2 condensing boilers

[mysteryman]

Forgive me for this - but the last two are getting their definitions wrong.

Secondary circulation means a pumped circulation of domestic hot water to keep instant hot water at all taps.

The device between a primary circuit for one or more boilers and the various heating circuits and cylinder primaries is called a low loss header. These are not, contrary to popular belief, just welded up boxes with connection sockets on them. There is much more to their design and construction if they are to work properly.

Reverse-return connections are used when you connect several devices in parallel - you take the return from the far end, and bring it back from there with the flow. This keeps the branch lengths to and from each device the same, so the flow rates should be the same.

 

[Onetap]

Forgive me for this - but the last two are getting their definitions wrong.

Secondary circulation means a pumped circulation of domestic hot water to keep instant hot water at all taps.

The term is used in that context,the primary (heat input) and secondary (distribution) sides of a heat exchanger. It's also used to mean heat input & heat distribution circuits with hydraulically disconnected circuits sharing the same fluid. The term is similarly used for chilled water pipework systems.

Might I suggest that you put the term "primary/secondary pumping" into your search engine and report back on what you find?

The device between a primary circuit for one or more boilers and the various heating circuits and cylinder primaries is called a low loss header. These are not, contrary to popular belief, just welded up boxes with connection sockets on them. There is much more to their design and construction if they are to work properly.

Stunning. What more is there to them, other than the fact that there is low/negligible frictional pressure loss due to flow along the length of the pipe? It's also called the 'common pipework' since it is shared by both primary and the secondary circuits. I've designed lots of them and soldered and brazed a few too.

 

[looneyfitter]

Forgive me for this - but the last two are getting their definitions wrong.

Secondary circulation means a pumped circulation of domestic hot water to keep instant hot water at all taps.

The device between a primary circuit for one or more boilers and the various heating circuits and cylinder primaries is called a low loss header. These are not, contrary to popular belief, just welded up boxes with connection sockets on them. There is much more to their design and construction if they are to work properly.

Reverse-return connections are used when you connect several devices in parallel - you take the return from the far end, and bring it back from there with the flow. This keeps the branch lengths to and from each device the same, so the flow rates should be the same.

Its the same thing... We all know what were talking about. No need to be a smart arse.....

Reverse- Return, Reverse Header, whats the difference? I have had numerous boiler house plans forwarded to me from various designers/architechts (spelling?) calling them as above but the design is the same.

 

[Biggles..]

It's been installed exactly as Keston advise,the manual gate valve is in place of the auto bypass..

No. It's a primary/secondary system. The boiler pumps circulate water in the primary pipework, the secondary circulation is dealt with by the secondary pump. The ABV is not there because it isn't necessary; there will always be primary flow through the boilers even if the secondary circulation is completely stopped by all TRVs being shut.

3rd pump or system pump as it's being used is there for belts and braces

It's the secondary circulation pump.

thats complete tripe.

its been installed exactly as the book,the gate valves not there to make it into a primary circuit at all,if it was youd have shown the keston diagram for connecting their boilers into a LLH.In stead the boilers are piped reverse return.

For what ever reason,probably cost, the gate is there instead of the ABV shown in the installation drawings,which as you are probably aware has to be the same size or larger than the boiler connections or the heat exs warp and split.

If the gate wasnt there and and all TRVs shut there would be no flow what so ever,boiler pumps would dead head.If it was connected via a LLH then i would agee that youd always have primary flow.

The lack of CV's(cant see them) will at worst probably cause some reverse circulation through the boilers but hey ho.

As for the external pump,its not a secondary pump, its just as Keston call it a system pump.

Id presume that the boilers are set with the stats at slightly different temps so that both fire initially then one satisfies and shuts down leaving one to run alone.

View media item 15353
thats 2 kestons pipe onto common rails into a LLH,deffinately nothing like what the OP has.

 

[Norcon]

For what ever reason,probably cost, the gate is there instead of the ABV

But if an ABV was fitted all the secondary flow created by the secondary circulator (or system pump as Keston call it) will be forced through the primary circuit.

If say boiler 1 was firing then boiler 2 will be adding a diluted flow temperature to the primary flow at the T where the common primary flows are connected together due to the influence of secondary circulator.
Thats what I see happening any way. But I could be wrong. I was wrong once before. LOL

 

[Biggles..]

Ignore secondary and primary it's not piped or deigned that way.

If there was just one boiler would you still be saying it,there could be 10 boilers it makes no difference,the abv is there to stop the pumps dead heading if all valves close,exactly the same as any bog standard heating system.The gate valve isn't there to effectively make it two loops.

 

[looneyfitter]

It's been installed exactly as Keston advise,the manual gate valve is in place of the auto bypass..

No. It's a primary/secondary system. The boiler pumps circulate water in the primary pipework, the secondary circulation is dealt with by the secondary pump. The ABV is not there because it isn't necessary; there will always be primary flow through the boilers even if the secondary circulation is completely stopped by all TRVs being shut.

3rd pump or system pump as it's being used is there for belts and braces

It's the secondary circulation pump.

thats complete tripe.

its been installed exactly as the book,the gate valves not there to make it into a primary circuit at all,if it was youd have shown the keston diagram for connecting their boilers into a LLH.In stead the boilers are piped reverse return.

For what ever reason,probably cost, the gate is there instead of the ABV shown in the installation drawings,which as you are probably aware has to be the same size or larger than the boiler connections or the heat exs warp and split.

If the gate wasnt there and and all TRVs shut there would be no flow what so ever,boiler pumps would dead head.If it was connected via a LLH then i would agee that youd always have primary flow.

The lack of CV's(cant see them) will at worst probably cause some reverse circulation through the boilers but hey ho.

As for the external pump,its not a secondary pump, its just as Keston call it a system pump.

Id presume that the boilers are set with the stats at slightly different temps so that both fire initially then one satisfies and shuts down leaving one to run alone.

View media item 15353
thats 2 kestons pipe onto common rails into a LLH,deffinately nothing like what the OP has.

WOW ive got pics of a job i did somewhere that looks remarkably like the pic above

 

[Biggles..]

That was near Manchester,but I wouldn't put your name to it as it was a bag of sh...
There's something quite significant missing from the primary side that's caused no end of issues.plus numerous other things .

 

[Norcon]

The gate valve isn't there to effectively make it two loops.

Then why is it there?
Kestons diagram posted by Onetap shows an ABV but their assumption is that the boiler pumps are capable of meeting system demand.

A "system pump" was added to the OP's arrangement.

Are you saying you would leave the ABV in place with such an arrangement?

 

[Biggles..]

Read the foot note at the bottom of the drawing that tells you about adding another pump and what it's called.

 

[Norcon]

Read the foot note at the bottom of the drawing that tells you about adding another pump and what it's called.

I have ......

The System resistance is within the spare capacity of the boiler pumps. Otherwise
consideration should be given to the use of an additional System pump.

Would you leave the ABV in place with the OP's arrangement?

 

[Biggles..]

Not where it is,in my opinion it should be after the system pump.

It's just a standard setup,nothing special.

 

[mysteryman]

A low loss header connected to a condensing boiler needs to be designed so as to prevent return temperature raising. It is not just a box or length of tube.

I advocate the use of weather compensation because it is the simplest and best form of control, especially for condensing boilers.

Mock on, lads, if you must, but please don't mislead the DIYers.

 

[ajrobb]

Id presume that the boilers are set with the stats at slightly different temps so that both fire initially then one satisfies and shuts down leaving one to run alone.

Should I run the boilers at different temperatures?

OK, at least one boiler must be set high enough to satisfy the HW thermostat (say 65°C). Can I turn one boiler off in the summer or should I just turn it right down to keep its pump running?

 

[Onetap]

It's been installed exactly as Keston advise,the manual gate valve is in place of the auto bypass..

No. It's a primary/secondary system. The boiler pumps circulate water in the primary pipework, the secondary circulation is dealt with by the secondary pump. The ABV is not there because it isn't necessary; there will always be primary flow through the boilers even if the secondary circulation is completely stopped by all TRVs being shut.

3rd pump or system pump as it's being used is there for belts and braces

It's the secondary circulation pump.

thats complete tripe.

its been installed exactly as the book,the gate valves not there to make it into a primary circuit at all,if it was youd have shown the keston diagram for connecting their boilers into a LLH.In stead the boilers are piped reverse return.

Tripe? You think so?

So you didn't put "primary/secondary pumping" into Google as I'd suggested? If you had, you might have found something like this, courtesy of Raypak;

http://www.raypak.com/Raypak_GetCad.cfm?File=8358.pdf

The gate valve is there to allow the system to be used in the event of the failure of either a primary or the secondary pump. Closing it will make the system operate with the working primary/boiler pumps (of which there could be 0, 1 or 2 operating) connected in parallel, to operate in series with the secondary pump. I can tell you, from experience, that doesn't work very well. The gate valve should be wide open; it is not essential.

The main problem with the system you're suggesting, lcgs, is that you would be trying to stuff the flow produced by two boiler pumps (probably Grundos 15-50s with stupid fittings so you have to buy replacements at extortionate prices from the boiler makers) through one system pump (probably a 15-50). Common sense should tell you that that is not a good plan.