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?

 

[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.