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.