Gravity DHW pumped CH radiator problem.

[jimk280]

Hi Guys, called to property with oil fired pressure jet boiler. loads of people looked at it in the past apparently but not sorted it. V old one pipe system with 10 rads, some old some new. Most recent somebody renewed the circ pump which is on the CH return to boiler about 1m away. The gravity DHW side (28mm) is absolutely fine. Problem appears to be that 2 rads never get hot (1 up and 1 downstairs), and then sporadically others do not get hot then on other days do get hot. Others mainly upstairs require to be bled regularly.

Our response was to drain system, clean F&E, renew all rad valves with lockshield & TRV sets, chemically powerflush via circ pump connections.

End result not much improvement! Did notice that after bleeding all rads that the rads only really bled properly with the system off. If we tried to bleed with the system running (to push air through quicker) then there was no air escaping or water to the rads?? as soon as pump was switched off air came hissing out. Quite confusing this one.

My next step is to recommend ripping old out and fitting new fully pumped but would welcome anyone's experience on this.

Jim

 

[BLOD]

Did you fit trv's suitable for a one pipe system?

 

[dancinplumba]

http://www.ukplumbersforums.co.uk/p...pumped-ch-radiator-problem.html#ixzz2BOCM842Y

 

[jimk280]

Thanks Blod, we fitted Honeywell that can be fitted on either flow or return i.e bi-directional?

 

[CXR100]

why did you put trv's on a one pipe system??

 

[CXR100]

it doesent mater if they fit on either flow or return, if room gets hot and trv closes off automaticaly this will stop the circulation in the central heating!!! then tey will have no heating at all will they??

 

[dancinplumba]

they need to be suitsble for one pipe, as blod has said. they have a larger bore " normal ones are to restrictive

 

[dancinplumba]

it doesent mater if they fit on either flow or return, if room gets hot and trv closes off automaticaly this will stop the circulation in the central heating!!! then tey will have no heating at all will they??

it will still circulate, its a continuous pipe, tho rads will be cold

 

[jimk280]

No chance CXR??? even on a one pipe, if a TRV closes, water can still pass to the next.

 

[CXR100]

what if its a one pipe system that just enters the rad and then out the next end into the next radiator , the type where you cant turn the radiators off? the the water would have cooled down alot by the time it reached the 9tha nd 10th rad wouldt it????

 

[Best]

it doesent mater if they fit on either flow or return, if room gets hot and trv closes off automaticaly this will stop the circulation in the central heating!!! then tey will have no heating at all will they??

Only shuts off the rad. Other rads have the flow going past them, not through them. One pipe is same as two pipe usually except the colder water continues to next rad in circuit.

 

[dancinplumba]

guess you are right in that type, but personally havent come across one only in france

 

[CXR100]

worth lifting a few boards up to have a look i suppose wouldnt it?

 

[Best]

Did notice that after bleeding all rads that the rads only really bled properly with the system off. If we tried to bleed with the system running (to push air through quicker) then there was no air escaping or water to the rads?? as soon as pump was switched off air came hissing out. Quite confusing this one.

Jim

Got to watch when bleeding a system with the pump on. Sometimes the vent is drawing air in! Try putting drop of water on end of your finger & pressing against air vent & water will suck in.

 

[jimk280]

Already aware that the one pipe is standard one pipe, ie is in a continuous circuit with tails to rads. I'm thinking that continuous requirement to bleed is due to negative pressure somewhere. two rads not getting hot maybe down to blockage somewhere?

 

[jimk280]

Hi Best, we did check that but all seemed ok, that's why I started head scratching as it's not every day we come across these old systems

 

[dancinplumba]

with pump on return, circuit is always under negative pressure and pulls air in which will cause corrosion. mebe flush these two rads, one pipe needs good balancing imo

 

[tamz]

Your powerflush did very little if anything to the rads. It would have just pumped around the circuit.
The rads on one pipes work by gravity circulation.
The system is under negative pressure so trying to bleed the rads with the pump on isn't going to work.
Check the pipe runs to the rads that don't work especially if they are additions to the original system and fit the correct valves.

 

[dancinplumba]

f an e will probs come off gravity circs

 

[Best]

Hi Best, we did check that but all seemed ok, that's why I started head scratching as it's not every day we come across these old systems

If the two rads that don't work well have long run of pipes from the main pipe, then they won't work well, if at all, I would think.
Better it's all ripped out! I have rarely seen these systems, but did come across a one pipe job a few weeks ago, that some idiot plumbers had taken long legs off the original rad pipes & obviously it didn't work as they had plugged it.

 

[Mike Jackson]

It's worth checking the pump speed. If it's too fast the rads won't heat up properly because there isn't time for the hot water to gravity circulate into the radiators.

 

[jimk280]

thanks Mike will take a look at that