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# Heat Pump Sizing?

• Posts: 3,842 Forumite
edited 27 June at 12:30PM
michaels said:
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?
If you know you need 50C flow temps at 0C to give 21C room temps, then you can calculate the exact heat output from the ratings of your current radiators. That gives you your heat loss at 0C. Then you can factor in the effect of changing a couple radiators. The larger the radiators, the lower you can run the flow temps to give the required output.
I haven't looked at the specs of anything other than the Samsung unit I have, but I doubt (m)any will publish the minimum output the unit can modulate down to. The rule of thumb seems to be that modern compressors can modulate down to around a 1/4 of their rated output, so a 10kW unit may be able to modulate it's output down to around 2.5kW.
Note this is complicated by the fact than many manufactures use the exact same hardware for many units in their range, and just limit the output in the firmware - e.g, our Samsung 12kW is identical to their 16kW unit, only limited to 12kW in firmware. Thus we should apply the 1/4 rule of thumb rating to the 16kW figure, not the 12kW rating of the unit, giving 4kW minimum, not 3kW, in my case.
The best place to obtain data is probably from other owners - they should be able to tell you how much power the unit draws on minimum output - I know mine reports 800-900W input when it's just ticking over. Also, look on the open monitoring website, and you may be able to see real life data for your proposed unit and look at the minimum current draw.
My OH was concerned about noise output, but I do not see it as an issue. It's outside, and during periods of continuous operation doors and windows are closed (it's cold), so we cannot hear it indoors. During the summer, it's only on for 30mins max to reheat the water, and it's not that loud. Indoors, the noise of the pump(s) is far quieter than our previous oil boiler and pumps, so overall I assess it as being significantly quieter than what it has replaced. Currently I run mine in quiet mode which limits the power output to around 70% (thus reducing the noise output), and usefully keeps the max power draw down to around 3.3kW allowing the DHW cycle to be largely covered by our solar output, assuming the sun is out - bonus!

• Posts: 28,361 Forumite
edited 27 June at 1:42PM
Thanks

Funnily enough I was just pouring over the specs of the Samsung 12kw unit (is yours the Gen 6 or Gen 7?) - I think the 8kw would be too small - lots of info on the max output at different temps but nothing on the minimum.  Seems to be 59DB sound power.

Others that are catching my eye are the Viessmann 10kw unit and also a Trianco (anyone heard of them?) 5-15kw unit.  Probably paired with a 250l or 300l tank and possibly a 50l buffer.  Viessman seems to show output goes down to about 3-3.5kw at 15C, not finding that sort of info for the others.
I think....
• Posts: 28,361 Forumite
I am thinking I want to go for the following and would be grateful for any input:
• A weather comp curve with 55c at minus 10 and 35C at plus 15 (0.8 slope).  This gives about 50C at the -3 design reference temp.
• Rad changes to give the required room output at a delta T of 25 (probably about 5 rads out of 18 to be changed).
• A heat pump capable of 10kw of actual output at -3C at 50C flow and with as much modulation as possible so support minimal cycling at lower heat demand.
• A 250l or 300l hot water cylinder with a 50l buffer.
• Probably dual heating circulation pumps as the circuit is long and half is only 15mm
• I want the heat pump to be installed on the side flat roof even though this will require planning permission (no idea if it might be forthcoming) but it meets the MCS noise calcs comfortably as this puts it out of the way and requires a much shorter external pipe run (2 or 3 m rather than 10m)

I am wondering if this heat pump and this cylinder might be suitable, there is a viessmann that sounds even better but costs nearly twice as much:
Samsung Ehs 12Kw Heat Pump High Temp R290 Monobloc Ae120Cxydek/Eu (cityplumbing.co.uk)
Tempest 300L Indirect Heat Pump Cylinder With 50L Buffer (theheatpumpwarehouse.co.uk)
I think....
• Posts: 1,539 Forumite
That 12Kw Samsung is actually a software limited 16kW unit I think? It won't modulate down to what you want I suspect.

50c flow will be awful, you should be aiming for 40c or less, probably more like 35c

The Samsung controls are pretty terrible from what I have read.

Two circulation pumps will require hydraulic separation, best avoided if possible.

What do you want a buffer tank for?

A Viessman heat pump will be miles better although with your heat loss the larger Vaillant units are really efficient and would be most knowledgeable people's choice.

I would get advice from a respected installer though that has actually been to your home.

• Posts: 4,692 Forumite
My heat pump was installed for 50 C flow (at -3 C) in late 2020.  It was replacing an oil boiler so I had to fit the new radiators where the old ones had been.  With hindsight I should have tried harder to go for a lower flow temperature but I didn't know enough about heat pumps then.  And in some rooms there really wasn't the space.  Luckily the heat loss from my house had been over-estimated so it turns out that I only need about 44 C flow at -3 C to keep the house warm enough.  My system has a 4-port buffer but with hindsight I would opt for a volumizer if I was starting again.  The heat pump warranty requires that a minimum water volume is maintained, 50 l I think.
Reed
• Posts: 28,361 Forumite
edited 11 July at 9:30PM
That 12Kw Samsung is actually a software limited 16kW unit I think? It won't modulate down to what you want I suspect.

50c flow will be awful, you should be aiming for 40c or less, probably more like 35c

The Samsung controls are pretty terrible from what I have read.

Two circulation pumps will require hydraulic separation, best avoided if possible.

What do you want a buffer tank for?

A Viessman heat pump will be miles better although with your heat loss the larger Vaillant units are really efficient and would be most knowledgeable people's choice.

I would get advice from a respected installer though that has actually been to your home.

There is no way I am getting big enough rads at deltaT 15 or deltaT 20 - 25 is pushing it.  I am used to weather comp where the flow temp goes up a the outside temp goes down, currently it needs about 52C at zero with a slope of 1.2 so about 55/56 at -3 so I think even with rad upgrades will likely need somewhere between 45 and 50 at -3.

I know the COP will be poor at those sort of temps but that is not that often each winter.

Yes it is the 16kw unit but the rated sound power of the 12 is 60db vs 65db when it is unlocked to 16 output.  The samsung controls support a heating curve plus an on off via a smart controller.

I understand a buffer is useful in avoiding short cycling?

Adding two circulation pumps is to help achieve a suitable flow rate given the pipe bore restrictions in part of the circuit.

I am struggling to find any technical data regarding the Vaillant heat pumps in terms of max outputs, cop etc by air vs water temp

I have had 6 'expert' companies out and received quotes for 6 very different solutions....
I think....
• Posts: 4,692 Forumite
michaels said:

I know the COP will be poor at those sort of temps but that is not that often each winter.

That's not really the point.  What you want is a good average over the year, a good SCOP.  If you can use 45 C flow instead of 50 C flow that will improve your SCOP at all the outside temperatures you encounter throughout the year.  40 C flow would be better still but harder to achieve with a retrofit, 35 C even better and even harder etc etc.
Reed
• Posts: 1,539 Forumite
edited 12 July at 7:00AM
michaels said:
That 12Kw Samsung is actually a software limited 16kW unit I think? It won't modulate down to what you want I suspect.

50c flow will be awful, you should be aiming for 40c or less, probably more like 35c

The Samsung controls are pretty terrible from what I have read.

Two circulation pumps will require hydraulic separation, best avoided if possible.

What do you want a buffer tank for?

A Viessman heat pump will be miles better although with your heat loss the larger Vaillant units are really efficient and would be most knowledgeable people's choice.

I would get advice from a respected installer though that has actually been to your home.

There is no way I am getting big enough rads at deltaT 15 or deltaT 20 - 25 is pushing it.  I am used to weather comp where the flow temp goes up a the outside temp goes down, currently it needs about 52C at zero with a slope of 1.2 so about 55/56 at -3 so I think even with rad upgrades will likely need somewhere between 45 and 50 at -3.

I know the COP will be poor at those sort of temps but that is not that often each winter.

Yes it is the 16kw unit but the rated sound power of the 12 is 60db vs 65db when it is unlocked to 16 output.  The samsung controls support a heating curve plus an on off via a smart controller.

I understand a buffer is useful in avoiding short cycling?

Adding two circulation pumps is to help achieve a suitable flow rate given the pipe bore restrictions in part of the circuit.

I am struggling to find any technical data regarding the Vaillant heat pumps in terms of max outputs, cop etc by air vs water temp

I have had 6 'expert' companies out and received quotes for 6 very different solutions....
You seem to be looking for a heat pump that performs like your current heat source.

The best way to avoid short cycling is to fit the correct sized heat pump and emitters running at the lowest possible flow temperatures.

Your COP will be poor all of the time compared to the same heat pump properly installed and operated as you will always be running at higher flow temperatures.

I have only heard negative comments about Samsung controls, they are not as good as others is all I hear.

I mentioned that it is a 16kW heat pump, not because of noise, but because of modulation.

The 12kW won't modulate down to a very low output, I believe about 4.5kW?

Samsung heat pumps like to work at high flow rates so I believe, you already sound like this could be an issue?

Two circulation pumps for 10kW of heat is excessive.

You have been directed here before, there are loads of monitored Vaillant heat pumps you can look at!

https://heatpumpmonitor.org/

No heat pump will work very well if you insist on those flow temperatures.

You seem to know best so I can't really understand what input you are searching for?
• Posts: 28,361 Forumite
edited 12 July at 2:51PM
michaels said:
That 12Kw Samsung is actually a software limited 16kW unit I think? It won't modulate down to what you want I suspect.

50c flow will be awful, you should be aiming for 40c or less, probably more like 35c

The Samsung controls are pretty terrible from what I have read.

Two circulation pumps will require hydraulic separation, best avoided if possible.

What do you want a buffer tank for?

A Viessman heat pump will be miles better although with your heat loss the larger Vaillant units are really efficient and would be most knowledgeable people's choice.

I would get advice from a respected installer though that has actually been to your home.

There is no way I am getting big enough rads at deltaT 15 or deltaT 20 - 25 is pushing it.  I am used to weather comp where the flow temp goes up a the outside temp goes down, currently it needs about 52C at zero with a slope of 1.2 so about 55/56 at -3 so I think even with rad upgrades will likely need somewhere between 45 and 50 at -3.

I know the COP will be poor at those sort of temps but that is not that often each winter.

Yes it is the 16kw unit but the rated sound power of the 12 is 60db vs 65db when it is unlocked to 16 output.  The samsung controls support a heating curve plus an on off via a smart controller.

I understand a buffer is useful in avoiding short cycling?

Adding two circulation pumps is to help achieve a suitable flow rate given the pipe bore restrictions in part of the circuit.

I am struggling to find any technical data regarding the Vaillant heat pumps in terms of max outputs, cop etc by air vs water temp

I have had 6 'expert' companies out and received quotes for 6 very different solutions....
You seem to be looking for a heat pump that performs like your current heat source.

The best way to avoid short cycling is to fit the correct sized heat pump and emitters running at the lowest possible flow temperatures.

Your COP will be poor all of the time compared to the same heat pump properly installed and operated as you will always be running at higher flow temperatures.

I have only heard negative comments about Samsung controls, they are not as good as others is all I hear.

I mentioned that it is a 16kW heat pump, not because of noise, but because of modulation.

The 12kW won't modulate down to a very low output, I believe about 4.5kW?

Samsung heat pumps like to work at high flow rates so I believe, you already sound like this could be an issue?

Two circulation pumps for 10kW of heat is excessive.

You have been directed here before, there are loads of monitored Vaillant heat pumps you can look at!

https://heatpumpmonitor.org/

No heat pump will work very well if you insist on those flow temperatures.

You seem to know best so I can't really understand what input you are searching for?
Thanks for the link, I have been pouring over the outputs and the discussion there.  I have not been able to find much it he way of detailed documentation for the vaillants which makes them a bit of an unknown quantity.  Price wise they seem to be half way between the Viessman and the Samsung, one question is the payback period for the extra 1.5k / 3k.  One advantage of the Samsung is that it is not a double height unit.  It would be useful if you could link some of the issues with the Samsung controls and how they impact performance. (Edit: Found this with some familiar names - Struggling with performance of Samsung Heat Pump - Hardware / Heatpump - OpenEnergyMonitor Community, OP was having issues with weather comp (water laws) and short cycling as maximum modulation was down to 4kw and the weather comp was being very aggressive with maintaining the flow temp via frequent on/offs)

Just looking at one room as an example, currently  have a vertical radiator - 450 x 1800 double panel no fins.  To achieve the required output at deltaT25 I would need to replace this with a 600 x 2000 radiator, there is nothing I could fit that would support a deltaT20 or deltaT15 (40 or 35 flow temp).  Are you suggesting that if I can't get such a low flow temp then getting a heat pump would likely be uneconomic?  I would rather not go back to gas but if that is the logical choice then I am not wedded to a heat pump solution.

IN terms of circulation, the options seem to be using additonal pumps to ensure a good circulation or doing some major work to split into two shorter loop zones.  Because of our floors this would have to be done to the upstairs from below by removing ceilings so I am loathe to do this if an additional pump would solve the problem.

Apologies if you think I am not listening to the really helpful input here, if I knew what to do then I would not be asking.  Having had these different 'expert' quotes with so many different suggested solutions it is hard to know which if any are realistic.
I think....
• Posts: 4,692 Forumite
Gas and electricity prices vary but if you divide the price you pay per kWh for electricity by the price you pay per kWh for gas then that is the SCOP you will need to make the heat pump slightly cheaper to run (since your gas boiler is not 100% efficient).  If you cannot operate a heat pump at conditions that would achieve that SCOP then a heat pump is likely to be uneconomic, at current energy prices.

But the chances are you will be able to achieve that SCOP or better quite easily.  After that it comes down to a trade-off between running costs and the inconvenience/expense of squeezing in suitably large radiators.
Reed

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