Central heating technical problem

[jangler]

I have a problem with the Central heating and water not getting hot.

System:
Gravity feed system, with H configuration and 3-port valve, Baxi Boiler, no drain off points except from radiator. No pressure gauge. No flling loop.

History:
System has worked fine for past three years, when I removed a radiator, I suspect air got into the system, pump became noisy, rads and water would not heat. Plumber replaced pump, saying it had failed, although I now suspect it had lost its prime. A week later pump ran but no hot water/rads, same plumber called again, said it was a loose connection in boiler causing it to trip out. Plumber got the system working and a week later it failed again, the over temperature switch had tripped on the boiler.
Spoke to plumber he said H configuration pipe work needing changing….lost faith with plumber. The loose connection was a red herring. As a Spark I later checked out boiler electrics no problem found. After further investigation it appears that the system looses its pressure. If I bleed a radiator then the system will loose pressure and result in no water/rad heat despite running pump (boiler over temp kills pilot light).

Action:
Header tank is full to brim. I have put cleanser through system and sludge inhibitor.
I opened 3-port valve and blasted system with hosepipe until water runs out of header tank overflow, bled pump and rads. System will run for around two weeks and fail due to loss of pressure.

Concern:
Water system top up via header tank is on feed side of pump so I’m not sure if pump inhibits the gravity pressurisation of the system (I am not sure that this has ever worked).

Note:
Plumber said that the H configuration has tendency to suck air into system via vent to header tank, but this had never happened until I took radiator off, so doubt it.
During the summer the system will happily run heating the water, only when I try to run CH does it fail.

Suspect:
Water pressure top-up failure due to pump?
Sludge builds up on radiator circuit return?
Air locked system?

Any clues would be greatly appreciated.

 

[ArtfulBodger]

Loses pressure? how would you know?

The "H" configuration:- if you mean the vent and the cold feed are within 6" of each other immediately before the inlet (upstream) to the pump then this is the standard accepted way of doing it. It shouldn't be drawing down (the vent) and certainly cannot pump over. There is a possibility that the pump is set too high causing it to draw down.

When you removed the radiator did you drain the entire system down and refil (thereby testing for a blocked feed) or did you just remove after isolating the rad? if so, was the rad replaced or do you just have the tails left?

Also, the F&E tank brimming over does not sound right. If fact this sounds more like the vent and feed are on the return side of the configuration and under positive pressure and the system is pumping over.

You didn't have to back-fill by any chance did you?

 

[jangler]

Hi Artfulbodger,

Loss of pressure was suspected due to vent plug releasing air, then no water coming out, am I right in this assumption? The vent is a vertical pipe close to the hot water tank tank slightly above the height of the pump.

Yes, the description of the 'H' system sounds correct. The Pump is a Grunfoss pump, set at mid-position. I have been informed that it is more powerful than the previous one. That I take it means more flow capacity.
When selected on low setting the boiler would trip out on high temperature, so I guess that it wasnt shifting the hot water away from the boiler quick enough.

No I did not drain the entire sytem. When I removed the radiator, I only drained that radiator down and replaced it after tiling, some weeks later. The system was running during that period, only after reinstating did the system exhibit problems.

My guess was the F&E tank is brimming due to re-filling the system with a hose, via the downstairs radiator.

I will post a photo of the configuration later today, it might make things clearer.

By back fill I assume that you mean filling the system with water with a hose pipe, then yes. The system will work for 2 weeks after dosing this exercise.

I hope this clarifys things.

 

[ArtfulBodger]

OK you have back filled your system which isn't very wise. I would suspect at this point that you have a blocked cold feed from your F&E tank where is tee's into the flow. This must be corrected as water has no way of refilling your system due to loss by either evaporation, or, pockets of air finding their way out of your system.

I would suggest the following:-

Before doing anything else, check that the ball valve in the F&E tank is working OK - it migvht just be stuck. Assuming it isn't .....

Shut down and electrically isolate the boiler. Drain the system. Turn off the stopcock to the F&E tank. Nip up to the loft and get as much water out of the F&E tank as you can. Remember, this might get messy!. Go to you airing cupboard and at a convenient point, as close to the tee as possible, slice through the cold feed. Oh, have a bucket ready - sorry forgot about that one. If I'm wrong you will be rewarded by a few pints of filthy water missing the bucket but somehow finding you and your wifes' clean towels (best move those on second thoughts).

If I'm right then you wont get anything coming out. In which case you will have to cut away and refabricate the cold feed tee and then turn the water back on and fill up as normal.

When all of the rads have been bled normally, you should be able to release all of the air from both your vent point and the pump.

Turn everything back on and it *should* work.

Good luck.

 

[ChrisR]

Agree with the above but another possibity - coil in indirect HW cyl is leaking. Why else would the f/e tank be full? Also you get lots of air sometimes when that happens.

What is the level of the large CW cistern's water in relation to the brim of the f/e tank. If they're the same - it isn't a coincidence!

 

[jangler]

Hi Artful,

Assuming that I remove a possible blockage from the cold water feed (I remember that there was a film in the f&e tank, so you may have a point!), will the cold water feed be enough to re-fill the system or should I attempt another backfill?

I will follow your instructions on Saturday, thankfully there is a coupling and a valve so no slicing required on the cold water feed, will keep you posted.

Thanks for the help.

 

[jangler]

Ok, have been in loft isolated f&e tank, siphoned down. Split cold water feed pipe to pump and checked small amount of water passed. Switched
on supply to f&e tank, successful water flow into bucket in airing cupboard.

Reconnected cold water feed and ran HW system, boiler tripped out on high temperature, after a while hot water flow began and the boiler overtemperature was reset and system left running.

All radiators bled, no air, Central Heating system started, hot water flow on CH side of three port valve. Only one radiator nearest to the airing cupboard gradually getting warm, none of the others getting hot.

System has been running on full for past twenty minutes, the radiator closet to airing cupboard is wamest but the return from the radiator is cold. Two other upstairs radiators have the return side hot and the feed is cold.

Checked f&e tank level ok and no return on vent. Pump switched to 'high' setting to see if radiators getting hot, still cold.

Does this sound like a sludge/air blockage? If so how do I clear it?

 

[kevplumb]

have you tryed switching the three port valve manually there is a lever on the top

 

[ChrisR]

Blockages in the feed from the f/e tank are usually worst right at the T where it joins the system - your tap may have been above that?

Normal air locks can be shifted by turning off all the working rads so the max pressure is available from the pump. So I suspect you have a lump of grot somewhere blocking the pipe. Have known it to be inside the pump.
Does the water look very heavy in the black stuff?

Because its easy (with working valves) I'd take the body off the pump at the Allen screws and have a look inside.

If no help I'd take off one of the non-working rads and force the mains through.

eg : empty the f/e tank, connect its mains feed (from its stop cock,(fit if missing!)) to its output pipe, and cap the tank's outlet.
To do that you need a couple of speedfit connectors and a bit of pipe.
When you turn the water on any crud in that part of the system will come up the vent pipe into the f/e tank.

Then I'd send the mains down the heating pipes:
Put a Speedfit cap on the end of the vent pipe,
take off one of the first non-working rads (check colour of water!) and connect a garden hose to the rad valve. Usually non-trv rad valves are the same size 3/4" as a garden tap so the hose fits.
Open rad valve and turn on the mains at the f/e connection, and you should see some grot going out.
I always have a collection of flexy tap connectors and speedfit adaptors which make it easy to connect to the trv size 1/2" too - or use a different garden hose adaptor.

Once you have a bit of flow round the entire system then a desludger can work - it wont find its way through a blockage.

 

[jangler]

Heating has been running for near an hour now and the upstairs rads are hot, albeit one of them is cooler in the bottom centre. The downstairs rads are still cold.

ChrisR, yes the tap is located above the T. Difficult to say what the colour is because I backfilled the system a couple of weeks ago, so is pretty clean.

I'm not sure whether backfilling the system is simply moving a slug of gunk around, via a downstairs radiator and hose pipe.

Will start by closing down upstairs rads to maximise pressure. Then off to B&Q to get bits to bypass the f&e tank as outlined.

Kev, the 3 port looks fine (less than year old) and hot water on all sides.

 

[ChrisR]

SOrry I though you only had one rad working. Cold downstairs is classic sludge problem.

A lot of it will be in the rads , which won't be shifted by clearing the pipes, though you can shift some by just turning on one rad at a time. Problem can be that there's another route the water can go if the motorised valves are open - which you'll need to use the flow side pipes.

If you have nowt better to do you can take the bad rads off and hose them in the garden, but you really need a power-flush. You can hire the machine.

 

[jangler]

Pump checked, some slight air and blackened water. I have isolated all upstairs radiators and downstairs have come on.

Two of the five downstairs radiators are lukewarm and the pump sounds a little noisey.

 

[ChrisR]

" pump sounds a little noisey"

Buy a new one. They DO slow down, and the black stuff wears the impellor so they don't pump.

Apart from that, good, it sounds like you don't have anything unusual wrong! Power flushing machine called for I think - but note that the blockages you get in the feed pipe do not dissolve in any of the descaler chemicals - I've even tried HCl. Some are magnetic - see if a small magnet sticks to the pipe.

 

[jangler]

Hi ChrisR,

The pump is new (just over a year old) and so is the three port valve!

I suspect the noisey pump is a result of labouring due to the downstairs blockage? It was quiet with the radiators on upstairs.

Just put the water on at 7.00pm and the boiler is again tripping out on overtemperature.

How does the power flush work and where do I get one from.

I intend to empty f&e tank, plug up cold water feed and back fill at mains water pressure via a downstairs rad and look at the contents in the f&e tank via the vent and hopefully there will be a bucket load of gunk.
Also, check the tee where the cold water feed connects to the pump to ensure there are no blockages.

Tomorrow...........the pub is calling !!

 

[ChrisR]

"I intend to empty f&e tank, plug up cold water feed and back fill at mains water pressure via a downstairs rad "

On reflection, you already did more or less that.

What make's your pump? A Wilo eg will wear out quite quickly in dirty water.

A power flush machine is "sort of" a big pump in a bucket that you can fix hoses to. You normally connect it where the CH pump goes. Key thing is that it enables you to reverse ther water flow very quickly, which gets the grot into suspension. There's quite a lot about it on the Kamco site.