Heat pump ready cylinders, are Vaillants small coil good enough?

[Bruce]

We are about to get a new hot water cylinder in the loft that we will run on immersion until we get the heat pump installed later this spring and our plumber has recommended the Vaillant 200l heat pump-ready cylinder, uniStor. I suspect he has recommended this mainly because they also install heat pumps under Vaillant umbrella scheme. These cylinder seems quite popular and it has the advantage that it is a bit shorter than many others of the same size, this is good as we can only fit a 1400mm tall cylinder with a 50-55cm in diameter.

My concern is that these cylinders have a coil size of 1.4m2 whilst others from Gledhill, Telford etc have about 2-3.5m2 coil area, quite a lot bigger. Does this actually affect the SCOP of the heat pump or is it more of a matter that it takes longer to heat up the water when it has a smaller coil and such the house sits without heating for longer?

Any other reason to not use a Vaillant cylinder when not knowing what heat pump we might end up with later this year?

[TonyT]

Is 200litres enough for your needs is the first question?

 

cylinder manufacturers generally advise on how many kW required to heat the cylinder from cold to hot in 20 minutes I recall 

 

larger coil size = less reheat time.

 

do you have PV?

Will you have top/bottom immersions too to provide back up and utilise excess PV generation?

 

can the hatch be made larger, a day’s joinery work will Have the hatch enlarged, giving more options for larger diameter cylinders?

 

will the loft take the weight?

 

Good luck 

[JohnMo]

They seem to work, OSO use a similar sized coil also.

 

I get one with a 3m2 coil, a Gledhill one badged by Ideal, comes with a 3 port valve etc. and was about £400 cheaper than the Gledhill one, although exactly the same.

[sharpener]

If you can, it would be better to decide what HP you are going to have as they (or their installers) can be picky about what cylinders they will work with. Installers will also want to make sure you have a big enough tank, one popular formula is (no. of bedrooms + 1) x 45 litres, and may argue that even if you need less HW than that now a subsequent purchaser might want more.

 

You do not say what size HP you are planning (for the purposes of calculation 1 kW provides 861 litre-degrees of water heating per hour, so about 4 deg/hr for a 200 l tank). Vaillant have in the past said that they need a 3 sq m coil for any of their HPs, but their own compatibilty chart does not actually require it.

 

In reality it will almost certainly be OK. After a lot of work (see my and @JamesPa's postings dating back months) I finally secured Vaillant's agreement that my existing OSO 210 l tank can be used with their 12kW HP, even though it has an assumed 0.75 sq m coil, provided I fit a bronze secondary circulation pump to increase the heat transfer.

 

The recovery time is in any case not very important if you heat the bulk of the water overnight at cheap rate, while the heating is on setback anyway. OTOH if you have a high demand in the early evening for 4 ppl showering before going out (another recent thread), or for childrens' bathtime, then you might need a re-heat after lunch, when the air temp will be higher and so the CoP a fraction better. And Octopus Cosy is cheap again.

 

 

 

Edited January 30 by sharpener

[ReedRichards]

3 hours ago, Bruce said:

is it more of a matter that it takes longer to heat up the water when it has a smaller coil and such the house sits without heating for longer?

 

I think that is it, essentially.  But if the calculations are done rigorously then if you spend, say, 90 minutes per day heating your hot water then that leaves 22.5 hours to heat your dwelling and that might just tip you over the edge into needing a bigger heat pump than if it took 45 minutes a day to heat your hot water, leaving 23.25 hours for space heating.  Or it might make no difference whatsoever.

[JamesPa]

10 hours ago, sharpener said:

If you can, it would be better to decide what HP you are going to have as they (or their installers) can be picky about what cylinders they will work with. Installers will also want to make sure you have a big enough tank, one popular formula is (no. of bedrooms + 1) x 45 litres, and may argue that even if you need less HW than that now a subsequent purchaser might want more.

Unfortunately I have to agree with @sharpener here, I have encountered some pretty obstinate installers in my attempts to get a heat pump installed.  This the question is effectively not a technical one, its about installer/heat pump manufacturer obstinacy & preference.  Oh, and the argument 'well its my house' doesn't work.

 

There is always the work-around of fitting an external plate heat exchanger/circulation pump in place of or in addition to the coil (as fitted by Mixergy, Mitsubishi and at least one other heat pump manufacturer).  There are actually some arguments that such an arrangement is anyway preferable, but I fear many installers wont play ball because its not in their book.  That said, the installer I am hoping to use (if I eventually beat my LPA into submission) actually does this pretty much by default, combining a PHE plus pump with a bog standard cylinder,  but he is in the minority

 

10 hours ago, sharpener said:

my existing OSO 210

Vaillant tanks look suspiciously similar to OSO ones.  Also, like OSO, they have an o/d of 600mm not 550mm which means that they are shorter from any given volume, an advantage in some circumstances (including my own).

[sharpener]

9 hours ago, ReedRichards said:

Or it might make no difference whatsoever.

 

A 210 l tank full will take about 9 kWh to heat. That's 1.5 hrs with a 6kW HP. No amount of coil area is going to reduce it below that. So it is probably not the rate limiting step unless you have say a 10 - 12kW unit and are desperate to get the recovery time as low as possible.

 

But a 12 kW one will generate HW continuously at over 5 litres/min which is rather better than an electric shower. So timing it for when you are actually using the HW might mean it does not deplete the stored water at all.

[Bruce]

12 hours ago, TonyT said:

Is 200litres enough for your needs is the first question?

 

cylinder manufacturers generally advise on how many kW required to heat the cylinder from cold to hot in 20 minutes I recall 

 

larger coil size = less reheat time.

 

do you have PV?

Will you have top/bottom immersions too to provide back up and utilise excess PV generation?

 

can the hatch be made larger, a day’s joinery work will Have the hatch enlarged, giving more options for larger diameter cylinders?

 

will the loft take the weight?

 

Good luck 

Yes it should be enough, it is a small 3 bedroom semi-detached with only one bathroom with a shower, so never any bathtubs to fill. No PV, it is on the which list but it will probably take a couple of years until we can afford that.

 

The loft hatch is 68x78cm and is as big as it can get due to wall limitations. The loft should be able to take the weight, we have a plan to put up more 100x47's and put a platform on top made of 18/25mm plywood, this will be on top of a wall and just on the side on another T-connecting wall. I have made a post about it but think it was a bit confusing perhaps.

 

10 hours ago, sharpener said:

If you can, it would be better to decide what HP you are going to have as they (or their installers) can be picky about what cylinders they will work with. Installers will also want to make sure you have a big enough tank, one popular formula is (no. of bedrooms + 1) x 45 litres, and may argue that even if you need less HW than that now a subsequent purchaser might want more.

 

You do not say what size HP you are planning (for the purposes of calculation 1 kW provides 861 litre-degrees of water heating per hour, so about 4 deg/hr for a 200 l tank). Vaillant have in the past said that they need a 3 sq m coil for any of their HPs, but their own compatibilty chart does not actually require it.

 

In reality it will almost certainly be OK. After a lot of work (see my and @JamesPa's postings dating back months) I finally secured Vaillant's agreement that my existing OSO 210 l tank can be used with their 12kW HP, even though it has an assumed 0.75 sq m coil, provided I fit a bronze secondary circulation pump to increase the heat transfer.

 

The recovery time is in any case not very important if you heat the bulk of the water overnight at cheap rate, while the heating is on setback anyway. OTOH if you have a high demand in the early evening for 4 ppl showering before going out (another recent thread), or for childrens' bathtime, then you might need a re-heat after lunch, when the air temp will be higher and so the CoP a fraction better. And Octopus Cosy is cheap again.

 

 

 

 

Deciding on specific heat pump brands is hard, we have had huge problems getting quotes from heat pump installers, dragging out on time which is why we now have to install the cylinder separately so we can move into the house and accept that we have to live without heating in the house as we had to remove the gas boiler as part of the refurb and was promised by two companies that they would be able to quote and install the heat pump before end of Feb (We where promised this in late November last year as soon aw we knew we where actually getting the house). The size should be fine, see my response to TonyT above. The heat pump size I have calculated should be between 6-7kW. 

 

9 hours ago, ReedRichards said:

 

I think that is it, essentially.  But if the calculations are done rigorously then if you spend, say, 90 minutes per day heating your hot water then that leaves 22.5 hours to heat your dwelling and that might just tip you over the edge into needing a bigger heat pump than if it took 45 minutes a day to heat your hot water, leaving 23.25 hours for space heating.  Or it might make no difference whatsoever.

That is a good comment, I wonder if going from a 1.4m2 to 3m2 coil actually halves the reheating time or if it is more like 25% quicker, I would love to see some testing of this!

[JohnMo]

9 hours ago, ReedRichards said:

 

I think that is it, essentially.  But if the calculations are done rigorously then if you spend, say, 90 minutes per day heating your hot water then that leaves 22.5 hours to heat your dwelling and that might just tip you over the edge into needing a bigger heat pump than if it took 45 minutes a day to heat your hot water, leaving 23.25 hours for space heating.  Or it might make no difference whatsoever.

You need to add to this how and when you heat the cylinder, and how your house reacts to the heating being switched off on a cold day. Also if you are using standard rate electric a time if use tariff.

 

Taking 90 mins off heating could be fine, in some cases in many you could get 3 to 4 degree temp drop in the house, which takes all day to recover on WC. In that case big coil and a few small tops would be better spread over the day. My 210L cylinder with a 3m2 coil, takes 35 mins to reheat to 50 deg based on a standard thermostat differential, from a 6kW heat pump. It generally does this twice a day.

 

With a coil half the size, you could spend a lot of time not heating the house.

[JamesPa]

1 hour ago, JohnMo said:

You need to add to this how and when you heat the cylinder, and how your house reacts to the heating being switched off on a cold day. Also if you are using standard rate electric a time if use tariff.

 

Taking 90 mins off heating could be fine, in some cases in many you could get 3 to 4 degree temp drop in the house, which takes all day to recover on WC. In that case big coil and a few small tops would be better spread over the day. My 210L cylinder with a 3m2 coil, takes 35 mins to reheat to 50 deg based on a standard thermostat differential, from a 6kW heat pump. It generally does this twice a day.

 

With a coil half the size, you could spend a lot of time not heating the house.

All true but...

 

• Many, if not most, installers oversize heat pumps
• The loss of a couple of hours heating time matters only in the very coldest weather
• you could always supplement using the immersion on those couple of days per year
• and its rare that a whole tank is reheated anyway.

 

We dont have to engineer everything for the six-sigma (or even three sigma) circumstance, and the industry has no right to force us to do so.

 

Its another one of those things where the homeowner should be allowed to make an informed choice, not installers allowed to force a particular solution down their throat!  Why shouldn't the OP be able to fit a reasonably specified cylinder now, provided he /she is happy to live with the consequences when he subsequently comes to install a heat pump?

 

Edited January 31 by JamesPa

[JamesPa]

1 hour ago, Bruce said:

Deciding on specific heat pump brands is hard, we have had huge problems getting quotes from heat pump installers, dragging out on time which is why we now have to install the cylinder separately so we can move into the house and accept that we have to live without heating in the house as we had to remove the gas boiler as part of the refurb and was promised by two companies that they would be able to quote and install the heat pump before end of Feb (We where promised this in late November last year as soon aw we knew we where actually getting the house). The size should be fine, see my response to TonyT above. The heat pump size I have calculated should be between 6-7kW. 

Probably the only 'safe' (in the sense that installers are unlikely to object) tank is one where the coil is at least 3 sq m and the size is at least 45l* (greater of number of bedrooms +1 or actual occupancy).

 

Unfortunately you may also get installers who insist on 28mm primaries to the tank, or using a pre-plumbed tank, so even this is not absolutely guaranteed (incidentally I would run a mile from any installer that wanted to use a pre plumbed tank unless it makes sense in terms of the layout of your house.  if they are shoehorning a pre plumbed tank in where it doesn't fit its because they use rookie plumbers and there is less chance of cock-up with a pre-plumbed cylinder ).

 

1 hour ago, Bruce said:

That is a good comment, I wonder if going from a 1.4m2 to 3m2 coil actually halves the reheating time or if it is more like 25% quicker, I would love to see some testing of this!

If you want to get a feel for it you could play with the simulation in the DHW tab in the attached workbook

 

With all the usual warnings that its a theoretical simulation only etc...

 

 

WC Simulation.xls

 

One of the more rational reasons for large coil size is the risk of cycling towards the end of the heating period if the coil is too small.  This is of course a particular problem if you have a high capacity, oversized, heat pump that doesn't modulate down well (as fitted by a fair few installers).  Cycling will extend the reheat time quite dramatically.  You say you are looking at around 6kW however - at that level you are very unlikely to have a problem with a 'Vaillant' coil size in practice, but of course that wont necessarily convince an installer.

Edited January 31 by JamesPa

[Bruce]

Many good comments here. Having a larger coil and the immersion element closer to the bottom both for now as we are going to use that as main heat source but also for better usage combined as a diverter with PV in the future sounds like good comments. The Gledhill 180 litre tank, PLUHP180 seems to be almost identical in actual usable volume as the Vaillant 200 litre version but with a 2.5m2 coil and has the immersion heater very close to the bottom. My understanding is that it has 28mm connection for the coil as well which should reduce risk of problems with the future heat pump installer.  We are not getting any installers that doesn't know have to plumb a tank.

[Bramco]

4 hours ago, JamesPa said:

Why shouldn't the OP be able to fit a reasonably specified cylinder now, provided he /she is happy to live with the consequences when he subsequently comes to install a heat pump?

 

@JamesPa This is exactly what we did. We'd had enough of the daft quotes for ASHPs, especially those that insisted that they would have to do a full heat analysis, when we already had that from our M+E consultants....  And then of course there was the MCS markup...   So we went ahead with the cylinders we'd specced in the M+E report. 300L DHW with 2 immersions and an UFH buffer tank also with 2 immersions. Both with about 3m2 coils.  The DHW tank is also heated from a solar PV diverter.

 

The plan was to use Octopus Gos 4 hour 9p window for both the DHW (if not enough solar the previous day) and for the UFH.

 

When we realised that the BUS grant was also available to new builds, we looked around again at ASHPs - in the end we had a Cool Energy 9kW installed which cost us a couple of hundred quid. They were happy with the heat analysis from the M+E report and the tanks that were already in place and did the grant submission as well as the MCS certification.  So for us a no brainer.  We now run the ASHP on heating for 4 hours at night instead of the immersions which except for really cold days is more than enough heat. The DHW is still on the immersions. We worked out that having the ASHP on heating only for 4 hours was the most economic way of doing things.  If we ran it for hot water outside the 4 cheap hours it would be more expensive than a scheduled run of the diverter for the 4 cheap hours..

 

@Bruce you should contact Cool Energy in Grimsby about a price for one of their 9kW ASHPs

 

Simon

[JohnMo]

7 minutes ago, Bramco said:

We now run the ASHP on heating for 4 hours at night

Out of interest what flow temp do you use?

[Bramco]

18 hours ago, JohnMo said:

Out of interest what flow temp do you use?

 

@JamesPa  Flow out is about 30C and back is around 25C.  The ASHP is set to 35C but even when it's not defrosting, it never reaches this, so it runs continuously for the full 4 hours.  When it has to defrost, you can see a dip in both the ASHP output and input and the UFH feed and return.

 

I think we'd set the ASHP setpoint to 35C to aim to get a flow temp approaching that but with the way things work, i.e. the ASHP never really reaching the setpoint, it's actually probably working optimally, in that when the ASHP doesn't have to defrost it runs continuously, so is probably getting a great COP.  Sadly can't see this on the Cool Energy units - I'm considering putting one of the open energy monitor ASHP kits in so that we can see what we're really achieving but that means draining down the ASHP loop to put a heat meter in, so will have to be done in the summer.

 

Graphs below to give you an idea of how things work - our build was almost passive house, very airtight, MVHR, insulated slab with embedded UFH loops, so we don't need much heat and when the slab is warm we don't lose heat v fast.

 

Warm night, so a bit above normal, no defrost.

 

 

Cold night with defrost. Given it was very cold, the immersion in the UFH tank we use for a boost when it's below 5C was probably also on.

 

[sharpener]

On 31/01/2024 at 11:15, Bruce said:

The Gledhill 180 litre tank, PLUHP180 seems to be almost identical in actual usable volume as the Vaillant 200 litre version but with a 2.5m2 coil and has the immersion heater very close to the bottom.

 

As noted upthread the Gledhill range is available branded as Ideal, identical but cheaper. Gledhill will only deal with the trade, not end users. Newark Cylinders are another established brand that were OTOH helpful and quick to respond when I asked them to quote, and will customise for multiple immersions and many other features.

 

But unless the price is an over-riding issue it might be  better to go with the Vaillant Unistor as originally suggested by the plumber in yr OP, then he cannot possibly complain later, and operation with the Arotherm Plus range is guaranteed (you will see that the 200l is compatible with their 10 and 12 kW HPs as well).

 

 

Their 7kW HP is a well regarded quiet unit with an exceptionally wide turn-down ratio, and at -2 OAT will actually put out 8kW with W = 55C (and 9.4 kW with W = 35!)

 

 

Alternatively you might get quotes from Octopus (Daikin or the new Cosy) or Good Energy (MIdea, cheap and cheerful), both are offering systems for small uncomplicated properties.

[HughF]

I’ve got the 1m2 vaillant cylinder… 9kW heat pump with a pwm pump driving into it. Works perfectly and recovers fast.

[Bruce]

Small update on this topic. We have now booked an install of the Gledhill 180 litre tank, PLUHP180, to be installed in a couple of weeks and will run it on immersion with a timer at the start. Is it standard practice to install a TMV to avoid scalding and minimise standing losses in pipes?

Thanks everyone for your input

Edited February 8 by Bruce

[JohnMo]

1 minute ago, Bruce said:

Small update on this topic. We have now booked an install of the Gledhill 180 litre tank, PLUHP180, to be installed in a couple of weeks and will run it on immersion with a timer at the start. Is it standard practice to install a TMV to avoid scalding and minimise standing losses in pipes?

Thanks everyone for your input

Look at the price of an Ideal branded cylinder they are exactly the same, several hundred pounds cheaper 

[JohnMo]

If you have mixer taps no need for a TMV

[sharpener]

Even if you don't have mixer taps TMV still only required for vulnerable people e.g. in retirement homes. One more thing to go wrong. We managed perfectly well without them for decades. Now the fashion in public places is to put up notices saying Danger, hot water may be hot. FGS!

[JohnMo]

24 minutes ago, sharpener said:

Now the fashion in public places is to put up notices saying Danger, hot water may be hot.

And most places you wish it was actually hot, most the time it's just cold.

[sharpener]

 

2 hours ago, JohnMo said:

3 hours ago, sharpener said:

Now the fashion in public places is to put up notices saying Danger, hot water may be hot.

And most places you wish it was actually hot, most the time it's just cold.

 

If the expectation is that it will be cold then the correct form of words is "Danger, hot water might be hot."

[MoDo]

On 31/01/2024 at 15:03, Bramco said:

 

@JamesPa This is exactly what we did. We'd had enough of the daft quotes for ASHPs, especially those that insisted that they would have to do a full heat analysis, when we already had that from our M+E consultants....  And then of course there was the MCS markup...   So we went ahead with the cylinders we'd specced in the M+E report. 300L DHW with 2 immersions and an UFH buffer tank also with 2 immersions. Both with about 3m2 coils.  The DHW tank is also heated from a solar PV diverter.

 

The plan was to use Octopus Gos 4 hour 9p window for both the DHW (if not enough solar the previous day) and for the UFH.

 

When we realised that the BUS grant was also available to new builds, we looked around again at ASHPs - in the end we had a Cool Energy 9kW installed which cost us a couple of hundred quid. They were happy with the heat analysis from the M+E report and the tanks that were already in place and did the grant submission as well as the MCS certification.  So for us a no brainer.  We now run the ASHP on heating for 4 hours at night instead of the immersions which except for really cold days is more than enough heat. The DHW is still on the immersions. We worked out that having the ASHP on heating only for 4 hours was the most economic way of doing things.  If we ran it for hot water outside the 4 cheap hours it would be more expensive than a scheduled run of the diverter for the 4 cheap hours..

 

@Bruce you should contact Cool Energy in Grimsby about a price for one of their 9kW ASHPs

 

Simon

Did you install the Cool Energy heat pump yourself? Is the 'couple hundred' you refer to, the materials required to install or the additional cost over the grant after covering professional installation? Trying to make a decision on this myself as with a trade account a Valliant is actually cheaper than a Cool Energy R290 unit - but if I've understood your post correctly, Cool Energy were able to claim the grant (did they arrange installation)? 

[Bramco]

@MoDo

 

Cool Energy's installers did the install, the MCS paperwork and the grant application - the couple of hundred quid was what the bill came to less the £5k grant.  Obviously now that's £7.5k.

 

Cool Energy have now taken over their installers business and are running things themselves - think a couple of the guys moved to Octopus to start their ASHP installation business.

 

I think but I'm not sure that you can apply for the grant yourself but then you'd need to get the MCS paperwork done - that's if that is important to you, lot's of folks just install their heatpumps themselves without the MCS paperwork and associated uplift.

 

Simon