Vaillant Unistor cylinder - or not

[mk1_man]

Hi, apologies if this topic has been covered previously.   We are just in process of installing a Vaillant Arotherm 7.0kW and need to change existing cylinder.  My default go to option was the Vaillant Unistor 250l cylinder but having checked the specs it only has a 2.2m2 coil and also uses 22mm pipe as opposed to other dedicated heat pump cylinders such as Joule that have 3m2 coil and 28mm connections.  I like the fact that the Vaillant is a shorter (600mm wide) tank which is beneficial as it is going into a place with restricted height and they are a neat looking unit with hidden wiring etc.  Am I worrying to much about the smaller coil dimensions, surely they wouldn't sell a product not up to speed?    I understand that regardless of coil size if the heat pump only outputs x amount then the coil can only transfer so much heat so a bigger coil doesn't necessarily mean faster heat recovery, also it is better to run cooler and longer to improve COP so again bigger coil not such a benefit?

 

Thanks 🙂

[sharpener]

Vaillant are not very consistent in their advice about coil sizes. However 7 kW is not the biggest o/p so I think you will be OK, and you might not need a new tank at all, see under. IIRC their tanks are made by OSO and it might be cheaper from them.

 

You need 2.9 litre/min to transfer 1 kW with a delta T of 5 deg so 20.3 for 7 kW. In a 22mm Cu pipe this will require a flow of 1.1 m/s which is just over the 1.0 max usually recommended.

 

1 kW will heat 250 litres at ~3.6 deg/hr so to heat it from 19 to 55C will take yr HP 10/7 = 1.4 hours

 

To improve the CoP while heating DHW you can put a Vaillant into Eco mode where it runs at 50% power so the approach temperature is better but of course it takes twice as long.

 

If you are happy to do this (depending on occupancy and usage pattern) then you could probably get away without changing the tank at all, a normal coil will handle 3.5 kW without difficulty.

 

Alternatively you could do what I am planning, fit a bronze pump on the secondary side to recirculate the water and improve the heat transfer. Or fit an external plate heat exchanger. Either would be a lot cheaper than a new tank.

 

@JamesPa has a spreadsheet which will model all this and perhaps will chip in.

 

BTW for a retrofit you do not need to fit a new 3-port diverter either, existing S-plan 2-port valves work just as well, I have this confirmed by Vaillant tech.

[JohnMo]

1 hour ago, mk1_man said:

uses 22mm pipe as opposed to other dedicated heat pump cylinders such as Joule that have 3m2 coil and 28mm

22mm will be fine for that size of heat pump. But regardless I would always go for 3m2 or bigger.  The smaller the coil the bigger the CoP hit doing hot water.

 

My 210l 3m2 slimline cylinder takes about 40min to reheat to 50 between thermostat hysterisis set points. 6kW ASHP max flow temp is 56.

[sharpener]

9 minutes ago, JohnMo said:

The smaller the coil the bigger the CoP hit doing hot water.

 

I doubt any improvement in CoP would repay the capital cost of fitting a new tank if it is not strictly necessary. @JamesPa would I am sure agree (name check in previous post doesn't seem to have worked).

[JohnMo]

48 minutes ago, sharpener said:

existing S-plan 2-port valves work just as well, I have this confirmed by Vaillant tech.

But wired to priority hot water, not both together assume. Grant have a similar setup as standard, but a note saying the timing has to such that both dhw and heating cannot occur at the same time. My cylinder actually came with diverter valve as standard.

[sharpener]

Yes, @mk1_manhas Vaillant, DHW priority by default, they also tell me all space heating circuits are automatically de-energised when any DHW heating is going on so S-plan is fine. If you have only got one heating cct and so no VR70/71 wiring centre you might need to add a £10 relay.

 

To avoid the heating being off for too long there is a programmable max time for DHW, and a programmable lockout period before it will re-start, which OP may find useful esp as he has a big tank and a relatively small HP.

[mk1_man]

Thanks all for the advice.  It is frustrating as my existing cylinder is a Warmflow solar 250l with twin coils, I currently have these plumbed in series on my gas boiler and the re-heat time is very quick however all of the MCS installers are saying that I must upgrade to a ' heat pump ' cylinder.  I am hoping I can convince them that my existing cylinder is fine and swap it out afterwards if proven not.  The Vaillant R290 shows good efficiency even at 65 - 75 degrees, if that is the case I don't see why anyone would have to swap out their cylinders.

 

I guess an option is to upgrade the existing by running an external heat exchanger connected to secondary input running in series / parallel with the coil ?

[sharpener]

28 minutes ago, mk1_man said:

I am hoping I can convince them that my existing cylinder is fine and swap it out afterwards if proven not. 

 

If the current 240l model listed here on p10 is anything to go by, the two coils in series will handle 30kW which is the same as the HP model on p11. Though it does not state the coil areas directly I imagine a call to Warmflow would quickly elicit it, and maybe an email from them would confirm its suitability.

 

28 minutes ago, mk1_man said:

I guess an option is to upgrade the existing by running an external heat exchanger connected to secondary input running in series / parallel with the coil ?

 

I notice the Warmflow HP models show an external heat exchanger. Perhaps worth discussing with W whether this can be added to yr existing cyl and if it is worthwhile to do so. Anything has to be better than unnecessarily scrapping off kit worth £1000+.

 

Yr installers may not know about the Eco mode either which if you can afford the extra heating time will for sure solve the problem. Also as you say the HP can achieve 75C and can have a programmable offset of as much as 40C between flow temp and DHW target, it is in Installer Settings|Hot Water in this simulator.

[JohnMo]

1 hour ago, sharpener said:

Warmflow HP models show an external heat exchanger.

I spoke to them last year, if it wasn't connected to their heat pump they weren't interested and said it only designed for their heat pump. 

[JamesPa]

8 hours ago, sharpener said:

 

I doubt any improvement in CoP would repay the capital cost of fitting a new tank if it is not strictly necessary. @JamesPa would I am sure agree (name check in previous post doesn't seem to have worked).

Agreed.  

 

4 hours ago, mk1_man said:

Thanks all for the advice.  It is frustrating as my existing cylinder is a Warmflow solar 250l with twin coils, I currently have these plumbed in series on my gas boiler and the re-heat time is very quick however all of the MCS installers are saying that I must upgrade to a ' heat pump ' cylinder. 

And that ultimately is the problem.  It seems that most of the industry can't think beyond what it says in their plumbing by numbers manual.  It's disgraceful but sadly where we are.  There are a few who can but they are definitely (judging by what we hear here, and personal experience) a small minority.  This pushes up retrofit prices and disruption thus putting people off.

 

I found only one supplier who will offer the obvious (in cases where an increase in heat capacity really is necessary) ie to retrofit a phe and pump to the existing cylinder.  Thats Pete Miller of Conga.  However they concentrate on a very limited geographical area.

 

 

Edited April 11 by JamesPa

[JamesPa]

2 hours ago, JohnMo said:

3 hours ago, sharpener said:

Warmflow HP models show an external heat exchanger.

I spoke to them last year, if it wasn't connected to their heat pump they weren't interested and said it only designed for their heat pump. 

What on earth does 'designed for their heat pump' mean.  It's just a tank with water in it and a heat exchanger for goodness sake.  Do Warmflow heat pumps follow different laws of physics?  

Edited April 11 by JamesPa

[JamesPa]

Just to add there are afaik two considerations related to coil size.  The first is reheat time, the second is cycling as you approach the target temperature.  The latter will occur if the coil doesn't dissipate into the water at least the minimum output if the heat pump.  Obviously this problem is worse if the ft is low, and more or less eliminated if the hp can operate at a high ft like the Vaillant and other R290 models.

 

Here we get into the industry trap again...many suppliers oversize the heat pump by a large amount and as a result it won't modulate down as far as it could if correctly sized.  This clobbers efficiency of space heating, and drives a requirement for large coils in dhw cylinders and even 28mm primary feeds to the cylinder, even if the customer doesn't care too much about reheat time. 

 

So then the cylinder has to be replaced and floors ripped up to substitute entirely unnecessary hardware for existing perfectly functional components.  Of course it also enables the supplier to specify a pre-plumbed cylinder, which means that they can employ rookie plumbers whilst still charging top dollar for unnecessary work.

 

Edited April 11 by JamesPa

[sharpener]

42 minutes ago, JamesPa said:

even if the customer doesn't care too much about reheat time. 

 

I have twice had put to me the argument "but a subsequent purchaser of the house might".

 

In one case it was as noted to fit a new cylinder and in the other to justify putting radiators in the utility room FGS as well as the kitchen. As I told them, if a subsequent purchaser is going to remove the AGA it is likely to be in the context of a £20k kitchen refit so adding a rad or two is small change. And they might want skirting rads or fancoils in the kickspace under the units, who knows?

 

They tried to argue the utility room was habitable and therefore MCS required it to be heated (we have had some discussion on this and I see it was in yr submission to MCS @JamesPa).

[JamesPa]

7 hours ago, sharpener said:

 

I have twice had put to me the argument "but a subsequent purchaser of the house might".

 

In one case it was as noted to fit a new cylinder and in the other to justify putting radiators in the utility room FGS as well as the kitchen. As I told them, if a subsequent purchaser is going to remove the AGA it is likely to be in the context of a £20k kitchen refit so adding a rad or two is small change. And they might want skirting rads or fancoils in the kickspace under the units, who knows?

 

They tried to argue the utility room was habitable and therefore MCS required it to be heated (we have had some discussion on this and I see it was in yr submission to MCS @JamesPa).

All of which and more is why permitted development rights should not be linked to MCS so that the industry can move asap to the point where grants aren't necessary and the customer is what matters. 

 

The current situation is nothing short of deplorable.  Costs will come down once MCS get out of the way and some innovators move on.  Sadly I doubt it will happen because government is listening to MCS but not to informed of customers who don't have a voice.

Edited April 12 by JamesPa

[JohnMo]

8 hours ago, JamesPa said:

minimum output if the heat pump

You make the assumption that the heat pump modulates during hot water production, mine explicitly states it does not. It runs flat out to get DHW production done ASAP.

 

The warm flow cylinder has a modulating pump speed controlled by the heat pump I believe, same principle as mixergy.

[JamesPa]

13 minutes ago, JohnMo said:

You make the assumption that the heat pump modulates during hot water production, mine explicitly states it does not. It runs flat out to get DHW production done ASAP.

 

The warm flow cylinder has a modulating pump speed controlled by the heat pump I believe, same principle as mixergy.

You are right I did make that assumption because it seems obvious that they should and they already do so for space heating with a very simple feedback loop, so all the functionality is there.

 

If they don't, why not is the obvious question?  Perhaps those that aren't so fussy about coil sizes do modulate down?

Edited April 12 by JamesPa

[JamesPa]

11 minutes ago, JohnMo said:

The warm flow cylinder has a modulating pump speed controlled by the heat pump I believe, same principle as mixergy.

I assume you refer to the phe circulator pump.  What's the advantage of modulating it?

[JohnMo]

13 minutes ago, JamesPa said:

I assume you refer to the phe circulator pump.  What's the advantage of modulating it?

Yes referred to phe pump, not sure if the advantage but that's two companies that do.

 

This what my manual says about DHW heating. There is some modulation but not until 1 Deg of target temp.

 

My Atag boiler seemed to operate exactly the same as the heat pump, I noticed if you tried to heat a cylinder (thermal store with heating coil) at too low a set temperature against small coil, it just wouldn't get there instead it would just cycle a lot. Cylinder temperature would remain static, boiler would run for a minute overshoot target and shutdown for 10 mins. I over came the issue with a huge PHE. 

 

 

[JamesPa]

25 minutes ago, JohnMo said:

Yes referred to phe pump, not sure if the advantage but that's two companies that do.

 

This what my manual says about DHW heating. There is some modulation but not until 1 Deg of target temp.

 

My Atag boiler seemed to operate exactly the same as the heat pump, I noticed if you tried to heat a cylinder (thermal store with heating coil) at too low a set temperature against small coil, it just wouldn't get there instead it would just cycle a lot. Cylinder temperature would remain static, boiler would run for a minute overshoot target and shutdown for 10 mins. I over came the issue with a huge PHE. 

 

 

Interesting.  So what happens with a non modulating heat pump, I wonder, if the tank hasn't reached temp -1C but the heat transfer to the cylinder is less than hp output.  Is it the same as your atag - ie it cycles, or does the flow temp rise until there is sufficient heat transfer (obviously still at a ft less than the coolant temp).  I guess this depends on the hysterisis allowed on the ft?

 

Actually I think the last part of the extract you posted answers this.  The hp cycles if flow temp goes above 60.

 

So as long as there is sufficient heat transfer to the tank when ft=59 and tank is at the required temp, all should be well.  That sets a minimum coil/heat exchanger size.

Edited April 12 by JamesPa

[sharpener]

I have had a quite lengthy interchange with Vaillant tech about retaining an existing cyl on a 12kW installation and at no point have they said it will not modulate on HW, some of the calcs I shared with them explicitly assumed that it will turn down to 6kW (summer mode, no parallel heating of the thermal store).

 

Also there is the Noise Reduction mode you can set to be at the same times as the DHW, you can programme the output to be 40...60% of the full rating, see the charts here.

 

The rest of that thread is also quite relevant to this discussion as the starting point was the Vaillant 200l tank, which has only 1.4m^2 coil but the chart shows is compatible with the 12kW HP. Proportionality would therefore suggest a 7kW unit will go on a 0.75 coil and the OP's solar tank is at least twice that.

[JoeBano]

I’ve got a megaflow 180l with a small coil but is corrugated, heat pump never cycles hits max flow temperature of 56c to heat my tank to 50c takes average of 45 mins to hit desired temperature. Grant aerona3 10kw heat pump 

[JoeBano]

3 minutes ago, sharpener said:

I have had a quite lengthy interchange with Vaillant tech about retaining an existing cyl on a 12kW installation and at no point have they said it will not modulate on HW, some of the calcs I shared with them explicitly assumed that it will turn down to 6kW (summer mode, no parallel heating of the thermal store).

 

Also there is the Noise Reduction mode you can set to be at the same times as the DHW, you can programme the output to be 40...60% of the full rating, see the charts here.

 

The rest of that thread is also quite relevant to this discussion as the starting point was the Vaillant 200l tank, which has only 1.4m^2 coil but the chart shows is compatible with the 12kW HP. Proportionality would therefore suggest a 7kW unit will go on a 0.75 coil and the OP's solar tank is at least twice that.

I do have noise reduction activated on mine which will modulate mine to 60% so this does probably helps my setup.

 

[dpmiller]

CoolEnergy units modulate on DHW both by power and pump speed, and can do WC on DHW also

[JamesPa]

... yet 'the industry' for the most part insists on replacing functional dhw cylinders with 'heat pump cylinders'.  That's another £3k plus mate (more if they also replace 22mm primaries with 28mm).

 

Thank goodness that MCS, with it's  government sanctioned monopoly, is there to protect and ensure installation standards.

Edited April 12 by JamesPa

[sharpener]

1 hour ago, JamesPa said:

... yet 'the industry' for the most part insists on replacing functional dhw cylinders with 'heat pump cylinders'.  That's another £3k plus mate (more if they also replace 22mm primaries with 28mm).

 

Thank goodness that MCS, with it's  government sanctioned monopoly, is there to protect and ensure installation standards.

 

Well since we have to use them if we want the BUS grant I was agreeably surprised (as I re-started my quest today from the beginning(!)) that MCS has a useful database which lists ?all registered firms in increasing distance from your address, which you can filter by service required, and with links in most cases to the company website.

 

So in an hour I was able to contact five firms and one has already got back to me with a site visit for next week.

 

Original installers appear to have gone to ground completely.