Boiler advice

[Russtcm]

Hi, I am hoping there are some friendly folk on here that can help with a bit of advice.

I have a house with 18 rads, 3 bathrooms/shower rooms (one electric so effectively 2) and I have a conventional boiler set up at the moment.

I am wanting to change to a combi and I am looking at the Worcester 8000 range. Worcester themselves have recommended I match the boiler to my flow rate and if I have a high enough flow rate and can get the 50kw one then I should, as that will give me the best outcome should two showers be turned on at the same time.

Sounds straight forward enough but I am wondering if anyone actually has first hand experience of how these boilers perform when two showers are switched on? Does the 50kw manage two no problem or could I drop to the 40 or 45 and have pretty much the same experience?

They are apparently designed specifically for running two bathrooms at once but what’s the reality?

Trying to make sure before I make the switch. Thanks.

 

[ShaunCorbs]

What’s your mains flow and pressure ?

combis don’t like to run more than one outlet even with a 50kw that’s around 13-14lpm so less than 7 per outlet most showers require around 10lpm to be effective

 

[SimonG]

You need to be checking your flow rate, static pressure and dynamic pressure.

I dont care what manufacturers say, if it's more than one bathroom then it's not a combi for me.

 

[Russtcm]

What’s your mains flow and pressure ?

combis don’t like to run more than one outlet even with a 50kw that’s around 13-14lpm so less than 7 per outlet most showers require around 10lpm to be effective

Thanks for the quick response. I have ordered a flow cup to test the flow but using a jug for a rough estimate, I reckon it’s over 20lpm. The flow for the 50 is rated at 17.9lpm at 40 degrees. Presumably when mixed in with cold water in a mixer shower this then gives suitable flow for 2 showers when required?

Sorry if I’m not quite understanding but that’s why I’m here. Thanks.

 

[John.g]

40C is the temperature rise through the boiler.
If you are happy with a 40C showering temp then a 50 kw combi should give 20.46LPM mixed with cold water at 5C, 23.9LPM mixed with water at 10C and 28.64LPM mixed with water at 15C

 

[gmartine]

50kw, assuming your current gas meter is up to it also?

Don't listen to WB...they will sell you any old turd. If you're insistent on not having any large HW water storage then you should consider a storage combi like the Viessmann 111w or push the boat out with a 222F

 

[ShaunCorbs]

Thanks for the quick response. I have ordered a flow cup to test the flow but using a jug for a rough estimate, I reckon it’s over 20lpm. The flow for the 50 is rated at 17.9lpm at 40 degrees. Presumably when mixed in with cold water in a mixer shower this then gives suitable flow for 2 showers when required?

Sorry if I’m not quite understanding but that’s why I’m here. Thanks.

that’s the flow rate in summer winter will be a bit less would estimated the water temp at around 5 dc (worst case) so 18lpm at 45dc max temp if you want higher temp your flow rate has to slow / drop to around 15lpm for 60dc

 

[John.g]

I was assuming that the actual boiler output is 50kw, if not then forget my figures as they are based on this, a 40 kw storage combi will theoretically give the same flowrates but only for a minute or two.

 

[ShaunCorbs]

I was assuming that the actual boiler output is 50kw, if not then forget my figures as they are based on this, a 40 kw storage combi will theoretically give the same flowrates but only for a minute or two.

 

[John.g]

If they make the storage cylinder big enough they will then be almost as good as a heat only boiler, imagine just a 15 kw combi with a 300 litre storage cylinder, you still won't believe its output!.

Oil fired combis usually give the hot water capacity equivalent which is more useful IMO.

 

[Gasmk1]

we fitted one several years ago all 3 showers running and kitchen tap no problem

 

[Russtcm]

I watched the video. Thanks for the advice.

So that I understand, would a normal combi boost the pressure but the 111 won’t? In the video he mentions that it will only have the pressure as supplied from the road.

 

[Gasmk1]

no a normal Combi doesn't it just uses incoming mains same as

 

[gmartine]

I watched the video. Thanks for the advice.

So that I understand, would a normal combi boost the pressure but the 111 won’t? In the video he mentions that it will as only have the pressure as supplied from the road.

Nope neither "boosts" pressure as such but should provide you with a steady 20l/m across more than one outlet but it's still dependent on your incoming water pressure like any combi. The reheat on this unit is very fast so you shouldn't run short of hot water even during high demand. Also although you want something as compact as a combi this isn't it's about twice as large and as heavy but it is wall mounted like a combi.

 

[John.g]

This is how I see it,
Your 50kw boiler above will as stated give a continuous flow rate of 20.48 LPM at 40C from a cold water temperature of 5C. A 35 kw boiler non storage combi will give a flow rate of 14.33 LPM under the same conditions. The 111 above is, I think, a 35kw boiler with a (primary ) storage of 46 litres, I don't know the temperature but if 75C is assumed, then that 46 litres will supply 92 litres of water at 40C from cold water at 5C, if you want the same flow rate of 20.48 LPM at 40C then this 92 litres will last for 4.5 minutes, after that you either reduce the flowrate to 14.33 LPM to maintain the 40C showering temp or you leave the flow rate at 20.48 LPM and accept a showering temp of 29.5C.

 

[EvilDrPorkChop]

IMO storage combis are pointless, for the size, complexity of them and the limited amount of stored HW they can supply you then if you're having one of them you may as well have an Unvented cylinder.

OP - in reality will you ever use two showers at once?

I have an Unvented system and we can use two at once. In the 4 years of living here I think we've done it twice....

Combis are great if you haven't got the room, I don't always agree with the if you've got two bathrooms you need a cylinder - whilst it's always ideal to have an Unvented cylinder costs, space and installation issues don't always allow - in these situations a big output non storage combi will give you decent HW.

 

[Russtcm]

IMO storage combis are pointless, for the size, complexity of them and the limited amount of stored HW they can supply you then if you're having one of them you may as well have an Unvented cylinder.

OP - in reality will you ever use two showers at once?

I have an Unvented system and we can use two at once. In the 4 years of living here I think we've done it twice.***

Combis are great if you haven't got the room, I don't always agree with the if you've got two bathrooms you need a cylinder - whilst it's always ideal to have an Unvented cylinder costs, space and installation issues don't always allow - in these situations a big output non storage combi will give you decent HW.

That you all for your advice. It’s has been very helpful.

I think this reply sums up my situation. In reality it is only going to be the odd occasion where I need two showers running at once. Therefore I think a high output combi like the 8000 45 or 50 is probably going to be sufficient. If there happens to be two showers in use at once then they will still be satisfactory.

Presumably I will save money on gas and I will also be able to remove the old tanks to create more storage, so I think those pros outweigh the cons.

Would you agree with that thinking?

 

[EvilDrPorkChop]

That you all for your advice. It’s has been very helpful.

I think this reply sums up my situation. In reality it is only going to be the odd occasion where I need two showers running at once. Therefore I think a high output combi like the 8000 45 or 50 is probably going to be sufficient. If there happens to be two showers in use at once then they will still be satisfactory.

Presumably I will save money on gas and I will also be able to remove the old tanks to create more storage, so I think those pros outweigh the cons.

Would you agree with that thinking?

Yeah sounds about right.

As other's have said you need to check your water pressure, static and dynamic pressures and flow rates. Providing they are OK then it'd be worth you having a large output combi, I've seen people run two showers on a 40kw before now so a 45 or 50 should be fine. IMO they're probably a bit overkill, as your modulation range will be reduced for the heating however i'm not familiar with Worcesters new 8000 range so they may have a large modulation range now.

In regards to saving Gas, depends how long you're in the shower for! At 50kw it'll be using a far bit 😂

 

[Russtcm]

Yeah sounds about right.

As other's have said you need to check your water pressure, static and dynamic pressures and flow rates. Providing they are OK then it'd be worth you having a large output combi, I've seen people run two showers on a 40kw before now so a 45 or 50 should be fine. IMO they're probably a bit overkill, as your modulation range will be reduced for the heating however i'm not familiar with Worcesters new 8000 range so they may have a large modulation range now.

In regards to saving Gas, depends how long you're in the shower for! At 50kw it'll be using a far bit 😂

The 8000 45 model has Max heating output of 35kw a modulation ratio of 1:10 and therefore a min heating output of 3.5kw so it should be ok on the heating side Id imagine.

 

[gmartine]

We all have our favorites, I think if you trawled these boards WB wouldn't come out as one though for a variety of reasons but basically for the UK domestic market it's been more a triumph of marketing than performance/maintenance for them.

Seeing as your HW requirements aren't quite as what we assumed initially you might as well make the jump in quality to the 200w for around the same unit cost of just under £2K. It still knocks the spots off the 8000 from what I can tell, similar HW output flowrate but far superior heating modulation rate, stainless hex. etc controls and fully integrated weather comp which'll save you a bucket if you allow it.

 

[Russtcm]

We all have our favorites, I think if you trawled these boards WB wouldn't come out as one though for a variety of reasons but basically for the UK domestic market it's been more a triumph of marketing than performance/maintenance for them.

Seeing as your HW requirements aren't quite as what we assumed initially you might as well make the jump in quality to the 200w for around the same unit cost of just under £2K. It still knocks the spots off the 8000 from what I can tell, similar HW output flowrate but far superior heating modulation rate, stainless hex. etc controls and fully integrated weather comp which'll save you a bucket if you allow it.

The only thing about that is that it doesn’t have the flow rate of the 8000. So if I do have an occasion where two showers are on it won’t be as good presumably.

 

[John.g]

32kw I think, so 13.1 LPM from 5c to 40C.

 

[Russtcm]

32kw I think, so 13.1 LPM from 5c to 40C.

32kw heating and 34kw dhw. I have 18 rads so is this not a tad light on the heating side?

This is what I thought I was just getting. So is 34kw dhw going to run two showers satisfactorily? Or do I need the extra 10 kw for when that situation arises?

 

[gmartine]

32kw I think, so 13.1 LPM from 5c to 40C.

More than that last time I checked _more like over 16lpm and doesn't require huge power requirements because of hex efficiency.

https://webapps.viessmann.com/vibooks/api-internal/file/resources/technical_documents/GB/en/VDP/6154113VDP00001_1.PDF?#pagemode=bookmarks&zoom=page-fit&view=Fit

 

[gmartine]

32kw heating and 34kw dhw. I have 18 rads so is this not a tad light on the heating side?

This is what I thought I was just getting. So is 34kw dhw going to run two showers satisfactorily? Or do I need the extra 10 kw for when that situation arises?

Do you have the gas supply/meter to even feed a 50kw combi otherwise you're wasting your time considering one?

 

[Russtcm]

Do you have the gas supply/meter to even feed a 50kw combi otherwise you're wasting your time considering one?

I’m looking at the 45kw Max now and would get it checked obviously prior to install.

What would be great is a video of two showers running off of the various boiler outputs with the correct pressures and flow rates so that consumers know what the reality actually looks like.

 

[John.g]

32kw heating and 34kw dhw. I have 18 rads so is this not a tad light on the heating side?

This is what I thought I was just getting. So is 34kw dhw going to run two showers satisfactorily? Or do I need the extra 10 kw for when that situation arises/

Attachment above says 32kw to heating and 29.3 to HW, it doesn't matter how efficient the boiler is, if its output to HW is 29.3kw then you can only get 12 LPM continuously from 5C to 40C. If its a storage boiler, then depending on the store size you can 16LPM or higher but for a limited period, I would like to see how they arrive at this 16 LPM number.

If you have these energy saving showers installed which limits the flow rate to 6 LPM, then, yes, just about possible to run two showers, I would measure the shower flow rate as is and you can then decide, also remember even if you can install a 50 kw boiler (gas supply etc, as pointed out) which gives 20.38 LPM, you will then have to restrict the showers to 10 LPM even if only one in service to allow both in use on the rare occasions you require them, so measure the present flowrate which will decide things, one way or the other.

CH output is probably OK especially if you have zoning.

 

[John.g]

I’m looking at the 45kw Max now and would get it checked obviously prior to install.

What would be great is a video of two showers running off of the various boiler outputs with the correct pressures and flow rates so that consumers know what the reality actually looks like.

The reality is exactly (IMHO) as I have shown for the various boiler outputs, it cannot be otherwise. If you ran 5 showers off this ( 29 kw) boiler then the flow rate will be restricted to 2.4 LPM, each, not very pleasant.

 

[IND_Nick]

It is definitely impressive the flow rates large combis and storage ones offer.
My only consideration would be where do you go once they start to withdraw natural gas.(I appreciate that it may still be 10 yrs plus off.) Apparently other options produce considerably less energy and we may all have to look at stored hot water options and much more efficient homes. Just a thought, but may not be relevant on this decision.

 

[Scott_D]

Unless you had identical showers, flow and pressure in your home it would be pointless

What would be great is a video of two showers running off of the various boiler outputs with the correct pressures and flow rates so that consumers know what the reality actually looks like.

If you have limited space, budget or are happy To live with its limitations go for a combi.
If not - boiler and unvented cylinder.

 

[gmartine]

Current Viessmans

It is definitely impressive the flow rates large combis and storage ones offer.
My only consideration would be where do you go once they start to withdraw natural gas.(I appreciate that it may still be 10 yrs plus off.) Apparently other options produce considerably less energy and we may all have to look at stored hot water options and much more efficient homes. Just a thought, but may not be relevant on this decision.

All Viessmann 200 & 300's are H2 ready or basically ready to run on a part hydrogen gas mix.

There's a 45kw Viessmann 200, it's semi commercial and almost double the cost of your WB 6000 at £3.5k but offers large flow rates.

 

[John.g]

I’m looking at the 45kw Max now and would get it checked obviously prior to install.

What would be great is a video of two showers running off of the various boiler outputs with the correct pressures and flow rates so that consumers know what the reality actually looks like.

Couldn't attach a file so here are a few numbers that may help you to choose your combi.

​	​	​	​	​	​	​	​	​
​	​	​	Continuous	​	​	Required	​	Recovery​
Power​	Mains​	HW​	Flow​	​	Stored Vol​	Flow​	Duration​	Time​
Output​	Temp​	Temp​	rate​	​	at 75C​	rate​	Period​	to 75C​
Kw​	DegC​	DegC​	LPM​	​	Litres​	LPM​	Mins​	Mins​
30​	5​	40​	12.3​	​	45​	16​	5.6​	3.7​
30​	10​	40​	14.3​	​	45​	16​	6.1​	3.7​
​	​	​	​	​	​	​	​	​
35​	5​	40​	14.3​	​	45​	18​	5.0​	3.1​
35​	10​	40​	16.7​	​	45​	18​	5.4​	3.1​
​	​	​	​	​	​	​	​	​
40​	5​	40​	16.4​	​	​	​	​	​
40​	10​	40​	19.1​	​	​	​	​	​
​	​	​	​	​	​	​	​	​
45​	5​	40​	18.4​	​	​	​	​	​
45​	10​	40​	21.5​	​	​	​	​	​
​	​	​	​	​	​	​	​	​
50​	5​	40​	20.5​	​	​	​	​	​
50​	10​	40​	23.9​	​	​	​	​	​

 

[Brambles]

Over the last 2 months we have, at the customers request, installed 3 large ( 40kw plus ) combi boilers. Only one ( customer) is happy with the performance delivered.

I would add that none of the above are the WB 50kw offering, but only because the existing gas supply was not adequate

 

[John.g]

What is your suggested largest Combi from a gas supply point of view?.

 

[Brambles]

35kw

 

[Russtcm]

35kw

On this basis I hope you can see my conundrum. I have had 3 different boiler engineer come to my house and tell me 3 completely different things, which is why I am researching it myself.

So based on the likelihood of limited gas supply (yet to be determined) would you suggest a storage combi, if sticking to a no tank solution?

What were the customers not happy with? Flow rates?

 

[gmartine]

If you haven't got the gas rate ie the energy to heat the water fast enough then you won't get the HW flow rate, any engineer who suggests just sticking in a high powered combi without a gas meter upgrade should basically be ignored.

 

[EvilDrPorkChop]

If you haven't got the gas rate ie the energy to heat the water fast enough then you won't get the HW flow rate, any engineer who suggests just sticking in a high powered combi without a gas meter upgrade should basically be ignored.

Why would you need a gas meter upgrade? A standard meter provides 6m3/hr gas. Providing there’s no other appliances or just a small one such as a hob then no upgrade is required.

Obviously your pipe work has to be correctly sized, but I don’t see an issue and I’ve personally never had any issues in requiring the correct gas rate.

The issue would be more with your incoming water supply, on 15mm pipe work. If you’re wanting to require the correct flow rates you should be running a 22mm water main to the combi and 22mm off.

 

[gmartine]

It's tight isn't it, yes a G4 has max capacity of 6m3 but really it's 4m3 with loss plus as you say you'd expect other appliances to be online simultaneously. Also you can't dismiss Bramble's experience with installing oversized boilers using a standard domestic meter. It doesn't seem that you get the real world performance you hope and pay for.

 

[EvilDrPorkChop]

It doesn't seem that you get the real world performance you hope and pay for.

From my experience it isn’t that you haven’t got enough gas, it’s that the water pressure and pipe sizing which won’t give you the required flow rates to even get the figures they suggest.

For instance we’ve fitted a 35kw combi in a refurb, we’ve done a 22mm water main and 22 to outlets @ 2.5 bar it easily does 15lpm to 55oc. If that’d been 15mm pipe work I doubt we’ve be even getting 12.

 

[Brambles]

Of the three recently fitted, two were in a system to our design and one was a customer free issue. In the later case, on the day, the Customer provided a much larger boiler than they originally proposed - so it could no longer be concealed within the kitchen fitments.

On one if the systems to our design, we made a mistake with the incoming flow rate. We measured it at 30lpm, but in hindsight we think that Thames Water were working on a repair and the flow was artificially high - in reality it is 20lpm, which is too low to deliver the blended hot water to two showers. We will fit a pumped accumulator in the New Year.

My experience of large domestic - 40kw plus boilers are that in general terms they need to within 20m of the incoming gas supply - with the majority of the gas pipework in 35mm and you need incoming flow rates of at least 30lpm - that is purely my view from getting an installation that delivers at low risk of non performance. My go to solution for non performing Combis is a pumped accumulator.

From the Customer perspective - the normal comments are - it is much noisier that my old boiler and it is a lot bigger than I expected.

I try to treat large Combi’s as I do air source heat pumps and take the Customer to see an operating installation first. For airsource around 50 percent decide against it on the grounds of noise.

I hope these comments help

From an installer perspective - if they cannot tell you on the first visit about supply suitability (gas) and routing, go elsewhere. Similarly on incoming flow rates, it needs to be above the boiler spec with a decent margin.

Please don’t read this as being against Combis. I am not, but from an installers perspective I don’t want disappointed customers

 

[John.g]

On this basis I hope you can see my conundrum. I have had 3 different boiler engineer come to my house and tell me 3 completely different things, which is why I am researching it myself.

So based on the likelihood of limited gas supply (yet to be determined) would you suggest a storage combi, if sticking to a no tank solution?

What were the customers not happy with? Flow rates?

One of the recommendations above is a Viessmann 111W. Based on what I read, this has a 45 liter storage tank and has a extra circ pump that circulates cold water through the heat exchanger and into the top of the cylinder so assuming this cylinder is heated to 60C then this boiler will (by my calcs) give the equivalent of a 160 liter conventional hot water cylinder at 60C so should supply 20 LPM at a deltaT of 35C for around 12 /13 minutes, before reheating required which should take around 5 minutes. Or, will supply 14.33 LPM continuously.

 

[Russtcm]

One of the recommendations above is a Viessmann 111W. Based on what I read, this has a 45 liter storage tank and has a extra circ pump that circulates cold water through the heat exchanger and into the top of the cylinder so assuming this cylinder is heated to 60C then this boiler will (by my calcs) give the equivalent of a 160 liter conventional hot water cylinder at 60C so should supply 20 LPM at a deltaT of 35C for around 12 /13 minutes, before reheating required which should take around 5 minutes. Or, will supply 14.33 LPM continuously.

Yeah it’s on my shortlist but the 8000 45 has a similar flow rate and is a much neater unit. Yes it will need more gas but assuming I have the required gas flow then I think the 8000 is probably my preferred choice.

 

[Russtcm]

Ok I have chosen my boiler. Thank you very much for all your help. It was much appreciated and one of the highest response levels I have ever had on any forum!

My next question is on radiators. All mine seem to get nice and hot with my current 35 year old potterton but I suspect most of them are the same age. Some will be newer from 2001 as there was an extension done around then but still old obviously.

Should they be changed as a matter of course at the same time?

 

[ShaunCorbs]

That’s personal preference but you will have to change them later on depending on age can’t say how long they will last could do another 10 years might do 1 year if there functionally I would leave aslong as they heat the rooms

 

[Russtcm]

That’s personal preference but you will have to change them later on depending on age can’t say how long they will last could do another 10 years might do 1 year if there functionally I would leave aslong as they heat the rooms

I would probably prefer to do them at a different time from the boiler. Maybe in the summer so that I don’t have a double whammy with the cost. A good flush and a filter installed should protect the new boiler I guess.

 

[Russtcm]

Probably obvious but do combi boilers modulate on the hot water side as well as the heating. Presumably they don't use all of their "power" all of the time on that side either, if only a tap is lightly turned on?

 

[John.g]

Yes, AFAIK they do but normally a flow switch on the DHW side is arranged to to cut out the boiler or something like that corresponding to the minimum turn down of the boiler, the reason being I presume to prevent the HW running hot and cold with a very low flow demand and possibly for boiler Hx protection as well.

 

[Russtcm]

Ok. So I have had my 8000 45 combi installed and there has been no issue with either the gas supply or the water flow as both exceed requirements where I am. My showers are transformed into amazing power showers, really cannot believe the difference, so the hot water side is flawless.

On the heating side, I have a query for anyone that has specific 8000 experience...my heating side is set to 75 degrees and it seems to build up to it in stages. So the house was freezing after the install down in single digits centigrade. My expectation was that the boiler would heat up to 75c on heating side and hold it there until the thermostat in my hallway cut it off at the desired temp. However it seems to heat up to 50 odd, then 60odd and I have seen it go into the 70s once (not standing looking at it all day). The plume stops in between so it is going off...should it do that?

The radiators seem to get hot and then cool and then hot again, rather than it just remaining even. As the day is going on, it might be getting better but not sure yet so thought I would ask if any of this make sense?

 

[John.g]

Your expectations should be met.
Looks as if the boiler is cycling, can you open up all your rads, zone valves, TRVs, whatever fully, and see the boiler reaction, also can you see what the minimum output is on CH,. Even though you shoiuldn't have to do it IMO, you may be able to range rate the boiler on CH to 25kw or whatever the total rad output is, what does your installer say?.

 

[Russtcm]

Looks as if the boiler is cycling, can you open up all your rads, zone valves, TRVs, whatever fully, and see the boiler reaction, also can you see what the minimum output is on CH,. Even though you shoiuldn't have to do it IMO, you may be able to range rate the boiler on CH to 25kw or whaever the total rad output is, what does your installer say?.

The boiler has a 35KW output on the heating side but can go as low as 5.1KW and I have 18 rads, so this shouldn't be an issue, I wouldn't have thought.

I called Worcester this morning and they sent out their engineer but tbh, he didn't even know how to get into the installer setting on the LCD display. He hadn't seen one of the 8000 yet and I had to show him how to access and change the settings. He said that there are no error codes so it is fine but given he didn't know the first thing about the boiler, I am not convinced.

Is it possible, given the modern nature of this boiler, that it is "learning" what my house requires in terms of output and will settle over a few days?

 

[John.g]

The boiler should fire up and remain running until either heating demand satisfied or boiler minimum output of ~ 5kw is greater than the heating load, in which case the boiler will cycle, I would get the installer back and see if the pipe work etc is connected properly. Surely there were some commissioning checks carried out.

 

[Russtcm]

The boiler should fire up and remain running until either heating demand satisfied or boiler minimum output of ~ 5kw is greater than the heating load, in which case the boiler will cycle, I would get the installer back and see if the pipe work etc is connected properly. Surely there were some commissioning checks carried out.

The installer did do the commissioning checks and it was him that said something was wrong initially. He wanted to get Worcester out to see it as he feels it’s a fault with the boiler and has been told there is a software fault that can cause this. However the reset that normally fixes it, is not doing so in my case.

As it was late in the day yesterday, I called Worcester this morning.

Think I’m going to need to call them again and just say they need to send someone that actually has experience of this boiler.

Pretty poor that one of their own engineers doesn’t know how to access the engineer section on the boiler if you ask me.

 

[Owen__]

Hi Russtcm,

Did you get this issue resolved - I have some similar issues with a WB 8000?

 

[Russtcm]

Hi Russtcm,

Did you get this issue resolved - I have some similar issues with a WB 8000?

Yes I did. I spent a lot of time watching the boiler in the service menu to see what mode it was going into. It turned out that it was going into hot water mode every time that anyone or any appliance used cold water in the house. I have very high water pressure and flow where I am and the conclusion was that the back pressure when the cold water was switched off was activating the hot water mode and therefore interrupting the heating. The solution was to fit a non return valve and damper.

Since then it has worked perfectly.