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]

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

 

[ShaunCorbs]

no

So you have separation / a llh ?

 

[JohnHuson]

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.

To clarify, the extra 25/80 pump has been installed in the heat output line from the boiler so
it sucks from the boiler pump? Does that sound stupid? The boiler pump and the new pump send the pressure to the two zones via the 42mm pipework in the loft down to the rads below. I think that there was always a problem with the Concorde Boiler in getting the furthest rads hot.

 

[ShaunCorbs]

Yes so In series

 

[SJB060685]

To clarify, the extra 25/80 pump has been installed in the heat output line from the boiler so
it sucks from the boiler pump? Does that sound stupid? The boiler pump and the new pump send the pressure to the two zones via the 42mm pipework in the loft down to the rads below. I think that there was always a problem with the Concorde Boiler in getting the furthest rads hot.

The pumps as you describe are in series and this can cause a number of problems. Some means of separation is needed and I explained what that is in an above comment.
There’s only so much we can advise you through a website on a phone or laptop etc and we have. You need to contact several reputable firms both qualified and experienced in these scenarios and explain your problem and the advise you’ve had here.

 

[John.g]

To clarify, the extra 25/80 pump has been installed in the heat output line from the boiler so
it sucks from the boiler pump? Does that sound stupid? The boiler pump and the new pump send the pressure to the two zones via the 42mm pipework in the loft down to the rads below. I think that there was always a problem with the Concorde Boiler in getting the furthest rads hot.

Its certainly unconventional but I'd venture that there are systems with pumps installed like this and working OK, as pointed out above, two different head pumps in "series" may cause one to fight the other, they are really not in series in accepted pump jargon where both pumps are installed one immediately behind the other, but generally one would then install one pump with the required head.
In your case, the boiler circ pump pumps from the return, through the boiler Hx and then supplies the 8M pump.
I would suggest trying to establish what the flow rate is from other data to hand, the boiler flow and return temps and the boiler output, is all you need but these three values arn't often available from the boiler display but the flow/return temps can be measured on the pipes which will give some info at any rate, the boiler output is more difficult but if both zones are open then maybe can get a good idea. If that 8m pump displays its power in watts? (W) then flow rate can be derived exactly, from the pump curves.

 

[JohnHuson]

Excellent precise information, thank you and to all others who have contributed. Cheers, John.

 

[JohnHuson]

Thank you for all the responses. They are waiting for a Navien engineer to attend to replace
the heat exchanger under guarantee? Why does that need replacing?
In the meantime the boiler pressure can only be supplied if the cold water pressure supply is
being balanced by the pressure valve constantly sending cold water supply to the outflow
resulting in hundreds of gallons being wasted every day. I thought it was a local offence
to waste water? it is the only way he can get heating in the pub.
What a mess.

 

[John.g]

Are you saying that the Boiler PRV (safety valve) is lifting at 3 bar and discharging water which then has to be replaced with cold water?

 

[SJB060685]

Are you now saying the plate heat exchanger for HW has failed and the PRV is lifting?
If so then yes you will be losing water but hundreds of gallons a day I highly doubt.
This seems to be getting worse and worse. You need a reputable engineer to go over the whole system. I’m not going to read or comment on this thread further. We’ve given you the advice you need, you now need to get someone out who actually knows a thing or two.

 

[JohnHuson]

Are you saying that the Boiler PRV (safety valve) is lifting at 3 bar and discharging water which then has to be replaced with cold water?

Yes, stupid isn't it! They are going to replace the heat exchanger so it seems. Several seals have burst causing water leaks since installation. What mess. I find it difficult to accept the inept performance and lack of knowledge from the installers.