Cold Radiator - thwarted

[BobbyOllox]

I have tried everything to fix a single cold radiator which has never worked since I moved in

Changed the rad, the TRVs, removed some isolating valves I found on the spur. There is flow to the rad ( the flow pipe gets hot), there is flow on the return ( the system drains successfully through a drain on the return flow of this rad ). No airlock.

If I turn all other rads off, then it will eventually get warm, but not hot like the other rads on the system. The return pipe never gets hot.

I am wondering now if this is just bad layout . It is one of 4 rads added in an extension. The spur that feeds the radiator ( the only one on this spur), is 10 m, each way, and must have 6 x 90 degree turns on each flow. The final stretch of the return goes from ground floor to first floor. At least part of the spur is under concrete.

I just wonder if there is not enough pressure to push water round this convoluted path, through a 2000mm x 600mm double rad, and then back again.

Any thoughts ?

 

[RickMoore]

If you turn all other rads off and the radiator is only getting warm, what's the boiler doing ? Have you made sure the locksheild valve is opening and the washer isn't jammed in it restricting flow ?

 

[HERTS P&D]

Take off the radiator and do a bucket test. This will confirm water flow through the valves, then it might be new radiator time and re-balance.

Andy

 

[BobbyOllox]

Herts - its a brand new radiator and valves - I thought changing them would fix - but more or less the same symptoms.

The pump just ticks over, occasionally firing up, and then switching off again.

 

[D_Hailsham]

I just wonder if there is not enough pressure to push water round this convoluted path

Are all the rads in the extension fed from a single branch off the original system?

Were the pipes before the extension branch increased to allow for the extra flow?

What size pipe was used for the rads in the extension?

Details of the other rads in the extension - kW if known?

 

[BobbyOllox]

OK I will try. I only lived here 2 years.
The house is 100 years old, and obviously had CH dropped in at some point. Then in 95 a large 2 story extension was added, and I think that the boiler and pump were changed at that time.
The boiler is in the loft space, pretty much at the centre of the footprint of the house.
On the first floor, there are 3 original radiators and 4 in the extension. The pipework to supply them is under the floorboards. I cant access. I dont know how the 4 in the extension were added to existing system. They all heat up without problem.
Downstairs, there are 4 originals,. and 2 in the extension. They all ( apart from the non functioning one) are fed by 15mm flow and return dropped from the 1st floor, direct to the rad. They all heat up fine.
The non-functioning one is fed by a similar drop in the old part of the house, but then through a wall into the extension,changes feeds plastic pipe which loops round 3 walls of the extensin room for 8 metres , ( under kicthcn cupboards) and then under tiles for 2 metres, before finally popping up as copper flow and return to the rad.

The boiler is 80K BTU - the failing rad is a 2mx600mm double, which I think is 10K BTU. The other rads are smaller, some singles , some towel radiators - I would estimate their total consumption at around 60K max

Again just to reiterate - the problem with the rad was not fixed by changing the rad itself, and the valves.

Either there is a constriction on the return ( but water drains out of the return OK - so would need to be uni-directional constriction) or I am just asking too much of the pump/boiler, which look fairly standard. The more I learn about this spur though, the more I think it isnt so bad. It is long, but the plastic bends round the corners of the room, not angles. The connections from copper to plastic and back are not visible, but no sign of water leaks, so I assume they are sound. Just stumped/

 

[BobbyOllox]

I have researched as best I can, and it seems that a 20 metre spur ( 30m equivalent length) of 15mm pipe should be well within the ability of the pump - it is the index path, and should only take 0.03 bars, whereas the pump should deliver 0.6 bars typically. ( correct me if I got this wrong.

Drained it again this morning - used a hose to push mains pressure through the return pipe into the empty system - no blockage. Let it drain out of the return pipe, came out completely clear. I cant tink of anything else to do to that return pipe - it looks clear.

Before I changed the rad, I took the old one out and flushed it with mains water. Lots of black came out - so there is or has been sludge in the system.

So

Even though the flow pipe is delivering hot water to the rad, perhaps the flow pipe is sludged up a bit, reducing the pressure before the rad. , and the problem is in there rather than the return pipe which stays cold/tepid

OR

The pump is not working efficiently. Perhaps this is full of sludge ( grundfos 15 60 130 )

Any thoughts on my logic ? Has anyone got any other ideas what might be causing this or what else I can try ???

 

[darenmul12]

check the feed pipe, i had the same problem i thought the return was blocked or kinked and concentrated my efforts on this, I gave up and passed it onto the gas engineer who was fitting my boiler, he had the system drained and connected mains pressure to the feed and found that was the blocked line, when it cleared we got loads of sludge out and now the rad heats up lovely.

 

[xr4x4]

Take the rad off, join the flow and return with a piece of temp pipe.

And I'm guessing you've run a hose from each, (flow and return) with good flow?

 

[BobbyOllox]

thanks both - I have not really concentrated on the flow pipe, because it gets heat to the radiator - I just assumed the problem to be on the return, and now I know the return is clear. The flow gets hotter than the rad, but thinking about it, I dont know that the flow is strong or not - just hot.

I can take the rad off, and then I can access both end of the ground level flow pipe ( other end is a plastic push fit before it starts the rise up to first floor - will blast some mains through there and check the flow rate , and what comes out.

Excited now - this may be it finally. Thanks for direction.

 

[D_Hailsham]

I have researched as best I can, and it seems that a 20 metre spur ( 30m equivalent length) of 15mm pipe should be well within the ability of the pump - it is the index path, and should only take 0.03 bars, whereas the pump should deliver 0.6 bars typically.

The index path is the one with the greatest pressure drop from pump exit to pump entry. Read Copper Tubes in Domestic Heating Systems.

The pump will only deliver max pressure at minimum flow. The pressure it can deliver decreases as the flow rate increases.

 

[BobbyOllox]

Thank you DH - I had found the document, and looked at some of , I think your, worked examples - which were enormously helpful.

I paraphrase the document and calculation - this spur comprises part of the index circuit ( biggest radiator on the longest pipe run) and according to my calculations - this part of the index circuit ( 30 m and 3.6Kw @15mm pipe) should not be overly resistant, and I should expect the rad to function. Did I get it wrong do you think ?

 

[D_Hailsham]

this spur comprises part of the index circuit (biggest radiator on the longest pipe run)

Just because it is the largest rad on the longest run, it doesn't necessarily mean it is the index circuit.

Have you tried balancing the system?

 

[D_Hailsham]

Forgot to ask!

What is the P/N of your pump (top right of data plate)?

Which boiler do you have (make and model)?

 

[BobbyOllox]

DH - in which case then I definitely do not understand index circuit, so will leave te subject well alone. I assume then that a 30 m equiv length of 15mm pipe does not ring alarm bells.

I have not balanced the system - but I have turned off most of the radiators except for this bad rad. I think it had a positive effect - after running for several hours, thats when return pipe goes from cold to tepid.

The boiler is a Potterton Suprima 80L - the pump is grundfos p/n 59506151

I completed a heat emitter schedule based on the document, measuring the rads. It adds up to 23.9KW I read the suprima has 23.4KW output. Is that a problem ?