Sunamp heat battery

[Triassic]

53 minutes ago, Moira Niedzwiecka said:

Hi Nick,

Thank you for your advice.

So, if I had the 6kw eDuel PCM58's would I still need any sort of a cylinder or IBoost?

I would not be able to increase the PV array as no further room on the roof & nowhere else to install.

I have attached the initial SAP worksheet.

We have built out of a different construction than originally planned when this was produced but values are largely the same, improved on in some areas.

We haven't had an airtightness test done yet but I am expecting well to be below 0.6.

Sorry to be so dim but not really sure what I am doing in this area & not sure I fully understand the SAP figures.

Sap Worksheet - Pickwell.pdf

Good to see someone elses SAP figures, it allows my the sanity check my own figures, thankfully they are similar.

[Triassic]

21 hours ago, Moira Niedzwiecka said:

I have a 4kw PV array & under floor heating installed on the ground floor only, about 90 sq mtrs.

No tank installed at the moment for DWH & underfloor heating.

What sunamp would I need if I went down this route & would I still require a cylinder of some sort.

I have an Iboost to connect but it is still in the box & could be returned if not needed.

There will be only 2 people living in the house.

An interesting question.

[Triassic]

It would be good is a Jeremy Harris style Sunamp selector spreadsheet to play with.

[Moira Niedzwiecka]

It looks like the offer is over now anyway. I couldn't find any info on prices of the PCM58eDuels on the Sunamp website.. Doesn't seem to be a search option  on the website to look at these.

Think I'll have to go with plan A, the UVC.

[Triassic]

@Moira Niedzwiecka just sent you the current price list. 

 

I agree the Sunamp website is more of a sales brochure rather than a technical website. Maybe Sunamp are missing a trick here. I wouldn’t buy anything without understanding the technicalities!

[Nickfromwales]

50 minutes ago, Moira Niedzwiecka said:

It looks like the offer is over now anyway. I couldn't find any info on prices of the PCM58eDuels on the Sunamp website.. Doesn't seem to be a search option  on the website to look at these.

Think I'll have to go with plan A, the UVC.

And use what to heat it ?

[Moira Niedzwiecka]

@Triassic Thank you.

I am not altogether sure I would understand the technicalities if it meant a firing squad, but, at least I could show it to someone who would.

There is really no information on the website other than generalities.

What about water pressure for showers?

Too many questions & too little information for me, teamed with my limited understanding I think counts it out as an option.

I am a bit wary as my current heating system is a bit 'alternative' & although it has served me very well for 20 years the company no longer produces or supports it & I can't find anyone to service or repair it. Still works but the switch has gone & I know have to light it by hand. Ok but just a pain.

Also, if I ever have to sell the new build it may cause an issue.

[Moira Niedzwiecka]

Just now, Nickfromwales said:

And use what to heat it ? 

Electricity.

I have the PV to offset some of the cost.

I am all electric for cooking & hot water at the moment & use an immersion heater.

I currently pay £41pcm for my electric on a standard tariff.

Used to have E7 but really wasn't worth it.

[Nickfromwales]

12 minutes ago, Moira Niedzwiecka said:

Electricity.

I have the PV to offset some of the cost.

I am all electric for cooking & hot water at the moment & use an immersion heater.

I currently pay £41pcm for my electric on a standard tariff.

Used to have E7 but really wasn't worth it.

And space heating ?

[Moira Niedzwiecka]

In current house oil. I use about 1200 litres a year.

In new house upstairs  a couple of electric towel rails.

I have wired for a couple of small panel heaters but will not install unless really needed.

Ground floor wet UFH & will run it off the UVC as & when needed.

I may also have a bio ethanol stove.

We will have MVHR of course but I am not expecting it to account for heating.

We have large glazed south facing patio doors & should benefit from some solar gain in winter when the sun is low.

Although maybe not as much as I had originally thought. If the triple glazed patio doors are closed the screed remains quite cool. Solar coating doing its job.

The house was surprisingly warm over the winter even though we did not have any floor insulation in, just the beam & block.

I am pleased to find that it has remained really quite cool over the last couple of weeks. The insulation doing its job well keeping heat out.

Should hopefully keep the heat as well in during the winter.

[PeterW]

@Moira NiedzwieckaYou can’t run UFH off a UVC - you need a thermal store to do that. 

 

Who has designed the system ..??

[Gone West]

6 hours ago, Dreadnaught said:

 

@PeterStarck, why do you say "in a PH larger than 130m2"?

 

That is around the size of our house and it has worked fine for us but it hasn't been tested for a long cold spell. Our house has a very low space heating requirement but a PH could have a higher requirement and still be a PH so taking that into account I wouldn't like to recommend it for a PH larger than ours.

[Jeremy Harris]

5 hours ago, Triassic said:

It would be good is a Jeremy Harris style Sunamp selector spreadsheet to play with.

 

If I get some spare time in the next week or two I'll have a look at knocking something up.  Won't be for a while though, as we've found ourselves in the midst of a few conflicting activities, like selling our old house, officially moving to the new one (at long last) and dealing with my late mother's estate (and all the associated family conflicts that's creating).

[Nickfromwales]

1 hour ago, JSHarris said:

 

If I get some spare time in the next week or two I'll have a look at knocking something up.  Won't be for a while though, as we've found ourselves in the midst of a few conflicting activities, like selling our old house, officially moving to the new one (at long last) and dealing with my late mother's estate (and all the associated family conflicts that's creating).

Another day at the office . Have a glass of red ....medicinal oil for the cogs . 

[Jeremy Harris]

Two glasses of a rather pleasant Pinot Grigio down so far, then hot chocolate, a choccy biccy and bed...

[Nickfromwales]

7 minutes ago, JSHarris said:

Two glasses of a rather pleasant Pinot Grigio down so far, then hot chocolate, a choccy biccy and bed...

Living the dream ?

[pdf27]

21 hours ago, Nickfromwales said:

More expensive SA units ( combined kW heat energy storage ) with far lower losses, 1/3 the physical size by comparison, and zero service / maintenance requirements.

Quite expensive if you need the PCM34, which you only need if you have an ASHP, but in a passive standard / other equally low energy dwelling and PV I am finding myself recommending SA heat storage and ditching the HP. The money saved on the HP goes towards more kW of SA storage and it is, to coin a phrase, "a very elegant solution".

 

You could certantly go for an ASHP to drive your UFH, with it also giving you DHW from an UVC. That would all be at a reasonable CoP and be "free" when the PV can support it, but then you also have the capital cost of the two cylinders, the glycol in the primary water circuit ( brine ), the costs when the glycol needs disposal and replacement, the lifespan of the ASHP and then the 'ugly factor' where do you put it and would you hear it, the latter consideration more so only when its providing DHW in reality as when driven softly these are quite quiet. 

I've got that going around in my head at the moment and as much as I like the simplicity of the concept I really struggle to make the logic work for a well insulated/low energy house:

• They've usually got a long time constant (decrement delay) with a significant heat capacity - so little or no benefit from adding volume from a buffer tank unless the heat pump can't take it. Provided the heat pump is a small one and the volume in the floor pipes is large enough, there shouldn't be any requirement for this - just connect them up directly as @jack did. That takes you down to one cylinder, and hopefully the mixing valve on the UFH goes as well improving the COP a bit.
• Vacuum insulation always makes me nervous with regards to long-term lifetime - this is a vastly better application of it than using it structurally within a building, but I spent 10 years of my life as a vacuum engineer looking for leaks so am suspicious of how long the insulation in an unpumped system will last, given that no matter what we did we always found more leaks. The manufacturers claim that when manufactured correctly the expected leakage will cause the U-value to double over 30 years - as I understand the process this isn't checkable however and relies on the build process being well controlled.
• Most (emphatically not all) people building a low energy house do so to reduce energy consumption rather than increase comfort. Delivering heat at a COP of 3 is a huge advantage here - to achieve the same performance you need to reduce energy loss by the same factor of 3, which is likely to be expensive.
• The PCM34 is in theory well suited to use as a hot water preheat tank. Problem is, you can do nearly the same thing with a shower heat exchanger at lower cost, which would lead to the heat delivered from the PCM34 being pretty minimal (when providing hot water to a shower the uplift to 34°C would be from ~25°C rather than 5°C - a 2/3rds reduction in power). That means the majority of hot water comes from another system - either a hot water tank (in which case why bother preheating) or a PCM58 unit run on resistance heat (minimum charge temperature 65°C - too much for a heat pump). Even if you only achieve a COP of 2 on the heat pump, that's a huge difference to the energy consumption for hot water and goes back to the question of what type of house you're trying to build.
• When they eventually commercialise the PCM43 material, I suspect all this will change. 43°C is hot enough for just about all DHW applications and eliminates the need for a TMV to deal with scalding, while the minimum charging temperature will be ~50°C which is comfortable for a heat pump.
• I think split heat pumps are probably underrated on here - finding an air conditioning company to make the connections can't be that hard (there are about a dozen within a 5 mile radius of me, so costs **should** be reasonable), you eliminate the need for glycol or equivalent and reduce the length of the primary hot water circuit to nearly zero with an associated reduction in losses.
• Heat pumps can do cooling - where the additional cost over an alternative solution is modest, that starts looking very attractive in weather like this.

Essentially my current view is that the current Sunamp PCM34/58 lineup doesn't play well with heat pumps, and that this is a critical flaw if trying to combine them with the design of a low energy consumption house (note: not the same as a low energy import house). With that in mind, my order of preference would be:

1. Heat pump with PCM43 Sunamp for hot water.
2. Heat pump with unvented cylinder for hot water.
3. Heat pump with standalone PCM58 Sunamp for hot water.

I would expect quite a significant performance gap between the three options. A Uniq rHW fitted with PCM43 would I suspect be an extremely good solution for a low energy house.

 

21 hours ago, Nickfromwales said:

It is also relative to the PCM, as in the PCM34 is temp / performance relative. For eg a PCM34 run off an ASHP that is delivering 36^oC will see less stored energy accumulate in the PCM, whereas a PCM34 heated by an ASHP at higher temps would see more storage and better kW yield. Hopefully I will be getting my promised training and accreditation soon so I will then hopefully be privy to the calculators that are needed to work that ? out. For now I remain blissfully ignorant and factor for the worst case  

Should be fairly straightforward - the PCM is a bit like water in that changing phase from solid to liquid needs a lot of heat while the material stays at the same temperature, while heating it up after that still takes a fair amount of heat. However, planning on heating it much above the phase change temperature turns it into what is essentially an expensive hot water cylinder.

[Jeremy Harris]

On 08/07/2018 at 08:41, pdf27 said:

I've got that going around in my head at the moment and as much as I like the simplicity of the concept I really struggle to make the logic work for a well insulated/low energy house:

• They've usually got a long time constant (decrement delay) with a significant heat capacity - so little or no benefit from adding volume from a buffer tank unless the heat pump can't take it. Provided the heat pump is a small one and the volume in the floor pipes is large enough, there shouldn't be any requirement for this - just connect them up directly as @jack did. That takes you down to one cylinder, and hopefully the mixing valve on the UFH goes as well improving the COP a bit.

 

 

The problem you run into is that there are no small (as in 2 to 3 kW output) ASHPs around, and even the smallest ones can't modulate down to the 200 to 500W heating requirement that's the typical average needed in winter for a well insulated and airtight house.  The rate of heat flow from a low temperature UFH system into the slab is just far too slow for the heat pump to deal with.

 

As a consequence, the heat pump gets the UFH up to temperature in a few minutes, then has to shut off.  Most have an in-built anti-short cycling feature; in our case ours only allows three starts per hour.  The heat capacity of the water in the UFH system is small, due to the low volume, so it quickly gives up it's heat to the slab, whilst the ASHP stays turned off until the end of the anti-short cycle timer.

 

Having a buffer completely fixes this.  Our 70 litre buffer sits at around 40 deg C, and the UFH runs at around 23 deg C.  The ASHP can charge the buffer and then shut down for a couple of hours whilst the buffer feeds the UFH demand.  This completely removes the short cycling problem, and also means that the ASHP should last longer, as it's the starts and stops that limit life more than the total hours run, I believe.

[Nickfromwales]

1 hour ago, pdf27 said:

I've got that going around in my head at the moment and as much as I like the simplicity of the concept I really struggle to make the logic work for a well insulated/low energy house:

• They've usually got a long time constant (decrement delay) with a significant heat capacity - so little or no benefit from adding volume from a buffer tank unless the heat pump can't take it. Provided the heat pump is a small one and the volume in the floor pipes is large enough, there shouldn't be any requirement for this - just connect them up directly as @jack did. That takes you down to one cylinder, and hopefully the mixing valve on the UFH goes as well improving the COP a bit.
• Vacuum insulation always makes me nervous with regards to long-term lifetime - this is a vastly better application of it than using it structurally within a building, but I spent 10 years of my life as a vacuum engineer looking for leaks so am suspicious of how long the insulation in an unpumped system will last, given that no matter what we did we always found more leaks. The manufacturers claim that when manufactured correctly the expected leakage will cause the U-value to double over 30 years - as I understand the process this isn't checkable however and relies on the build process being well controlled.
• Most (emphatically not all) people building a low energy house do so to reduce energy consumption rather than increase comfort. Delivering heat at a COP of 3 is a huge advantage here - to achieve the same performance you need to reduce energy loss by the same factor of 3, which is likely to be expensive.
• The PCM34 is in theory well suited to use as a hot water preheat tank. Problem is, you can do nearly the same thing with a shower heat exchanger at lower cost, which would lead to the heat delivered from the PCM34 being pretty minimal (when providing hot water to a shower the uplift to 34°C would be from ~25°C rather than 5°C - a 2/3rds reduction in power). That means the majority of hot water comes from another system - either a hot water tank (in which case why bother preheating) or a PCM58 unit run on resistance heat (minimum charge temperature 65°C - too much for a heat pump). Even if you only achieve a COP of 2 on the heat pump, that's a huge difference to the energy consumption for hot water and goes back to the question of what type of house you're trying to build.
• When they eventually commercialise the PCM43 material, I suspect all this will change. 43°C is hot enough for just about all DHW applications and eliminates the need for a TMV to deal with scalding, while the minimum charging temperature will be ~50°C which is comfortable for a heat pump.
• I think split heat pumps are probably underrated on here - finding an air conditioning company to make the connections can't be that hard (there are about a dozen within a 5 mile radius of me, so costs **should** be reasonable), you eliminate the need for glycol or equivalent and reduce the length of the primary hot water circuit to nearly zero with an associated reduction in losses.
• Heat pumps can do cooling - where the additional cost over an alternative solution is modest, that starts looking very attractive in weather like this.

Essentially my current view is that the current Sunamp PCM34/58 lineup doesn't play well with heat pumps, and that this is a critical flaw if trying to combine them with the design of a low energy consumption house (note: not the same as a low energy import house). With that in mind, my order of preference would be:

1. Heat pump with PCM43 Sunamp for hot water.
2. Heat pump with unvented cylinder for hot water.
3. Heat pump with standalone PCM58 Sunamp for hot water.

I would expect quite a significant performance gap between the three options. A Uniq rHW fitted with PCM43 would I suspect be an extremely good solution for a low energy house.

 

Should be fairly straightforward - the PCM is a bit like water in that changing phase from solid to liquid needs a lot of heat while the material stays at the same temperature, while heating it up after that still takes a fair amount of heat. However, planning on heating it much above the phase change temperature turns it into what is essentially an expensive hot water cylinder.

Youve missed out the whole point of the SA integration  Solar PV. Yes go for a heat pump if youve E10 and no PV, but if youve Pv then the sums shift. Heat energy is the largest consumer of energy in even the best performing dwellings, so offsetting that with 'free' PV is where storage ( and capacitance ) is attractive. If heating is typically required at night ( comfort ) then you'll be 180^o out of phase with solar generation, so you'd ideally want to store whats available during the day for use through the night, as space heating uplift, and in the morning as DHW. Direct drive off a HP will still just chew through grid electricity, the cost of which is only set to rise.

Good points, but missing that critical factor.

PV through a HP however is very attractive for much larger dwellings, so horses for courses. As we've said before, no two cases are identical.

 

1 hour ago, pdf27 said:

Most (emphatically not all) people building a low energy house do so to reduce energy consumption rather than increase comfort

Bit of a contradiction there, as reducing energy costs will increase comfort. eg pay for you to be on the beach abroad more   

[pdf27]

On 08/07/2018 at 08:48, JSHarris said:

The problem you run into is that there are no small (as in 2 to 3 kW output) ASHPs around, and even the smallest ones can't modulate down to the 200 to 500W heating requirement that's the typical average needed in winter for a well insulated and airtight house.  The rate of heat flow from a low temperature UFH system into the slab is just far too slow for the heat pump to deal with.

 

As a consequence, the heat pump gets the UFH up to temperature in a few minutes, then has to shut off.  Most have an in-built anti-short cycling feature; in our case ours only allows three starts per hour.  The heat capacity of the water in the UFH system is small, due to the low volume, so it quickly gives up it's heat to the slab, whilst the ASHP stays turned off until the end of the anti-short cycle timer.

 

Having a buffer completely fixes this.  Our 70 litre buffer sits at around 40 deg C, and the UFH runs at around 23 deg C.  The ASHP can charge the buffer and then shut down for a couple of hours whilst the buffer feeds the UFH demand.  This completely removes the short cycling problem, and also means that the ASHP should last longer, as it's the starts and stops that limit life more than the total hours run, I believe.

@jack does exactly this with a 5kW heat pump though, and both Panasonic and LG do ~3kW units. There is typically a minimum water volume requirement in the UFH circuit - for Panasonic that's 30 litres. 16mm pipe has a 12mm bore - that means any layout with more than 265 metres of pipe in it is stated by the manual to not require a buffer vessel. That's a pretty small house. Note also that some units (Panasonic and Samsung I know about, not sure on others) can modulate down to 25°C for a big improvement in COP over 40°C.

 

If the heating capacity is 3.2kW, 5°C dT gives 4.186 kJ/kg.K - 20.93 kW@1 l/sec, and thus a hot water flow rate of 9.2 l/min (I have a Panasonic manual for a 7kW unit which gives 20 l/min at max flow rate which is consistent with this). An 80m² ground floor slab with 16mm OD pipes at 150mm centres would have 5 x ~100m loops of pipe in it, with each pipe containing 11 litres of water - so even with no heat transfer to the slab at all and no modulation on the heat pump that's ~6 minutes of heat pump running (0.3 kWh heat input before there is any possibility of the outlet temperature increasing), giving a heat pump which runs once per hour to meet the typical heating demand. If there is an anti-short-cycle limit then it will only be hit if the heat demand is >~1kW (12W/m²) and there is no heat transfer between the slab and water. The simplest remedy for this would be to increase the flow temperature slightly - increasing the flow temperature to 30°C would double the heat dumped to the system per cycle - thus doubling the heat input per cycle with no heat transfer to the slab but also greatly increasing the heat transfer rate between water and slab.

So if you have a heat pump which can modulate down to a low enough temperature that you don't need a mixing valve, a buffer tank is superfluous in the vast majority of circumstances. I'm not quite sure how it works if you go down the mixing valve route however - I suspect that matching the power output of the heat pump to the rate at which the slab can accept the heat will need some playing around with circulation pump flow rates, mixing valve settings, etc. and that if they aren't right then short cycling will be the result. If I've understood correctly, most circulating pumps will be in the 20 l/min zone, at which point you'd see temperature rises in the return line after 2-3 minutes and thus a reduction in the dT seen by the heat pump.

[ProDave]

16 minutes ago, pdf27 said:

@jack does exactly this with a 5kW heat pump though, and both Panasonic and LG do ~3kW units. There is typically a minimum water volume requirement in the UFH circuit - for Panasonic that's 30 litres. 16mm pipe has a 12mm bore - that means any layout with more than 265 metres of pipe in it is stated by the manual to not require a buffer vessel. That's a pretty small house. Note also that some units (Panasonic and Samsung I know about, not sure on others) can modulate down to 25°C for a big improvement in COP over 40°C.

 

This is precicely the logic I am using to initially try mine from a 5KW ASHP with no buffer. If it proves problematic I can add a buffer later.

 

Also, I know the blending valves on the UFH manifolds won't go to a low enough temperature, so I am intending to set the ASHP water flow temperature in heating mode very low indeed to regulate the UFH water temperature.

 

I will report back when I eventually get it running.

[pdf27]

5 minutes ago, Nickfromwales said:

Youve missed out the whole point of the SA integration  Solar PV. Yes go for a heat pump if youve E10 and no PV, but if youve Pv then the sums shift. Heat energy is the largest consumer of energy in even the best performing dwellings, so offsetting that with 'free' PV is where storage ( and capacitance ) is attractive. If heating is typically required at night ( comfort ) then you'll be 180^o out of phase with solar generation, so you'd ideally want to store whats available during the day for use through the night, as space heating uplift, and in the morning as DHW. Direct drive off a HP will still just chew through grid electricity, the cost of which is only set to rise.

Good points, but missing that critical factor.

PV through a HP however is very attractive for much larger dwellings, so horses for courses. As we've said before, no two cases are identical.

 

Bit of a contradiction there, as reducing energy costs will increase comfort. eg pay for you to be on the beach abroad more   

Depends how much you integrate the heat pump with a PV system. Most heat pumps now have a "SG Ready" contactor, which lets you turn the thermostat up whenever it is contacted across - most obviously when there is excess PV available. There is going to be a minimum power level at which this makes sense, so for small PV systems it won't work very well and the Sunamp solution would be better - resistance heating is much more flexible in the rate at which it can absorb heat - but in those cases the majority of your energy isn't going to be coming from PV anyway. A number of inverters (SMA certainly do) now have the ability to trip a relay whenever export goes above a certain level, so implementing this shouldn't be hard. I've been playing around with a spreadsheet that takes daily average air temperatures (used to predict the COP) plus half hourly PV data to work out self-consumption rates. A bit crude, but it suggests that this approach should provide >50% of annual hot water and a big chunk of the space heating too - can't remember the exact numbers and don't have it to hand right now, but using real-life data from 2015-18 gave "COP"s based only on the consumption of imported electricity of greater than 10:1 compared to heat demand. Given that heat demand was modelled for a 180m² Passivhaus using a fair bit of hot water, to me it suggests that this approach should work pretty well.

 

I would also take issue with the idea that heating is mostly required at night in a well-insulated house - you're normally in bed and would want a slightly lower temperature for comfort. With a well insulated & high decrement delay house the time constant for it to cool down is very long, so with no heating on overnight the temperature drop is likely only to be a degree or so, making the value of shifting space heat from day to night pretty minimal. Where you do need to shift heat is from summer to winter, which unless you go for the container sized storage units isn't happening with a Sunamp and there is significant value in increasing the COP since there won't be much surplus PV available after plug loads are taken care of.

 

Oh, and if I thought building a Passivhaus would lead to me spending more time on the beach I wouldn't even consider it ?

[pdf27]

2 minutes ago, ProDave said:

This is precicely the logic I am using to initially try mine from a 5KW ASHP with no buffer. If it proves problematic I can add a buffer later.

 

Also, I know the blending valves on the UFH manifolds won't go to a low enough temperature, so I am intending to set the ASHP water flow temperature in heating mode very low indeed to regulate the UFH water temperature.

 

I will report back when I eventually get it running.

http://heatpumps.co.uk/2014/03/06/getting-the-best-from-underfloor-heating/ is worth a read - it's clear that provided the heat pump will regulate low enough then blending valves make things worse rather than better.

[Jeremy Harris]

26 minutes ago, pdf27 said:

@jack does exactly this with a 5kW heat pump though, and both Panasonic and LG do ~3kW units. There is typically a minimum water volume requirement in the UFH circuit - for Panasonic that's 30 litres. 16mm pipe has a 12mm bore - that means any layout with more than 265 metres of pipe in it is stated by the manual to not require a buffer vessel. That's a pretty small house. Note also that some units (Panasonic and Samsung I know about, not sure on others) can modulate down to 25°C for a big improvement in COP over 40°C.

 

If the heating capacity is 3.2kW, 5°C dT gives 4.186 kJ/kg.K - 20.93 kW@1 l/sec, and thus a hot water flow rate of 9.2 l/min (I have a Panasonic manual for a 7kW unit which gives 20 l/min at max flow rate which is consistent with this). An 80m² ground floor slab with 16mm OD pipes at 150mm centres would have 5 x ~100m loops of pipe in it, with each pipe containing 11 litres of water - so even with no heat transfer to the slab at all and no modulation on the heat pump that's ~6 minutes of heat pump running (0.3 kWh heat input before there is any possibility of the outlet temperature increasing), giving a heat pump which runs once per hour to meet the typical heating demand. If there is an anti-short-cycle limit then it will only be hit if the heat demand is >~1kW (12W/m²) and there is no heat transfer between the slab and water. The simplest remedy for this would be to increase the flow temperature slightly - increasing the flow temperature to 30°C would double the heat dumped to the system per cycle - thus doubling the heat input per cycle with no heat transfer to the slab but also greatly increasing the heat transfer rate between water and slab.

So if you have a heat pump which can modulate down to a low enough temperature that you don't need a mixing valve, a buffer tank is superfluous in the vast majority of circumstances. I'm not quite sure how it works if you go down the mixing valve route however - I suspect that matching the power output of the heat pump to the rate at which the slab can accept the heat will need some playing around with circulation pump flow rates, mixing valve settings, etc. and that if they aren't right then short cycling will be the result. If I've understood correctly, most circulating pumps will be in the 20 l/min zone, at which point you'd see temperature rises in the return line after 2-3 minutes and thus a reduction in the dT seen by the heat pump.

 

 

All I'm saying is that it very definitely doesn't work for our 130m² house.  Turn the buffer off and the heat pump short cycles.  Turn the buffer on and it works perfectly.  Our heat pump will modulate down to about 1 kW to 1.5 kW minimum output, and that is just way more heat than the slab can absorb at the set UFH flow temperature.  If I turn the UFH flow temperature up, to increase the heat flow rate, then we get a massive room temperature over-shoot.

 

Turning the heat pump flow temperature down makes the problem massively worse, as the pump just shuts down within a couple of minutes of starting up, then stays shut down for around 20 mins.

 

Theory is great, but I've found that practical experience and experiment is a hell of a lot more accurate.

[PeterW]

5 minutes ago, pdf27 said:

Panasonic and Samsung I know about, not sure on others) can modulate down to 25°C for a big improvement in COP over 40°C.

 

Its a negligible improvement as CoP isn’t linear. 

 

23 minutes ago, pdf27 said:

I've understood correctly, most circulating pumps will be in the 20 l/min zone, at which point you'd see temperature rises in the return line after 2-3 minutes and thus a reduction in the dT seen by the heat pump.

 

The small Samsung’s have a max flow of only 14 l/min and that’s not going to be enough for this sort of scenario. 

 

Given a low heat requirement, ASHP is overkill anyway and a Willis Heater would work. Capex of £50 vs £2300 for the smallest Samsung means even with a CoP of 4-5 your payback is decades.