System or Combi in local pub?

[JohnHuson]

23 rads and one kitchen hot water supply plus two basins in toilets. Had an old Concorde system boiler which gave up.
Two zones supplying two large rooms 50' x 12' x 24' each. Local heating company recommended Nevien 42kw combi
boiler and things went wrong after that.
The installers have made excuses that the building needs the radiators, which worked fine before, at a cost of £1400
or they would disown the installation with much chattering of balancing the system etc. I have done that and vented a
number of times. ( I am an old fart who has worked and maintained many systems, but not the gas.) My problem boilers
were Worcester Bosch. I learnt to replace PCB parts at 30p from Maplins instead of paying £120 for a new board.
The boiler keeps blowing seals and is not happy with the system pressure.
Landlord was advised to install another pump as the heating took four hours to circulate to the lover zone radiators but
not to the extremities. The boiler has to lift the supply to the roof space and is a 40mm pipe work.
Its a mess and the Publican has lost customers owing to the lack of heating.
He has appealed to Nevien who have been helpful and initially said that the 42kw boiler will cope with the system.
They have since changed their position after their engineer changed the seals and said a system boiler was the best fit.
The original tank is still in place.
Nevien seem to be putting pressure on the local installers to sort things out but they are pratting around and making as many excuses
possible to avoid the issue.
Could you please advise me regarding the room volumes if a combi boiler should provide the heating system?

 

[ShaunCorbs]

heat load / capacity might be ok but too small of a pump and expansion vessel normally they have 10l onboard yours requires a min of 14 extra Litre’s of expansion eg 18l expansion vessel hence the problem with pressure and things going pop

25-80 pump required as the boilers own pump won’t have the capacity

 

[Last Plumber]

You need a commercial heating engineer to assess it properly in my opinion. There are a number of things that need looking at in order to decide what might be a suitable boiler.
The 23 rads could be any size and so could the pipework so you can't tell anything off that.
The building may be under 42 Kw but is that the heating output or output to hot water?

 

[SJB060685]

It’s hard to gauge a rooms heat loss knowing only the area. Yes a combi boiler sized correctly can happily heat your heating system, although by the sounds of it the system needs hydronically separating and having an additional pump.
My concern when considering what type of boiler to use know would be what’s your hot water demand like? A combi boiler supplies hot water to taps etc or energy to the heating system, not both at the same time. If your kitchen and basins are in constant/near constant use of hot water then I wouldn’t suggest a combi.

 

[JohnHuson]

Thanks to you both for the replies. The additional pump is the higher output 25/80 and has improved the circulation slightly but still taking a number of boiler hours to get warm water to 15 out of the 23 rads. the amount of domestic hot water is very little with no shower or bath, just two vanity basins and one kitchen tap. 95% of demand is the heating system.

 

[ShaunCorbs]

What does the boiler do with heating on temp wise stat around 40dc and rise slowly or straight to 60dc ?

Also the old boiler cxa?

 

[EvilDrPorkChop]

Your main issue is every time someone runs a tap, it'll switch the heating off then cycle for maybe 1-2 minutes. In a place like a pub it'll be being fired on HW every few minutes, so it'll never run on heating when needed.

Also agree regarding extra expansion and bigger pump. In all it's the wrong boiler though.

 

[JohnHuson]

Navien made contact today and stated it is the wrong installation and that the system
boiler is the desired spec. The original system tank is still there. The publican is prepared
to spend money to go for a system boiler replacement but is awaiting to hear from the
installers if they will take responsibility for the wrong installation of a combi to replace
a system boiler as what it was.

 

[JohnHuson]

The boiler takes at least four hours to heat the radiators but will only get warm with
the large rads nearest the boiler. The rads at the end of the zone remain cold and it is
not an issue of balancing or venting. Yes a 25/80 additional pump was installed and
improved things but not solved the problem.
Does anyone have a calculator to determine what KW is needed for the volume of the
rooms in question please?

 

[ShaunCorbs]

Size of rads is the easiest way but a Heatloss of the property is the correct and only guarantee way

 

[SJB060685]

The only sure fire way is as Shaun mentioned above, a proper survey of the property and calculate an accurate heat loss. There are quick cheat sheets available but they are only a guesstimate at best.
I appreciate you’re waiting to hear from the installer to see if they take responsibility and you are correct in doing so but I wouldn’t employ them to carry out the remedial work. The residual pump head of the pump is obviously not enough to reach the system extremities. An additional pump is and has been fitted but the system needs to be hydronically separated as well due to this additional pump. Separation is basically creating a primary and secondary loop each with their own pump or pumps. By separating the system this allows the pumps to work without having an influence on each other’s performance.

 

[JohnHuson]

Your main issue is every time someone runs a tap, it'll switch the heating off then cycle for maybe 1-2 minutes. In a place like a pub it'll be being fired on HW every few minutes, so it'll never run on heating when needed.

Also agree regarding extra expansion and bigger pump. In all it's the wrong boiler though.

Thanks I appreciate that. The pub does not have a busy trade and relies on Chinese takeaways so domestic hot water demand is minimal.

 

[JohnHuson]

The only sure fire way is as Shaun mentioned above, a proper survey of the property and calculate an accurate heat loss. There are quick cheat sheets available but they are only a guesstimate at best.
I appreciate you’re waiting to hear from the installer to see if they take responsibility and you are correct in doing so but I wouldn’t employ them to carry out the remedial work. The residual pump head of the pump is obviously not enough to reach the system extremities. An additional pump is and has been fitted but the system needs to be hydronically separated as well due to this additional pump. Separation is basically creating a primary and secondary loop each with their own pump or pumps. By separating the system this allows the pumps to work without having an influence on each other’s performance.

A most concise reply. I thank you.

 

[JohnHuson]

You nice people do a great job. Thank you.

 

[SJB060685]

Thanks I appreciate that. The pub does not have a busy trade and relies on Chinese takeaways so domestic hot water demand is minimal.

I thought this system sounded familiar. Hasn’t this problem been discussed several months ago?

 

[JohnHuson]

I thought this system sounded familiar. Hasn’t this problem been discussed several months ago?

Yes but not resolved. The old LPG Concorde system boiler was condemned and the local heating
company was contacted who suggested a Nevien combi boiler was the answer to the problem.
The installers have given many reasons why their £3000+ does not do the job.

 

[JohnHuson]

A most concise reply. I thank you.

The publican is prepared to spend money to get this fixed to do whatever is required to having a working heating system.

 

[SJB060685]

Yes but not resolved. The old LPG Concorde system boiler was condemned and the local heating
company was contacted who suggested a Nevien combi boiler was the answer to the problem.
The installers have given many reasons why their £3000+ does not do the job.

In my opinion a combi should never been suggested. Although you say the hot water demand is not high it is a pub/kitchen takeaway place and hot water will be needed regularly.
As for the heating problems you’re encountering I strongly suggest you have a thorough heat loss survey undertaken. When that is known then the boiler and system can be designed/specified.

 

[EvilDrPorkChop]

Worst case just use the combi as a system boiler, doesn't technically need the boiler replacing just the system reconfiguring.

 

[Scott_D]

Did you get any pictures?
Might need a low los header and external pump
Is it a one pipe system?
Is it a sealed system?

 

[JohnHuson]

Did you get any pictures?
Might need a low los header and external pump
Is it a one pipe system?
Is it a sealed system?

Hi, it is a two zone system feeding two large rooms. The heating rises to the loft space where the pipework is
42mm and vented in the two circuits, with two vertical fart valves. An extra 25/80 pump was installed close to the boiler
heating output but its a hell of a long line of piping and water volume to shift and get successful circulation to the ends
of the heating system? Thats the problem.
The 42kw boiler is capable of doing the job but in my simple logic it can not get the heat quick enough on demand then
circulate it around the system as required.
When the old Concorde boiler was in use the rads at the end of the line were only lukewarm. They have been balanced and vented may times.
Thanks for your help, John.

 

[JohnHuson]

Hi, it is a two zone system feeding two large rooms. The heating rises to the loft space where the pipework is
42mm and vented in the two circuits, with two vertical fart valves. An extra 25/80 pump was installed close to the boiler
heating output but its a hell of a long line of piping and water volume to shift and get successful circulation to the ends
of the heating system? Thats the problem.
The 42kw boiler is capable of doing the job but in my simple logic it can not get the heat quick enough on demand then
circulate it around the system as required.
When the old Concorde boiler was in use the rads at the end of the line were only lukewarm. They have been balanced and vented may times.
Thanks for your help, John.

In hindsight the old system only needed one pump to service the whole two zones, Now it has two.

 

[JohnHuson]

In hindsight the old system only needed one pump to service the whole two zones, Now it has two.

 

[ShaunCorbs]

Have the pumps been put in series though?

 

[SJB060685]

Have the pumps been put in series though?

He said the shunt pump is on boiler CH flow close to boiler? So yes in series.

@JohnHuson you need to know what the pressure loss across the index circuit is to size a pump or additional pump in this case. The index circuit is usually the pipework feeding the furthest rads but not always. If you had an idea of the length of pipework feeding the extremity rads, size and an approximate idea of the amount of fittings as well as an idea of radiator outputs you might be able to arrive at a reasonably close answer.

 

[JohnHuson]

Have the pumps been put in series though?

No, there is the pump in the combi boiler and a 25/80 pump installed in the heat output line to push the hot water to the central heating system. My limited knowledge says it needs one pump for each zone as suggested? Should the pumps be on the supply or return?

 

[SJB060685]

That would imply they’re in series and not parallel.
You say you’re confident the boiler is powerful enough and you might be right and a 25/80 pump is huge, should be more than enough. You also say this system has always been troublesome which leads me to question what size pipe work has been used. If you’ve always had problems getting energy to the furthest rads it might be the pipework is too small to carry the required load.

 

[Scott_D]

Are you able to post some pictures?
It’s going to help if you want suggestions on a way to improve things.
If it’s never worked properly it may need a repipe

 

[ShaunCorbs]

No, there is the pump in the combi boiler and a 25/80 pump installed in the heat output line to push the hot water to the central heating system. My limited knowledge says it needs one pump for each zone as suggested? Should the pumps be on the supply or return?

The problem is the boiler pump is adjustable and isn’t fixed also your boiler is limited in regards to lpm output so your external pumps needs separation which I think will fix your problem

 

[SJB060685]

The problem is the boiler pump is adjustable and isn’t fixed also your boiler is limited in regards to lpm output so your external pumps needs separation which I think will fix your problem

I’m not convinced separation will solve his problem. We both know it’s needed anyway but I still suspect there’s a problem on what would be the secondary side.
IF the boiler is suitably sized and separation is installed be it CCT’s or a LLH then there should be no mixing as the flow rates should be equal on primary and secondary. There has been no mention of UFH etc. A 25/80 is huge and I assume 8m head? That should be more than enough to cover even an abnormally large index circuit, yet they’re still complaining of Luke warm radiators at the moment.
I still maintain they should have the building and current pipework system surveyed. Without knowing the boiler output required and the current system configuration no engineer can accurately say this is the fix or that’s the fix. Once you have a picture of the current system (which I suspect was never designed properly at the start) then you can give them their options. There’s nothing saying the combi couldn’t stay in place and have a separate HW cylinder but with what little information we have all we are doing is just guessing, regardless of how experienced and competent I know we are.

 

[ShaunCorbs]

I’m not convinced separation will solve his problem. We both know it’s needed anyway but I still suspect there’s a problem on what would be the secondary side.
IF the boiler is suitably sized and separation is installed be it CCT’s or a LLH then there should be no mixing as the flow rates should be equal on primary and secondary. There has been no mention of UFH etc. A 25/80 is huge and I assume 8m head? That should be more than enough to cover even an abnormally large index circuit, yet they’re still complaining of Luke warm radiators at the moment.
I still maintain they should have the building and current pipework system surveyed. Without knowing the boiler output required and the current system configuration no engineer can accurately say this is the fix or that’s the fix. Once you have a picture of the current system (which I suspect was never designed properly at the start) then you can give them their options. There’s nothing saying the combi couldn’t stay in place and have a separate HW cylinder but with what little information we have all we are doing is just guessing, regardless of how experienced and competent I know we are.

Agreed but I suspect the boiler pump is fighting the fixed speed pump, the fixed speed pump is pulling the combi pump eg increasing the speed so the boiler is having a hard time as the water is flowing too fast through the hex or the other way round the boiler is strangling the 25-80

 

[SJB060685]

Agreed but I suspect the boiler pump is fighting the fixed speed pump, the fixed speed pump is pulling the combi pump eg increasing the speed so the boiler is having a hard time as the water is flowing too fast through the hex or the other way round the boiler is strangling the 25-80

Exactly the problem with pumps in series yes. A quick measure of the ΔT across the boiler would give you an idea of which.

 

[John.g]

In hindsight the old system only needed one pump to service the whole two zones, Now it has two.

Irrespective of the rad sizes, if they are not getting hot then no room heat.
You say 40mm piping, a system with a 42kw boiler with a rad dT of 12C will require a flow rate of 50LPM, 3.0M3/Hr, even if all the rads were located at the very end of a 50M run (100M total) the pipe friction losses are ~ 3M, a 8M pump will flow 50LPM @ a 6M head and assuming the boiler pump is similar to a UPS3, this pump will flow 50LPM @ a 2.8M head, maybe be worth looking at the boiler hx loss at a flow of 50 LPM, also condition of 40mm piping.

 

[John.g]

You need a commercial heating engineer to assess it properly in my opinion. There are a number of things that need looking at in order to decide what might be a suitable boiler.
The 23 rads could be any size and so could the pipework so you can't tell anything off that.
The building may be under 42 Kw but is that the heating output or output to hot water?

The NCB-40LWDE shows 40kw to hot water & 33kw to heating.
Also requires external E.vessel as only 6 liter internally.

 

[JohnHuson]

Have the pumps been put in series though?

no