System or Combi in local pub?

[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

 

[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.