British Gas heat pumps - some restrictions on their offering FYI

[markocosic]

For the curious I had BG out to survey for a heat pump install this morning. (If they'll do a - supervised - install for £3k that's eminently reasonable)

 

 

- The pre-install data gathering is name / address / photo of electricity meter and consumer unit for a load calculation (for looped supplies or over 60A on single phase and they'll refer to DNO before going anywhere - though in my case they've not spotted the looped supply and proceeded regardless)

 

- They're installing Daikin systems

- They won't fit the Daikin unvented cylinder setup unless you have 20 litres/minute at 1 bar (as measured at an outside tap)

- They will however fit a Mixergy setup

 

Water is 10L/min at 1 bar; or 15L/min at 1 bar with the ahem inline booster on a 1/2" copper main around these parts)

 

Wouldn't touch a hot water tank that runs Linux with a 40' barge pole either. That won't be around in 30 years time.

 

Nor have I seen any decent results with heat pump charging Mxergy cylinders to date. The integration here appears to be lacking.

 

So that pretty much ended the process for me.

 

 

Sounded tempting at £3k but they won't do the basic unvented unless you have an excellent water supply / suspect they're trying to upsell Mixergy cylinders.

 

[JamesPa]

3 minutes ago, markocosic said:

Wouldn't touch a hot water tank that runs Linux with a 40' barge pole either. That won't be around in 30 years time.

 

I assume thats the Mixergy one.  Linux probably will be around in 30 years (and will still be working perfectly, just doing its thing - it seems to be bomb proof if my Linux based synology NAS is anything to go by), but mixergys back end servers defo not.

 

Have you managed to get any concrete detailed info about Daikin performance.  They are very cagy with answering technical info and keep telling me to ask my installer (who doesnt know).  I get the impression from such little info as I can find that 

 

1. all their units in any given range are identical apart from a sw limit on max output

2. modulation range is poor  approx 2-2.5:1 with the top end unit in a range, thus even worse with lower end units in a range

 

They also, bizarrely, specify a min cylinder coil area of 1.1sq m for both their HT and LT models, which makes absolutely no sense and wipes out the idea of running their HT unit at say 70 for DHW only to allow an existing cylinder to be retained.  But they refuse to engage in explaining because Im not my installer.

 

Daikin are well renowed in the a/c industry so Id expect their products to be good.  However Im beginning to wonder.

 

Have you or anyone else got any insight into this?

[markocosic]

Mixergy; yes.

 

Supporting an internet connected device for the next 30 years isn't happening.

 

Mixergy will withdraw support by then even if they are still about.

 

 

We didn't get as far as asking which units they propose or diving into the Daikin specs.

 

A minimum coil area of 1.1 m2 is small. It makes sense that both the LT and HT models have the same minimum. "coil needs to be at least thing big to transfer at least this much heat with a deltaT of maximum that across flow/return"

 

I'd be looking for 2-3 m2 in a cylinder before considering using it. And a larger cylinder to allow it to operate at a lower temperature whilst having enough hot water. Can understand why, given they're busy enough already, they're not interested in bespoke setups.

 

 

[JamesPa]

11 minutes ago, markocosic said:

A minimum coil area of 1.1 m2 is small.

Typical UK cylinder has a coil size of 0.6-0.8sq m, so for retrofit this is what you are faced with.  Gas boilers running at 70-75 have been working with this coil size for decades, so why not a (HT) heat pump?  It may not be optimum in all cases, but it works well enough for most.

 

11 minutes ago, markocosic said:

"coil needs to be at least thing big to transfer at least this much heat with a deltaT of maximum that across flow/return"

If the HT unit is run at a higher temperature (for DHW only, the penalty is quite small compared to the cost of swapping the cylinder) then the heat transfer from the coil to the water is more, so I don't get your argument (or Daikins).  That may mean or may not equate to a larger flow-return delta T depending on flow rate.

 

11 minutes ago, markocosic said:

Supporting an internet connected device for the next 30 years isn't happening.  

 

Mixergy will withdraw support by then even if they are still about.

 

Totally agree.  Why would anyone want an internet connected hot water cylinder

Edited August 1, 2023 by JamesPa

[Beelbeebub]

As a thought,

 

You say you have crap water pressure anyway, why not a vented hot water cylinder with a high recovery coil and a booster pump

 

This cylinder is about £450

 

https://mwphs.co.uk/product/ultra-high-recovery-117-litre-vented-hot-water-cylinder-900-x-450/

 

With 2m² coil

 

And I have this water pump 

 

https://www.anglianpumping.com/product/water-boosting-pumps/stuart-turner-mainsboost-variq

 

That is excellent, very quiet.

 

Can both be installed by regular plumber with no unvented regs.

 

[markocosic]

1 hour ago, JamesPa said:

Typical UK cylinder has a coil size of 0.6-0.8sq m,

 

In a house that needed a small heat pump that could modulate down whilst delivering high temperatures that might function.

 

Combis would be in the ones that could tolerate a small heat pump. Remaining cylinders tend to be in older, larger, houses. Those would need a heat pump too large to modulate down far enough to work with a small coil.

 

Also all the previous discussion about cylinders being too small to work at low temperature, too full of open vented legionella and rat pish to work at low temperature, and it not making any commercial sense to do more work and take on more risk for no extra money.

 

BG not being interested unless water supply adequate enough to guarantee no flow rate complaint possibility or consumer prepared to spend extra on an absurd tank will be for similar reasons. 🙂

[JamesPa]

8 minutes ago, markocosic said:

Combis would be in the ones that could tolerate a small heat pump. Remaining cylinders tend to be in older, larger, houses. Those would need a heat pump too large to modulate down far enough to work with a small coil.

I'm not sure your argument stacks up.

 

If you are only heating the dhw to say 50 with a ft of 70 you have a decent coil  to water DT.  Same ratio of DT to coil area as a 55C heat pump and a 2.4sq m coil.  So I'm not at all sure you are right.  3sq m is a throwback to older refrigerants so far as I can see.

 

 

Edited August 1, 2023 by JamesPa

[markocosic]

6 minutes ago, Beelbeebub said:

As a thought,

 

You say you have crap water pressure anyway, why not a vented hot water cylinder with a high recovery coil and a booster pump

 

This cylinder is about £450

 

https://mwphs.co.uk/product/ultra-high-recovery-117-litre-vented-hot-water-cylinder-900-x-450/

 

With 2m² coil

 

And I have this water pump 

 

https://www.anglianpumping.com/product/water-boosting-pumps/stuart-turner-mainsboost-variq

 

That is excellent, very quiet.

 

Can both be installed by regular plumber with no unvented regs.

 

 

10L/min at 1 bar (or 15L/min at 1 bar with existing booster pump) is adequate enough for a terraced house - combi is the main restriction and it'd be happy enough on an unvented cylinder. 

🙂

 

Gravity tank and a vented cylinder that then gets pressure boosted is an interesting suggestion. Back to the old school. I think I'd prefer a sealed accumulator on the mains side if the property were large enough to care/water pressure low enough to care. More robust/fewer moving elements etc.

 

More money than the setup than you suggest though so I can see why folks might choose that where water supply is meh.

[Beelbeebub]

I get why I vented and a booster seems less attractive.

 

And I have experience with a really noisy crap old booster.

 

But recently (about 2.years ago) I have fitted the pump linked to provide cold water mains pressure for 8 properties (from one pump!). It has an additional 50l expansion vessel to buffer it's output a bit, but to be honest it probably doesn't need it. It's very fast responding and just a mild "rustle" sound. If it was on an insulated pad in an airing cupboard you probably wouldn't notice it at all.

 

It made me rethink my strategy for DHW cylinders a bit. For less than the cost of an unvented cylinder you can get a vented cylinder and pump and not have to bother with any invented regs or checks.

[JamesPa]

4 minutes ago, Beelbeebub said:

It made me rethink my strategy for DHW cylinders a bit. For less than the cost of an unvented cylinder you can get a vented cylinder and pump and not have to bother with any invented regs or checks.

Which is one reason I am becoming more resistant to upgrading my own dhw at the same time as I fit a heat pump.  I just am not convinced we yet have the best solution, my existing dhw is pretty new albeit vented and the extra electric cost is about the same as the G3 annual inspection.

 

Sure I can afford to do it, but is now the time to do it. 

 

For the mass retrofit market getting over the idea that hp=new dhw makes the difference between fitting being a major job and it being a relatively simple one.  An installer should be able to do 2 per week not 1 so no loss of profit.

[markocosic]

8 minutes ago, JamesPa said:

I'm not sure your argument stacks up.

 

The heat pump has a minimum output

 

It needs to dump that output into a load

 

The upper limit is compressor discharge temperature.

 

The dT on the coil and the dT from coil to water and the temperature of the and water dictate all this.

 

High water temperature (because needing to pasteurise legionella and pish; because stored volume small); high water to coil dT (because minimum output of large heat pump is high AND coil is small); gives high compressor discharge temperatures.

 

If the dT on the coil itself is also high due to lower primary flow than is ideal you further skyball compressor discharge temperatures.

 

If you only heat a small tank to a low temperature (too low to pasteurise the pish, too low to have an adequate stored volume) AND the heat pump happens to be small so you don't get the compounding nasty of a large house minimum turndown to add to the mess then maybe.

 

That's niche though. Small space heat load AND abnormally low DHW demand AND happy not to heat pushy water.

 

Using round numbers:

 

3kW at 57hp:50water on 2m2 - fine 

6kW at 57hp:50water on 4m2 - fine (if we assume scales linearly)

6kW at 64hp:50water on 2m2 - ok if hp happy at 64 mean water temperature)

6kW at 78hp:50water on 1m2 - you start to see how it's a double whammy for small coil and high minimum output)

3kW at 64hp:50water on 1m2 - ok this may fly as badly as 6 kW on 2m2 but the chances of having a small space heat load and a cylinder with a larger than default coil are small

1.5 kW at 64hp:50w on 1m2 - this may fly as badly as 6 kW on 2m2 but the chances of having such a small space heat load are small

[markocosic]

1 minute ago, JamesPa said:

For the mass retrofit market getting over the idea that hp=new dhw makes the difference between fitting being a major job and it being a relatively simple one.  An installer should be able to do 2 per week not 1 so no loss of profit.

 

Best of luck!

 

I said the same to DECC when they were setting up the RHI and decided to exclude air to air heat pumps (minisplits) for off gas properties from any grant funded promotion.

 

Why?

 

"It's already cheaper than oil or lpg. We see no reason for it needing to be subsided further..."

[JamesPa]

34 minutes ago, markocosic said:

 

The heat pump has a minimum output

 

It needs to dump that output into a load

 

The upper limit is compressor discharge temperature.

 

The dT on the coil and the dT from coil to water and the temperature of the and water dictate all this.

 

High water temperature (because needing to pasteurise legionella and pish; because stored volume small); high water to coil dT (because minimum output of large heat pump is high AND coil is small); gives high compressor discharge temperatures.

 

If the dT on the coil itself is also high due to lower primary flow than is ideal you further skyball compressor discharge temperatures.

 

If you only heat a small tank to a low temperature (too low to pasteurise the pish, too low to have an adequate stored volume) AND the heat pump happens to be small so you don't get the compounding nasty of a large house minimum turndown to add to the mess then maybe.

 

That's niche though. Small space heat load AND abnormally low DHW demand AND happy not to heat pushy water.

 

Using round numbers:

 

3kW at 57hp:50water on 2m2 - fine 

6kW at 57hp:50water on 4m2 - fine (if we assume scales linearly)

6kW at 64hp:50water on 2m2 - ok if hp happy at 64 mean water temperature)

6kW at 78hp:50water on 1m2 - you start to see how it's a double whammy for small coil and high minimum output)

3kW at 64hp:50water on 1m2 - ok this may fly as badly as 6 kW on 2m2 but the chances of having a small space heat load and a cylinder with a larger than default coil are small

1.5 kW at 64hp:50w on 1m2 - this may fly as badly as 6 kW on 2m2 but the chances of having such a small space heat load are small

I understand the constraints however I'm not sure what your numbers are.  

 

Heat transfer at the coil is proportional to coil to water DT and coil area.   So exactly the same amount of heat is transferred from the coil to the water with a 3sqm coil at ft55 and water temp 50 as with ft70 and coil area 0.75 at the same water temp (to be strictly accurate the first case will be slightly less than the second because of the temp drop along the coil, but for simplicity that is ignored)

 

So, unless the minimum output from the hp increases materially between ft55 and ft70, a (high temp capable) hp that works at ft55/3sq m (the figure generally quoted in the UK for a  'heat pump coil) will work also at ft70/0.75 sq m.

 

Are you sure you aren't propagating thinking that arises from older refrigerants than R32 or 290?

Edited August 1, 2023 by JamesPa

[Beelbeebub]

54 minutes ago, markocosic said:

 

The heat pump has a minimum output

 

It needs to dump that output into a load

 

The upper limit is compressor discharge temperature.

 

The dT on the coil and the dT from coil to water and the temperature of the and water dictate all this.

 

High water temperature (because needing to pasteurise legionella and pish; because stored volume small); high water to coil dT (because minimum output of large heat pump is high AND coil is small); gives high compressor discharge temperatures.

 

If the dT on the coil itself is also high due to lower primary flow than is ideal you further skyball compressor discharge temperatures.

 

If you only heat a small tank to a low temperature (too low to pasteurise the pish, too low to have an adequate stored volume) AND the heat pump happens to be small so you don't get the compounding nasty of a large house minimum turndown to add to the mess then maybe.

 

That's niche though. Small space heat load AND abnormally low DHW demand AND happy not to heat pushy water.

 

Using round numbers:

 

3kW at 57hp:50water on 2m2 - fine 

6kW at 57hp:50water on 4m2 - fine (if we assume scales linearly)

6kW at 64hp:50water on 2m2 - ok if hp happy at 64 mean water temperature)

6kW at 78hp:50water on 1m2 - you start to see how it's a double whammy for small coil and high minimum output)

3kW at 64hp:50water on 1m2 - ok this may fly as badly as 6 kW on 2m2 but the chances of having a small space heat load and a cylinder with a larger than default coil are small

1.5 kW at 64hp:50w on 1m2 - this may fly as badly as 6 kW on 2m2 but the chances of having such a small space heat load are small

I get the point but a 16kw r32 HP has a minimum output of 6kw at 65C water temp. 16kw should be enough for most properties

 

From the cylinder's point of view, all it knows is that it's connected to a 65C flow source and will extract the same heat. If anything, it will extract more with a HP as the return temp is lower thus the average coil temp is higher.(65/58 Vs 65/45)

 

6kw is well below the output of any gas boiler so any coil that works with a gas boiler, IE can dump enough heat to stop the boiler cycling, should be fine with a HP.

 

The only thing I can think that might cause a problem is the follow through the coil being higher if the resistance is to much. But 22mm copper should be big enough and I can't imagine anyone using 15mm feeds for the coil in a large house.

 

With old r410a pumps limited to lower temps, so I get the issue.

 

But r32 and r290 especially can replicate the flow temps of a gas boiler.

 

The could even do a legionella cycle and do away with the immersion heater. That would mean there is no possibility of the cylinder boiling. That works mean we won't need expensive g3 cylinders, installer ticket or annual checks.

[markocosic]

1 hour ago, JamesPa said:

I understand the constraints however I'm not sure what your numbers are.  

 

Mean water temperatures on the heat exchange surfaces.

 

 

1 hour ago, JamesPa said:

Heat transfer at the coil is proportional to coil to water DT and coil area.   So exactly the same amount of heat is transferred from the coil to the water with a 3sqm coil at ft55 and water temp 50 as with ft70 and coil area 0.75 at the same water temp (to be strictly accurate the first case will be slightly less than the second because of the temp drop along the coil, but for simplicity that is ignored)

 

You're suggesting:

 

Base case 55:50 (MTD 5) with 3m2 coil

High case 70:50 (MTD 20) with 0.75 m2 coil

 

Same power is transferred. That's what I've worked through above.

 

 

Short of setting up an example lets's start with a best/base case and work backwards using simplified assumptions shall we?

 

 

[edit: the below is wrong - was looking at electrical input not heat output]

 

Let's look at a modest (7 kW nameplate; 8.6 kW at 45C flow temp at design condition) heat pump with the wick turned down to minimum (2.4 kW at current ambients) delivering DHW to a very modest 45C target in a cylinder with a hulking great big coil. (Vaillant's largest heat pump cylinder IIRC)

 

https://emoncms.org/app/view?name=Primary&readkey=eec7334e58f2a6927a9ff7b27c8c7e63

 

 

 

Coil runs at 52.5 / 46.5 C (mean temp 49.5C) for water at a nominal temp of 45C. 2.4 kW.

 

Call it 5C dT on the coil and 5C to the water for arguments sake.

 

Bump that to your 50C setpoint. Heat pump 57.5/52.5 and 2.4 kW. 

 

Bump that to 4.8 kW. That's still lower than the largest of heat pumps but not out the question as a minimum for a 10-12 kW unit.

 

Coil of 3m2 must now be 10C above not 5C. Heat pump will be 62.5/57.5 IF it can maintain a 5C dT on the coil side by dialling up the pump. More if it can't. It probably can.

 

Drop the coil to 1.5 m2. Now you're 20C above not 5C. Heat pump will be 72.5/67.5 again IF it can maintain a 5C dT on the coil side by dialling up the pump. 

 

Drop the coil to 0.75 m2. Now you're 40C above not 5C. Heat pump will be 92.5/87.5 again IF it can maintain a 5C dT on the coil side by dialling up the pump. Clearly this isn't realistic.

 

Pretty sure our simplified assumptions are wrong (you're going to get better heat transfer to the water by driving larger convection currents with higher heating powers / the coil to water dT won't be as extreme) but you see how this can escalate quickly / compound quickly if you have (a) large minimum turndown AND (b) small coil area AND the icing on the cake is (c) you need to heat to higher temperatures.

Edited August 1, 2023 by markocosic
mistake made

[markocosic]

Try again.

 

 

Coil runs at 52.5 / 46.5 C (mean temp 49.5C) for water at a nominal temp of 45C. 8.8 kW to the water (not 2.4 kW)

 

Call it 5C dT on the coil and 5C to the water for arguments sake.

 

Bump that to your 50C setpoint. Heat pump 57.5/52.5 and 8.8 kW. 

 

Drop the coil to 1.5 m2. Now you're 10C above not 5C. Heat pump will be 62.5/57.5 again IF it can maintain a 5C dT on the coil side by dialling up the pump. 

 

Drop the coil to 0.75 m2. Now you're 20C above not 5C. Heat pump will be 72.5/67.5 again IF it can maintain a 5C dT on the coil side by dialling up the pump. 

 

 

Picking the right figures to work form I now agree this could work on a high temperature capable unit as a retrofit if you keep to a 50C cylinder temperature and say 6 kW input.

 

Increasingly marginal if you want to store at 60C though. And I still don't see the reduced hot water volume being stored as particularly viable. Or it being economic over 10-20 years to take the COP hit by operating at nasty temperatures. @Dan F has a Mixergy setup IIRC chose coil is too small.  COP 2-2.5? 15 years as COP 2 vs 4 costs?

 

 

Old post above saved for posterity!

 

Edited August 1, 2023 by markocosic

[sharpener]

51 minutes ago, Beelbeebub said:

I can't imagine anyone using 15mm feeds for the coil in a large house

 

That's the situation I inherited. It struck me as very unusual but I can only think that (with an S plan setup) it is so the coil cct (which is very short) does not starve the radiator cct when they are on at the same time.

 

Otherwise I agree with all your calcs, I have put similar to Vaillant - as they make a point of pushing their 75C flow in their advertising, wot is point if you cannot make use of it to avoid having to change your cylinder? Will report here in due course.

 

PS would not fit a Stuart Turner booster again. Despite what they say some products are rebranded Far Eastern imports. Have had a shaft seal fail, no spares stocked, zero support after 18 months. Full price replacement has recently developed an unexplained crack in the plastic impeller shroud, with drastic drop in o/p pressure, have now built one good pump out of the bits of two. Endless hassle with it, will buy DAB next time.

[Beelbeebub]

17 minutes ago, markocosic said:

Try again.

 

 

Coil runs at 52.5 / 46.5 C (mean temp 49.5C) for water at a nominal temp of 45C. 8.8 kW to the water (not 2.4 kW)

 

Call it 5C dT on the coil and 5C to the water for arguments sake.

 

Bump that to your 50C setpoint. Heat pump 57.5/52.5 and 8.8 kW. 

 

Drop the coil to 1.5 m2. Now you're 10C above not 5C. Heat pump will be 62.5/57.5 again IF it can maintain a 5C dT on the coil side by dialling up the pump. 

 

Drop the coil to 0.75 m2. Now you're 20C above not 5C. Heat pump will be 72.5/67.5 again IF it can maintain a 5C dT on the coil side by dialling up the pump. 

 

 

Picking the right figures to work form I now agree this could work on a high temperature capable unit as a retrofit if you keep to a 50C cylinder temperature and say 6 kW input.

 

Increasingly marginal if you want to store at 60C though. And I still don't see the reduced hot water volume being stored as particularly viable. Or it being economic over 10-20 years to take the COP hit by operating at nasty temperatures. @Dan F has a Mixergy setup IIRC chose coil is too small.  COP 2-2.5? 15 years as COP 2 vs 4 costs?

 

 

Old post above saved for posterity!

 

From what I have seen if HP performance tables the minimum power output is relatively flat as flow temp increases, actually falling abit.

 

All the coil caers about is the temp coming in. The temp going out is a function of flow rate.

 

Eg if you pumped 1lpm of 65c water in it would come out at whatever temp the s

DHW was.  If you pumped at 200lpm the water would come out at about 64C.  

 

The power dissipated is just the average dT between the DHW and the coil (and the area). 

 

If you double the average dT you double the power dissipated.

 

So a regular boiler might flow 65c water in at 10lpm and the return is 45C. For 14kw

 

If our HP could supply 65C at 10lpm with a return of 55C the power would be a bit above 7kw (because the average coil temp was now 60C and not 55C)

 

The issue was always that the little coil couldn't provide enough SA to dump enough power at 50C or whatever the old HPs could pump out.  The HP needed to raise the temp to dump enough heat to stop cycling, bit couldn't.

 

Yea the cop of hearing a cylinder using 65c flow temps is  it good, maybe 2.

 

But, so what. It just drags the SCOP.down a bit.  And, in time the cylinder cam be changed.  Meanwhile the heating is much more efficient.

 

The point is it is probably better for the UK to get more HPs in even if they aren't being super efficient at DHW, than people sticking with gas boilers because the cost of swapping is too much (because of the cylinder swap)

 

[sharpener]

11 minutes ago, Beelbeebub said:

The point is it is probably better for the UK to get more HPs in even if they aren't being super efficient at DHW, than people sticking with gas boilers because the cost of swapping is too much (because of the cylinder swap)

 

Just so. I have said to Vaillant that if they can relax the automatic requirement for a new cylinder it would be a competitive advantage for them as they will sell fewer cylinders but many more heat pumps.

[Beelbeebub]

5 minutes ago, sharpener said:

 

Just so. I have said to Vaillant that if they can relax the automatic requirement for a new cylinder it would be a competitive advantage for them as they will sell fewer cylinders but many more heat pumps.

Exactly, and I reckon that you could make most (not all) existing cylinders work with a (modern) HP. Initially you would use the "boiler emulator" setting of 65 or 70C flow and thermostat.  Not very efficient, but it works and the switchover job is much faster.

 

Then you could tune the system to work best with existing setup. Once you've got to maximum efficiency then, if it's still not high enough, you look to fitting a "HP coil" cylinder.

 

As I said, the magic number is around 2.5

 

As long as you overall scop is better than that the.UK burns less gas.

 

[JohnMo]

All the above is great in theory, but the ASHP doesn't put out a set temp all the time, it slowly ramps up as the graph above illustrated. It has taken 40+ mins to get to 52 degs flow temp.

 

Not sure how your small coil manages the heat transfer at the lower flow temp. The dT would too low the ASHP would ramp up temperature quickly and basically run out of temperature rise to manage dT and leave you with a slightly warmer cylinder, not hot or usable. It would then just continue to short cycle never achieving set point.

 

 

[Beelbeebub]

24 minutes ago, JohnMo said:

All the above is great in theory, but the ASHP doesn't put out a set temp all the time, it slowly ramps up as the graph above illustrated. It has taken 40+ mins to get to 52 degs flow temp.

 

Not sure how your small coil manages the heat transfer at the lower flow temp. The dT would too low the ASHP would ramp up temperature quickly and basically run out of temperature rise to manage dT and leave you with a slightly warmer cylinder, not hot or usable. It would then just continue to short cycle never achieving set point.

That would be true of an older pump using r410 similar. I think they tended to top out in the low 50's sometimes below.

 

But an r290 can go up to 70+, basically the same range as a gas boiler.

 

So it starts at 45/37 flow/return.  But the coil is too small and the return is 42C (say)

 

As you say, it ramps up, maybe tries 55/47. Still no good, the return is 49!

 

At this point an old HP would just quit (as you say).

 

But newer ones just try 65/57

 

At which point the cylinder can't tell the difference between 65C flow from.a gas boiler and 65C from a HP.

 

The only issue I can see is the higher flow rate of the HP might cause hydraulic issues.  But, as I said, 22mm pipes should be able to handle the 65/57 flow rates required to dump 6kw or so. There may be some cases where the run is long, has many bends and valves etc where we start to run into issues.

[Beelbeebub]

I had a look at a table for an R32 York machine, it can produce 60C water from -15C to 30C (so all usual UK situations).

 

At 0 it's min output at 60C is 3.6kw at a rather poor 2.3CoP

 

So we would only be getting "worse than gas" performance on the few days of the year we are below 0.

 

It only has to rise to 5C to start being more efficient than a boiler.

 

At 20C the min output rises to 7kw but the cop is 3.6

 

A standard cylinder coil should be easily able to handle 6kw transfer at 60C

 

 

[Beelbeebub]

Regarding CoPs, 

 

If you can run your (modern) HP at 50C to get your cylinder up to 45C (IE you have a big coil) then the CoP is likely to be above 3.5 all the way down to 10C again, most of the year.  Don't forget your HP will not spend all the.time at 50C it will creep up from the lowest temp it can sustain with sufficient heat transfer not to cycle.  So you COP should.be better than 3.5 and it should be cheaper than gas.

[JamesPa]

2 hours ago, sharpener said:

 

Just so. I have said to Vaillant that if they can relax the automatic requirement for a new cylinder it would be a competitive advantage for them as they will sell fewer cylinders but many more heat pumps.

Do they really say that.  In which case what is the point of a HT heat pump.  Using it at HT for space heating gives a lousy scop and upgrading radiators is cheap and low disruption  Using it at HT for dhw barely affects the scop and upgrading the cylinder is expensive and high disruption.  So the bang for the buck is to upgrade the radiators but keep the dhw cylinder.

 

Ok I've oversimplified, but manufacturers of HT heat pumps insisting on replacing the dhw tank clearly aren't in the real world. 

 

Edited August 1, 2023 by JamesPa