Combi's with a Single Pipe Rad System

[iamalyon]

Hi,
I recently moved into a house which requires modernisation, I would appreciate some advice regarding the CH system.

Currently the house has a Baxi Bermuda back boiler which is connected to a single pipe rad system, there is currently no control for selection of CH or hot water (the the heating is either on or off), which is pretty annoying!

I would like to rip out the back boiler and install a Potterton P28i Combi in the airing cupboard.

Does anyone know of any problems associated with connecting combi's to single pipe systems?

Would appreciate any help, many thanks.

 

[PEDANTICVINDICTIVEMAN]

Just fitted a 28Cdi Worcester onto a 5 rad one pipe system last week and put Danfoss ras D thermostaic valves onto the bedroom rads, all worked a treat.

 

[PTH]

Do you REALLY have a single pipe system in a house. This would be fairly unusual after 1950's.

Single pipe circuits are characterised by large bore pipework (for a Bermuda; 1" circuit) and large radiator connections and valves (I've seen 1 1/4" radiator valves on BIG radiators in schools, hospitals etc.)

A single pipe circuit is essentially a pipe loop sized to carry the total output of the boiler. Radiator connections are then taken directly off this loop and rely on convection current to transfer the heat energy from the pipework into the rads ie. both flow and return connections are taken from the same pipe normally about 5ft apart. This means that there is virtually no pump head available across the radiator connections and thermostatic valves cannot be used (too much resistance - flow through the rad will stop altogether).

So... if you have got a single pipe circuit you will be spending lots of money on the combi and not be able to control its output effectively. To get the most out of your system you will need to break the loop, run a return pipe and remake all your radiator return connections to this.

I'd start again.

 

[PEDANTICVINDICTIVEMAN]

I say again, the house I was at last week was a late sixties house with 15mm copper one pipe loop with 5 rads and worked absolutaly fine with a combi and flow and return temp diff was spot on after adjusting byepass.

 

[PTH]

Mr Pedant,
I don't dispute that it will work, after a fashion. I wasn't intending to argue with you - we were both typing at the same time.
I don't install systems, I only design them, so I'll bow to your superior knowledge. I'll just add that single pipe systems work best with very slow circulation velocities and tall radiators to generate the required thermal syphoning, effectively lifting the heat from the loop into the rad. They are also quite sensitive to air which can collect as bubbles around teh rad connections and within the rad valves increasing their resistance.

Iamalyon - I suppose the way forward is to try it and see. I'm quite happy to be proved wrong.

Good luck.

 

[iamalyon]

Thank you both for your input.

Still undecided on what exactly to do, my preferred option due to cost and ease would be to use the rad system thats in place but I know this is not ideal.

Could any installers give me a rough price/estimate on the approximate labour costs to run in new pipework to convert the system into a fed and return type?

 

[PEDANTICVINDICTIVEMAN]

It didn't work after a fashion it worked 100% all rads heated up fully and very quickly with correct temp differential.

£25000.00 an hour plus materials, got to put in a big price just to cover myself as I havent seen the job.