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... Also negative and positive pressures do not apply to a sealed system!
Oops forgot it's sealed. Actually there is negative and positive head even in a sealed system. I think, it's just half the pump head; negative up stream and positive down stream. If there's vent it can be negative to the value of the whole pump head if the vent is down stream and likewise positive the whole pump head if the vent is upstream. In the former case this occurs if it's a small static head and is the cause if ingress of air which "mysteriously" builds up in radiators suffering the negative head.

There is another possibilty - air locks. I get quite a lot of work from doing small re-pipes and fitting vents. Even in sealed systems, an inverted pipe loop without a vent can stop any system working.

Trace all pipe runs. Once it starts going up hill start looking for the high point ie just where it starts going down hill again. There must be a vent at each high point.

A "good one" I find is a wall-mounted boiler with flow and return on the top. The pipes come up from the floor, go over the boiler and drop down to the F&R couplings creating a couple of lovely air locks.
 
So the high point for the boiler will be the vent at the top of the heat exchanger or there abouts. I would recommend that and the remainder of the sytem is checked. There is no doubt that airlocks would cause this problem and if the plumber does not understand their significance they could be everywhere...
 
The cylinder is 12 meters away from the boiler and the water heats fine so no problem with air at the boiler or the cylinder (prepiped module). The problem lies with the circulation through either the module or the heating which is below the floors. It is not air.
 
Hello Folks

OK - Plumbers in this morning, checked all again, no blocked pipes. Put the spare 15/60 grundfos pump onto the return line - small change - can get most rads to heat bar 3. Any other ideas?
 
Does the system have two motorized valves, one for the CH? If a radiator is starting to get worm but return is still cold it could be the hot simply hasn't yet replaced all the cold in the radiator. Then boiler is overheating so cuts out. No pipe blockages found. Might be a time/temperature problem with CH motorized valve. It gets warm/hot and electric motor coil opens and it shuts hence intermittent blockage only when hot.
 
The Motorised Valve has already been checked and this is not faulty. All the rads in the house are now hot except the two up the stairs, but i am not bothered about these at the moment. The rads have taken about an hour for all to become hot. This seems like a very long time
 
ok dokay - The plumber has told me to leave all rads on full blast and he will be back on wed am to check them.

I prefer the bedrooms to be quite cold as these are only used once or twice a year, and the living areas to be warmer. Should i open the living areas fully and close down the rooms that are not used, or should i do it by the sequence the rads come on i.e. closest to the megaflo closed off and open come the end rad?
 
Essentially you want the Lockshield valve on the furthest away radiator open and slightly close them more and more as you get closer to the pump. This will balance the system.
It takes a bit of practice to get this right and is quite important if you want the best out of your system.
If you don't want as much heat in bedroom then you control this by opening and closing the TRV's to your requirements.
 
Did you ever satisfactorily resolve this. Re - your 2 pictures - is the grundfos mounted vertical as in pic 1 or horizontally as in pic 2.
 
Theres a story about the old penny trick ? usually when a custard has nt weighed in the plumber properly ? :) Id say track the flow and return til you spot the temp differ. to find the blockage ?
hiya

I was the plumbers apprentice today. We did use FX2 and we did each rad seperate at first whilst hitting them with a mallet, then did them all together for one final blast, it didn't shift the blockage at all. Whilst flushing each rad we listened for flow in the rads, and each rad seemed to be getting. Turn it to CH mode and the return pipe gets boiling hot to touch, boiler runs for 2.37 mins and cuts off, and return pipe flow dosn't seem to go anywhere. The plumber doing the works now, was not who installed it. The installed has done a runner and we can't get hold of him to rectify the fault, so were having to pay out even more money to get the original fault sorted. Our new plumber has said to leave the CH on full blast for the whole weekend, and he'll pop in past again Mon am to see if theres any change. This system was fitted in March and has never worked, but it wasn't such a problem in the summer months, now it 0 deg c here in the northof scotland and were freezing! thank god we have a gas fire or we'd really be us the creek without a paddle!!!!
 
Old thread I know but a couple of my own experiences with regards
situations like this. Its so easy when cutting a chopping old bits to new
bits to leave little bits of exsistng here and there due to being difficult
to get to, boxing, trying to save the customer some decorating
etc... One situation I had very simular to this was I spent hours on
an installation only to find a couple of old checks valves (non return)
on the old exsisting pipework and due to the re routing of pipework
they were now facing in the wrong direction, this were only found
after taking down boxing.

The other time and some thing that has taught me a good lesson
...."Good house keeping".... do not put all your fittings in one bucket,
as 15mm end feed caps have habbit of getting stuck inside other
fittings....lol.... I could have kicked myself.
 
ive had a piece of tube go past the stop on a tee effectively blocking the pipe sods law it was the lowest point of the system under a floor
 
is the pipe work new if so there could be a crossed flow and return on a strait coupler witch would bypass rads and have that sort of effect
 
Hello this sounds like a real nightmare. Don't want to sound condescending but I think the best thing to do with this is to go back to basics and start again. It sounds like the boiler is working ok but is the bypass set correctly try closing it and setting it again and are all the pipes, pump and valves fitted and connected correctly. You will have to check each one. Water will always take the easiest route. If you think its a blockage in the pipework try a magnet if it sticks to the pipes that's where the blockage is.
 
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