Integrated pump in a system boiler. Any reason for it?

[richardindorset]

If your house is anything like typical, no reason why it won't be fine with the built-in pump alone. If the speed is adjustable, use the minimum that works OK.
(you mean blowtorch, not soldering iron!)

soldering iron for the temp sensor, blow lamp for the original pump! (actually probably some compression fittings!)

 

[richardindorset]

Most system boilers have a internal bypass, vaillant's default setting is 2.5M but can be adjusted to a max of 4.5M, if the system pump is pumping into the boiler HEX then at max circulating rate the HEX pressure loss might be ~ 4.0M, depending on the bypass setting then the bypass will be shut at max flowrate if set to 4.5M but at very low flowrates can cause excessive bypassing with very high boiler return temperatures resulting in reduced boiler efficiencies, also most internal pumps wont give more than ~ 5/5.5M head at full flowrate resulting in "only" 1.5/2M available for system circulation.
A heat only boiler has no internal bypass so the external bypass can be set somehere below the max pump head, there are some instances where a 8M pump is required to give the required flowrate through some systems, for whatever reason, if this boiler/pump are replaced with a system boiler then a LLH/CCTs are required with extra pumps/expense.
Also the max flowrate through a system boiler is based on a flow/return of 20C, so a 30kw boiler will have a recommended max flowrate of 21.5LPM, if you wish to run your rads at a lower dT then you may have to install a LLH/CCTs to achieve this, you wont require this with a heat only boiler, high head pum , if the decrease in the required dT is modest, say 15C vs 20C, of course, in most cases the boiler output is way more than the required rad heat requirement so LLH/CCTs not required either with the system boiler.

There's a lot to unpack there. I'll get back to you. Thanks for the input though. I have the old and new boiler specs so I can check. I'll check for the bypass. The original system has an external bypass, near where the pump is.

 

[richardindorset]

John IMO you're overthinking and overcomplicating things. I think the best plan is to install it and look into some of that in the unlikely event that there's a problem.

Ha ha! Yes, suck it and see. Well I have nothing to lose, but I am going to do as much research as I can before I shut the heating off, at this time of year!

 

[fixitflav]

soldering iron for the temp sensor, blow lamp for the original pump! (actually probably some compression fittings!)

I'm sure you'll find use for both, and a few spanners! It was meant as a bit of a joke