Do I need a HP specific cylinder?

[sharpener]

14 hours ago, Gordo said:

Any feedback on how it performs would be appreciated

 

See this thread which I have already linked you to so no new information.

 

Did you get a quote for the HX kit? I am interested to know what they cost. IMO this will be the best technical solution.

Edited November 11, 2024 by sharpener

[Gordo]

On 11/11/2024 at 13:11, sharpener said:

 

See this thread which I have already linked you to so no new information.

 

Did you get a quote for the HX kit? I am interested to know what they cost. IMO this will be the best technical solution.

wow that post is i litttle over my head. it sounds like you opted to replace the DHWC in the end for a HP cylinder? I think my existing cylinder should work but would have to fit a booster pump to increase the heat transfer, or fit a plate heat exchanger. Im thinking id be best to just stick with the immersion heater on PV or E7 tarrif.

[sharpener]

11 hours ago, Gordo said:

it sounds like you opted to replace the DHWC in the end for a HP cylinder?

 

No, it was physically impossible to do short of a substantial amount of building work. Hence the quest for a workaround. The new tank is a heat store so I can heat with cheap rate electricity during peak hours.

 

11 hours ago, Gordo said:

I think my existing cylinder should work but would have to fit a booster pump to increase the heat transfer, or fit a plate heat exchanger.

 

As upthread I would suggest you pursue the HX option, hence my interest in the price they quote. I don't think a secondary destratification pump will be enough on its own.

 

11 hours ago, Gordo said:

Im thinking id be best to just stick with the immersion heater on PV or E7 tarrif.

 

Well depending on the cost of the HX I would think that might work out cheaper in the long run. Need to do a proper cost-benefit analysis. Octopus Cosy is good bc of the afternoon cheap period just before the evening peak so better than E7 for a HP.

 

Using your own PV is now suboptimal when you can sell everything you generate to Octopus at 15p and substitute it with cheap rate bought from them at 12p or Eon at 6.7 or Tomato at 5.

 

[Gordo]

11 hours ago, sharpener said:

 

No, it was physically impossible to do short of a substantial amount of building work. Hence the quest for a workaround. The new tank is a heat store so I can heat with cheap rate electricity during peak hours.

 

 

As upthread I would suggest you pursue the HX option, hence my interest in the price they quote. I don't think a secondary destratification pump will be enough on its own.

 

 

Well depending on the cost of the HX I would think that might work out cheaper in the long run. Need to do a proper cost-benefit analysis. Octopus Cosy is good bc of the afternoon cheap period just before the evening peak so better than E7 for a HP.

 

Using your own PV is now suboptimal when you can sell everything you generate to Octopus at 15p and substitute it with cheap rate bought from them at 12p or Eon at 6.7 or Tomato at 5.

 

Octupus not available im my area 😞 

I only get 7hrs E7 at 14p. Export at 10p so i try to consume myself before exporting.

 

Iwill do a cost benifit calculation to see if HX is worth it. But my gut says not

[Michael_S]

On 20/11/2024 at 12:59, sharpener said:

 

No, it was physically impossible to do short of a substantial amount of building work. Hence the quest for a workaround. The new tank is a heat store so I can heat with cheap rate electricity during peak hours.

 

 

As upthread I would suggest you pursue the HX option, hence my interest in the price they quote. I don't think a secondary destratification pump will be enough on its own.

 

 

Well depending on the cost of the HX I would think that might work out cheaper in the long run. Need to do a proper cost-benefit analysis. Octopus Cosy is good bc of the afternoon cheap period just before the evening peak so better than E7 for a HP.

 

Using your own PV is now suboptimal when you can sell everything you generate to Octopus at 15p and substitute it with cheap rate bought from them at 12p or Eon at 6.7 or Tomato at 5.

 

So I really want to keep my existing tank (Viessman Vitocell 180l) due to disruption issues but also because I think with a R290 heat pump (10kw mitsubushi) I could easily use the HP to heat to 45C and then the immersion to top up to 60/65 using 6.7p overnight electricity to give us enough stored energy for our morning showers.  How did you go about convincing an installer that this met BUS requirements?  Thanks

[Nickfromwales]

7 minutes ago, Michael_S said:

So I really want to keep my existing tank (Viessman Vitocell 180l) due to disruption issues but also because I think with a R290 heat pump (10kw mitsubushi) I could easily use the HP to heat to 45C and then the immersion to top up to 60/65 using 6.7p overnight electricity to give us enough stored energy for our morning showers.  How did you go about convincing an installer that this met BUS requirements?  Thanks

You’d do this in the winter to prevent freezing when already doing space heating, but surely during the remaining months you’d use the HP at night to double or treble the saving, even at the 60/65°c you’d not freeze it up.  

[JamesPa]

8 hours ago, Michael_S said:

So I really want to keep my existing tank (Viessman Vitocell 180l) due to disruption issues but also because I think with a R290 heat pump (10kw mitsubushi) I could easily use the HP to heat to 45C and then the immersion to top up to 60/65 using 6.7p overnight electricity to give us enough stored energy for our morning showers. 

Is there a reason to dismiss installing a PHE plus circulation pump as an add on the the cylinder, thats the best technical solution AFAIK to effectively to increase the coil size, which is the only material difference between a heat pump cylinder and any other UVC.  

 

Even without you may well be able to get the cylinder to 55/60 with an R290 heat pump operating at say 70C flow temp.  That will still be more efficient than the immersion and if you do the DHW run at night cheaper. 

 

Throwing away a perfectly good cylinder is, in many cases,  a madness foisted on us for no reason that has been properly explained IMHO.  If your chosen installer wont allow you to keep it, find one who will.

Edited July 30 by JamesPa

[sharpener]

On 30/07/2025 at 01:18, Michael_S said:

So I really want to keep my existing tank (Viessman Vitocell 180l) due to disruption issues but also because I think with a R290 heat pump (10kw mitsubushi) I could easily use the HP to heat to 45C and then the immersion to top up to 60/65 using 6.7p overnight electricity to give us enough stored energy for our morning showers.  How did you go about convincing an installer that this met BUS requirements?  Thanks

 

Key was reducing the heat flux to the cylinder to minimse the delta T, which is done (i) by sharing the HP output between cyl and the thermal store, charging them both at the same time (ii) putting a secondary bronze pump to recirculate the DHW to improve heat transfer on the outside of the coil (iii) not using the asinine Vaillant algorithm for DHW which has very high (essentially unlimited) flow temps. All this was agreed with Vaillant technical who produced bespoke schematics. Eventually I found a (sixth) installer who was prepared to go along with them. Even then it did not go entirely to plan, see the writeup from Sept 2024.

 

I don't think there is any aspect of this that BUS are directly concerned about. They were more worried about the wood burning stove not being connected, as this was listed as supplementary heating on the EPC. 

 

With a 10kW mitsi you will have less of a problem and may be able to turn it down natively to the point the delta T is reasonable. @JohnMo may be able to comment on this.

 

 

[Michael_S]

1 hour ago, sharpener said:

 

Key was reducing the heat flux to the cylinder to minimse the delta T, which is done (i) by sharing the HP output between cyl and the thermal store, charging them both at the same time (ii) putting a secondary bronze pump to recirculate the DHW to improve heat transfer on the outside of the coil (iii) not using the asinine Vaillant algorithm for DHW which has very high (essentially unlimited) flow temps. All this was agreed with Vaillant technical who produced bespoke schematics. Eventually I found a (sixth) installer who was prepared to go along with them. Even then it did not go entirely to plan, see the writeup from Sept 2024.

 

I don't think there is any aspect of this that BUS are directly concerned about. They were more worried about the wood burning stove not being connected, as this was listed as supplementary heating on the EPC. 

 

With a 10kW mitsi you will have less of a problem and may be able to turn it down natively to the point the delta T is reasonable. @JohnMo may be able to comment on this.

 

 

Thanks.  Where is the write up?

 

So what goes wrong if the cylinder coil can't give up heat quickly enough?

[JamesPa]

28 minutes ago, Michael_S said:

So what goes wrong if the cylinder coil can't give up heat quickly enough

Heat pump cycles, takes longer to heat the water and ultimately may not heat to quite as high a temperature and will do so less efficiently.

 

Not a complete disaster despite what some would have you believe, but at the same time not the perfect outcome that many expect to be guaranteed.  Hence (I suspect) the industry insistence on changing out cylinders.  

Edited July 31 by JamesPa

[JohnMo]

1 hour ago, Michael_S said:

So what goes wrong if the cylinder coil can't give up heat quickly enough

The way the heat pump operates doing DHW.

 

You have a target flow temperature, let's say 50 degs for ease. If the cylinder is at 30 degs, the heat pump will sense a return temp of 30 degs, and add a dT to flow temp (approx 5 degs). Now as the cylinder heats the return temp increases as does the flow temp to keep the dT constant. This gives a good CoP as temp stays low as long as possible.

 

Now the rub, if your heat transfer area is smaller than ideal, the coil doesn't extract as much energy as it should, so flow temp stays higher than it should. So the ASHP runs at an elevated temperature for a given cylinder temperature. 

 

The more flow and return diverge away from actual cylinder temperature, or more distortion you have between cylinder and return temp the worse the CoP. Worst case is you do not hit cylinder target temp in one hit, you could end up cycling the heat pump, but most will have a learning feature saying it don't succeed heating cylinder with ASHP, so I will just use the immersion instead.

 

Best situation is a huge coil 3m² or larger or a plate heat exchanger. An add on phe and pump is the easiest way to do things, choose a bigger PHE get a better CoP.

[JamesPa]

41 minutes ago, JohnMo said:

but most will have a learning feature saying it don't succeed heating cylinder with ASHP, so I will just use the immersion instead.

Interesting comment this one and doubtless true in many cases.

 

My Vaillant heat pump doesn't talk to the immersion at all so cant 'use the immersion instead'.  The heat pump also does its legionella cycle natively.  I wonder what it does if it doesn't like what the DHW loop throws at it.  Probably it just turns the flow temp up to 75, which is where a boiler might be,  and all (except COP) is good.   Interestingly some of the Vaillant heat pump cylinders have coils with quite a lot less than 3sq m coil area.

 

As you say:

 

41 minutes ago, JohnMo said:

Best situation is a huge coil 3m² or larger or a plate heat exchanger. An add on phe and pump is the easiest way to do things, choose a bigger PHE get a better CoP.

 

Edited July 31 by JamesPa

[marshian]

On 30/07/2025 at 01:18, Michael_S said:

So I really want to keep my existing tank (Viessman Vitocell 180l) due to disruption issues but also because I think with a R290 heat pump (10kw mitsubushi) I could easily use the HP to heat to 45C and then the immersion to top up to 60/65 using 6.7p overnight electricity to give us enough stored energy for our morning showers.  How did you go about convincing an installer that this met BUS requirements?  Thanks

180 Litre tank??

 

how many showers are required? (See note)

 

Just two of us in the house 115 litre tank we heat to target 50 deg C in the morning with a flow temp of 60 deg (takes 40 mins on average from it’s mid 30’s start point

 

the tank is old style copper with foam casing and not a very big coil so loses ~0.5 deg per hour (was much higher losses before a london loop was added)

 

this means we can still wash up and comfortably have a further two showers in the evening before the tank is fully depleted.

 

I could probably heat to a slightly lower temp maybe 47 or 48 but my old school tank stat has 50 deg as min - below that is off!!!! (I really should buy one that allows a lower set point)

 

Note - If Teenagers are involved all bets on water usage are off

[JohnMo]

4 minutes ago, marshian said:

180 Litre tank

The other bit in MCS rules to which the BUS is linked to has a specification for cylinder and number of bedrooms - is the cylinder big enough to comply?

[marshian]

17 minutes ago, JohnMo said:

The other bit in MCS rules to which the BUS is linked to has a specification for cylinder and number of bedrooms - is the cylinder big enough to comply?

Would have expected a water requirement would be based on occupancy not bed rooms - our 4 Bed house has had a 115 litre tank from new 

 

But it’s BUS grant and MCS so probably all a bit crazy

 

from an energy efficiency you only ever want to heat the water that’s needed??

[JohnMo]

35 minutes ago, marshian said:

expected a water requirement would be based on occupancy not bed rooms

Based on not just you, but future buyers of your house, as it's a bunch of tax payers money. So bedrooms, rather than sizing for me and my wife, in any number of bedrooms.

[Michael_S]

Yeah - so 5 of us showering, 180l tank at 70C lower immersion set temp gives enough hot water with no recharge. 

 

BUS says we need at least 250l I assume based on a much lower temp so much less cold mixed in.

[marshian]

21 minutes ago, JohnMo said:

Based on not just you, but future buyers of your house, as it's a bunch of tax payers money. So bedrooms, rather than sizing for me and my wife, in any number of bedrooms.

which is why (as my house is definately ASHP ready) I wouldn’t bother with the BUS grant.

 

a future owner if they wanted a higher volume of HW for a family of 4 or 5 could get a bigger cylinder fitted

 

I stand by my comment that it’s more economic to heat only the water you need.

 

I’m still thinking about changing the cylinder but Trevor quoted me £1400 for a 120 L cylinder which had a ASHP coil (smallest one they do apparently) I can’t see the point of fitting larger for our needs and we have no plans to move until we self build

[marshian]

2 minutes ago, Michael_S said:

Yeah - so 5 of us showering, 180l tank at 70C lower immersion set temp gives enough hot water with no recharge. 

 

BUS says we need at least 250l I assume based on a much lower temp so much less cold mixed in.

Then to qualify you are almost certainly going to have to go with a bigger tank as it’s very likely to be heated to a lower temp (maybe 50/55 max)

 

I’m keeping my eye out for a suitable pre-owned tank because I’m sure sooner or later one will pop up having been replaced as a result of the BUS grant 😉

[Michael_S]

2 hours ago, JohnMo said:

The way the heat pump operates doing DHW.

 

You have a target flow temperature, let's say 50 degs for ease. If the cylinder is at 30 degs, the heat pump will sense a return temp of 30 degs, and add a dT to flow temp (approx 5 degs). Now as the cylinder heats the return temp increases as does the flow temp to keep the dT constant. This gives a good CoP as temp stays low as long as possible.

 

Now the rub, if your heat transfer area is smaller than ideal, the coil doesn't extract as much energy as it should, so flow temp stays higher than it should. So the ASHP runs at an elevated temperature for a given cylinder temperature. 

 

The more flow and return diverge away from actual cylinder temperature, or more distortion you have between cylinder and return temp the worse the CoP. Worst case is you do not hit cylinder target temp in one hit, you could end up cycling the heat pump, but most will have a learning feature saying it don't succeed heating cylinder with ASHP, so I will just use the immersion instead.

 

Best situation is a huge coil 3m² or larger or a plate heat exchanger. An add on phe and pump is the easiest way to do things, choose a bigger PHE get a better CoP.

So tank is at 30C, HP sends out water at 36C expecting it to return at 31C but instead it returns at 34C as the coil is too small.  I would assume that it then either gets confused (thermostat says current temp is 30C so why is return so warm or perhaps it sees return at 34C and increases flow to 39C?

 

But basically it needs to either reduce output (modulate down) or accept a bigger difference between the flow temp and the tank temp.

 

One issue with the PHE+Pump solution is that you don't maintain stratification so well - at present I suspect we are still getting hot water at the shower when a lot of the tank is full of 12C cold water from the inlet and the average temp is well below 45C. However I can't see why this is not the preferred option over a tank replacement.

[marshian]

2 hours ago, JohnMo said:

Best situation is a huge coil 3m² or larger or a plate heat exchanger. An add on phe and pump is the easiest way to do things, choose a bigger PHE get a better CoP.

 

Hmm I'm starting to wonder if that's the answer for me too - Tanks with no coil at all are substantially cheaper than one with a HP suitable coil?

 

Yes I know you need a suitable circulation pump and a PHE but with both those fitted any increase in tank size is just another relatively cheap tank

 

Plus they normally have 2 immersions so if and when I go down the PV route........

[Michael_S]

4 minutes ago, marshian said:

Then to qualify you are almost certainly going to have to go with a bigger tank as it’s very likely to be heated to a lower temp (maybe 50/55 max)

 

I’m keeping my eye out for a suitable pre-owned tank because I’m sure sooner or later one will pop up having been replaced as a result of the BUS grant 😉

In a way the cost of the tank is only part of the story, not sure a new one would go through our loft hatch and even if it could it would then need to be plumbed.

 

For a phe/pump solution I assume it is the useable hot water that needs to go through the phe, I wonder if the tank has suitable bosses?

[JamesPa]

7 hours ago, Michael_S said:

For a phe/pump solution I assume it is the useable hot water that needs to go through the phe, I wonder if the tank has suitable bosses

Correct.  The usual retrofit arrangement I believe is to tee off the cw inlet pipe (after the prv) and the dhw outlet pipe, so no extra bosses needed

[JamesPa]

8 hours ago, marshian said:

Hmm I'm starting to wonder if that's the answer for me too - Tanks with no coil at all are substantially cheaper than one with a HP suitable coil?

One installer I contacted during my exploratory phase uses this arrangement as standard.  I think mainly for performance but also as you say it's cheaper.

[JamesPa]

8 hours ago, Michael_S said:

One issue with the PHE+Pump solution is that you don't maintain stratification so well - at present I suspect we are still getting hot water at the shower when a lot of the tank is full of 12C cold water from the inlet and the average temp is well below 45C. However I can't see why this is not the preferred option over a tank replacement.

Depends on your reheat regime.  If, like me, you reheat once a day it won't materially affect stratification.  Thats also the case if you reheat when the tank is basically close to empty.

 

If you do continuous reheat it will, but then stratification doesn't matter as much.

 

Edited August 1 by JamesPa