System or Heat Only Boiler

[John.g]

A (if) CH system has a very high head loss due to improperly sized pipework and long runs and possibly partially blocked pipes and requires a UPS2 pump running at its 8M head to feed 21 rads with a theoretical output of 25kw at a 20C dT.
A fairly old Vaillant 438 boiler is being replaced, the boiler is range rated to 24kw so a 25kw replacement boiler should be adequate.
One choice is a 25kw system boiler but because of the very high head required which will still, more than likely be ~ 6M after the system is cleaned, then a LLH or CCts will be required as the max head after passing through the (system) boiler HEX will probely only be ~ 2M at its rated flow rate of 17.92LPM, 1075LPH, also the internal bypass has a maximum setting of 4M, (before the HEX).
The other choice is to install a 25kw heat only boiler and use the existing UPS2 pump and the existing ABV. Question is, assuming a 8M head is still required (even though unlikely) at the rated flow rate, does this cause any problems for the boiler HEX, etc?.

 

[ShaunCorbs]

Either it all depends if the hex can take the rated flow and pressure loss (at that flow)

Me personally would go system and ccts with a 1-8 pump externally

 

[John.g]

There's also UFH (with its own manifold pump) + HW cylinder, would that entail 3 pumps in all, two new, one each for the UFH & the HW cylinder + the existing 8m for the CH?.

 

[ShaunCorbs]

Yes if using a system boiler or you could do one modulating pump and one of the ufh etc

 

[John.g]

Is there a particular way of arranging these flow/return Ts to avoid undesirable mixing?.

 

[ShaunCorbs]

Do it like a header system and cct the primary if you want I can draw it out etc

 

[John.g]

Do that, please Shaun, if you don't mind.

 

[Timmy D]

Sounds like the system is primed for a LLH.

 

[ShaunCorbs]

Do that, please Shaun, if you don't mind.

eg something like this

 

[John.g]

That looks like a conventional system to me.

 

[Timmy D]

That looks like a conventional system to me.

Usually the primary flow and return wouldn’t be linked. The boiler pump would serve all branches and could only get back to the return through a branch.

With Shaun’s diagram, the boiler pump, pumps in a loop around the primary flow/return.

There are 2 close tees as take offs feeding separate loops, each with separate pumps.

 

[John.g]

I thought the CCT arrangement in the context I mean is that it works like a LLH where the primary water can "circulate" via its own internal pump, it still provides the HW to the other circuits which circulate at their own required rates, in the event of say just the UFH being on then it can also provide the necessary recirculation required to avoid the burner tripping on high dT, thats the theory at any rate, seems popular in some countries but maybe for different reasons than acting as a cheap LLH.

 

[Timmy D]

I thought the CCT arrangement in the context I mean is that it works like a LLH where the primary water can "circulate" via its own internal pump, it still provides the HW to the other circuits which circulate at their own required rates, in the event of say just the UFH being on then it can also provide the necessary recirculation required to avoid the burner tripping on high dT, thats the theory at any rate, seems popular in some countries but maybe for different reasons than acting as a cheap LLH.

It is just a cheap LLH. Bigger pipe is obviously better

 

[ShaunCorbs]

I thought the CCT arrangement in the context I mean is that it works like a LLH where the primary water can "circulate" via its own internal pump, it still provides the HW to the other circuits which circulate at their own required rates, in the event of say just the UFH being on then it can also provide the necessary recirculation required to avoid the burner tripping on high dT, thats the theory at any rate, seems popular in some countries but maybe for different reasons than acting as a cheap LLH.

You could do that but you would need port valves and not the best practice your better off with a 4 pipe boiler eg Viessmann

 

[John.g]

Looks like either a system boiler with a LLH then or just replace the existing heat only boiler with a 25/30kw unit, the high pump head shouldn't be felt by the boiler HEX as the pump is pumping from the flow through the CH etc and then through the boiler HEX so the HEX entry head shouldn't be much more than normal and in the event of anticycling or pump overrun will be throttled right down by the ABV?.

 

[Daniel smith]

If you install a 25 kW system boiler with a high head requirement, there may be issues with the boiler's heat exchanger due to the lower maximum head capabilities. Opting for a 25 kW heat-only boiler and using the existing pump and bypass valve may be a better choice to avoid potential problems with the heat exchanger. However, it's important to address the underlying issues of improperly sized pipework and blockages in the CH system for optimal performance. Consulting with a professional is recommended for a thorough assessment and solution.

 

[John.g]

Any gas boiler's HEX can stand a static head of 30M, 3.0bar, because the PRV (pressure relief valve) is set to this, the question is, what is the maximum differential head they can tolerate, most will have 2.5M to 3.5m dP at their rated output at a dT of 20C. The boiler that one of these will be replacing is a (Vaillant) 438 ( heat only) which has run for years with a 8M pump, but this doesn't mean that the boiler HEX was operating at a dP of 8bar or anywhere near it as the dP is spread across both the CH system and the boiler HEX, if the pump is on the boiler flow then even less of a problem as the remaing head after a external ABV should be even less)

Even if the system (Vaillant) boiler is capable of withstanding a very high dP its a bit academic because its circ pump head (of 6/7M) is only capable of giving its max flow rate (of 17.92LPM for a 25kw boiler) at a dP of ~ 4.0/4.05M so depending on the boiler HEX dP will result in very little remaining head to drive it through the system., hence the requirement for LLH or the like if high flowrates are required. Vaillant system boilers as well have a internal bypass (ABV) that can only be adjusted to a max setting of 4M so again, (if I am interpreting correctly how it works) if a high system dP is required at even modest flowrates then the pump head will increase but not much use since it will just recirculate as the ABV is at its max setting of 4M.

A heat only boiler would do the task above very simply if it can tolerate a high dP.