Circulating pump in central heating system - now redundant?

Old 06-06-12, 03:16 PM
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Circulating pump in central heating system - now redundant?

I'm about to replace my system boiler (and the timer/thermostat). All the rest of my central heating system including radiators and hot water cylinder are staying put.

The replacement work is going to be done by qualified professionals, but I'm sourcing the hardware and I need to have all the right stuff on site before they start. So I'm trying to fully understand the system.

My problem arises because the existing central heating system has a Grundfos UPS pump in the circuit, and my specific issues are these:

1) No component in the new system appears to have the capacity to drive such a pump

2) The new boiler (Vaillant ECOtec plus 624) actually says it has an integrated pump, so it's no surprise that there seems to be no way to drive an external pump

3) Trouble is, the old pump is still in the circuit and I assume that it's actually going to impede the flow

4) As a matter of interest, I can't actually see anything in the old system that would drive the pump either, and I'd like to understand that before we do anything irreversible.

So, could someone please help me out, and kindly explain for me:

What would have been driving the pump previously? (The old Honeywell ST699 programmer is the obvious candidate, but I can't see that it has the functionality to drive a pump)

Is the pump definitely going to be redundant when the new boiler is installed?

Do I need to be getting the pump removed when the new boiler is installed, so that it doesn't impede the flow?

Is there anything else I need to know? I'm hoping I've understood the future setup right, but I'm still a bit baffled about what controls the pump right now.

Thanks in advance for your help!
Old 06-06-12, 03:48 PM
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Here is the user manual for reference. ( I have not read through it)

Your non US, right? Not sure the responses you will get.

I am looking for install manual, but the installer should know what you need. Give them a call and ask. That will probably be your best bet.

Here is the install manual I believe.

Mike NJ
Old 06-06-12, 11:54 PM
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Thanks Mike.

Yes, I'm in the UK.

I'm pretty clear (from the same docs you linked to) that the new boiler has an integrated pump. What the docs don't explain is what to do if you're installing the boiler into a system which already incorporates a standalone pump.

This has got to be a common problem, because the standalone pump is a very distinctive and familiar piece of kit that I've seen many times as part of a central heating system. If boilers are nowadays being supplied with integrated pumps, then there must be a lot of boilers being put into systems which already have standalone pumps in them.

So my main question is still: Will the old pump impede the flow around the circuit and do we need to take it out?

My other question to complete my understanding is: In systems with a standalone pump, like my old system, what controls the pump? Is it normally driven off the programmer unit? I just can't see that the programmer unit has the necessary connections to fire up the pump, but I can't see what else does it.

Thanks again.

PS the old system has a Honeywell ST699 programmer which is responsible for switching the valves between the radiator circuit and the hot water cylinder circuit. The boiler is made by Keston and (possibly) seems to be triggered into action by cool water flowing down the return pipe into the boiler - so I don't think it can be the boiler that triggers the pump but maybe the other way around. I just need to make sure I've understood everything that's there before we start ripping things out!
Old 06-10-12, 01:43 PM
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Without pictures it would be too hard to discuss how your system is layed out. But, normally the old pump is removed when the new boiler is installed. Dependent on the piping strategy they may keep the old pump in place and still use the new pump if piped primary/secondary.
Old 06-10-12, 11:40 PM
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Thanks rbeck,

I called Vaillant and they said the same thing. They said it's a common thing for the circuit to have a pump in it, and that it should be removed when installing a boiler which has a pump of its own. (I can see why you might want to keep it in the circumstances you describe, but that's not what I have here.)

Thanks very much for your reply.
Old 06-12-12, 06:05 PM
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if it is condensing boiler, just build a primary/secondary loop, keep the old pump in secondary loop,you are fine. If your have cast iron rads and black pipes, don't forget to put in dirt separator.

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