System/pump design when merging one-pipe and two-pipe systems

[garyo]

Hi,

The old part of our house runs a one-pipe heating system, and works reasonably well serving 3 bedroom and 2 floors. It has a grunfoss circulator that seems happy on speed 1 of 3.

The new extension has 5 radiators in it, and I've plumbed that as a standard two-pipe system.

I'm just thinking about how to join the two systems together.

* Should I fit an additional pump and then tee off the existing flow and returns (between existing boiler and pump) to keep the systems separate? A benefit here is that I could use the new pump as my zone control rather than having to fit a valve, but I guess it may be a waste of electricity?

* or would I be better off running the whole lot off the existing pump, and then install a gate valve on the one-pipe system so that I can balance it just like it's one big radiator?

I'm leaning towards the first option, but wonder if there's something I'm not considering?

Thanks

 

[Exedon]

First is your hot water heated by your boiler? If so gravity circulation to cylinder or pumped?
Two pumps can work but depending what you already have relay switches would be needed .

 

[garyo]

First is your hot water heated by your boiler? If so gravity circulation to cylinder or pumped?
Two pumps can work but depending what you already have relay switches would be needed .

Yes, the water is heated with a vented tank upstairs, and that is a separate loop off the same pump. This is a diagram of the current situation with the two motorized zone valves:

 

[Exedon]

I would keep it simple add the new 2 pipe as another zone with new zone valve (extended S plan) add a good quality gate valve on return just before connecting it back into existing heating return you will probably need that to balance down new 2 pipe to make existing work.

 

[garyo]

I would keep it simple add the new 2 pipe as another zone with new zone valve (extended S plan) add a good quality gate valve on return just before connecting it back into existing heating return you will probably need that to balance down new 2 pipe to make existing work.

Thanks, I think I'll probably do that then. I was thinking that a new balance valve would be required on the old one-pipe side because that's effectively a 22mm open loop that will steal all of the flow, while the new two-pipe side will have a balance valve on each rad so quite easy to balance down (for situations when both zones are open)?

 

[Exedon]

As your draining system why not add another gate valve on one pipe return? Covers all bases
I've done a number of systems with the 2 pipe section connect in first always had to balance the 2 pipe well down to make the one pipe loop work well.

 

[garyo]

always had to balance the 2 pipe well down to make the one pipe loop work well.

Interesting. I'll add a gate valve to the new two pipe loop too then and will have covered my bases as you say.