring/autumn.

Most heat pumps should modulate down to around 1/4 of their rated output. My Samsung 12kW is actually a 16kW unit limited in software, hence will only modulate down to around 4kW minimum.

So there are two sides to the coin - you need an output high enough to match your rated heat loss at design temps, but also a unit that will ideally modulate down to the heat output you require on the mildest of spring/autumn days where you would still like the heating on. It's a difficult balance, and you need radiators large enough that can actually dissipate that heat at as low flow temps as possible (hence why the lowest flow temps we can achieve is 33-34C, as that matches our radiators ability to emit that 4-4.5kW of heat output the heat pump is producing.

If you can find a heat pump that will give you the desired output at design temp AND will modulate down to allow low flow temps in spring/autumn AND you have large enough radiators to emit that heat at low flow temps (say 30C), then you have the basis for a potentially very efficient installation.

To give you some context on DHW, we have a 170L tank, and my 12kW unit will reheat to 48C from 32C in under 30mins in summer, from a cold start. I'd expect it to be quicker in winter as the circulating water is already at radiator flow temps so does not need heating from cold. You should be able to reheat a 190L tank in 30mins with a 12kW ASHP (not to 60C though). If your property is losing a significant amount of heat in 30mins whilst the heat pump is doing DHW, then you probably need to look at improving the insulation rather than buying a bigger heat pump.

Reed_Richards

NedS said:

... I cannot run my heat pump constantly at a low flow temp (say 30C), and instead have to run it for a few hours at a higher flow temp (33-34C is the lowest I can achieve), heat the house up, turn it off for a few hours, let the house cool down, rinse and repeat - which is not very efficient and does not allow us to maintain a 

The cycle time should be set by the controller.  I have a third party controller (Drayton Wiser) and when I tell it it is controlling a heat pump it limits the number of cycles per hour to 3.  If the leaving water temperature gets higher than specified the heat pump turns itself off, the water cools but the controller won't allow it to turn on again until 20 minutes after the start of the previous cycle.  This prevents both frequent short cycling and the need for a manually controlled long cycle, as done by @NedS .  And the water in the system cools much faster than the house so I don't perceive any room temperature fluctuations.     

michaels

Thanks again, looks like a bigger than needed heat pump to allow a smaller cylinder is not the answer due to the limited modulation issue.

I was looking at this resource and assuming the grey area on the chart on page 45 gives the range of output for a given outdoor temp then I can see that the 10kw model will give up to 10kw at -5(probably about what we need) and then at warmer temps will go as low as about 3kw at 15c.

Do other manufacturers supply this sort of level of detail re the performance of their heat pumps?

We currently have gas weather comp so I know that we currently need a flow temp of about 50c at zero degrees outside to give 21C room temp with a slope of about 1.3 (so flow temp of 37 at 10C).  We would probably upsize some rads that might knock a couple of C off this to lets say we want a max output of 10kw at -5 with a 55 flow rate and 42C at 5C, 29C at 15C.  Our boiler can do this without cyclin until the heat demand goes below about 2.5kw, are there heatpumps that can do the same and how can we find out?

Th Viessman kit is expensive but also is assessed at 56DB sound power, whereas most other kits seem to be 59 or 60db.

Thoughts?