Taking forever to pressurise central heating system. Why is this so?

[siliconeseal]

Hello,
I live in a two floor semi with a gravity fed conventional gas fired boiler (Potterton Kingfisher MF). The boiler is fed cold water from the loft water reservoir tank (which also feeds the cold water taps in the bathroom). There is an expansion vessel attached to this cold water feed to the boiler (the boiler, circulation pump, and two 2-way motorised valves, one each for hot water and central heating, are in the kitchen on the ground floor). The expansion vessel and the hot water storage tank are in the airing cupboard on the first floor, directly overhead of the boiler.
There is a braided flexible connector hose between the flexible PVC cold water feed tubing from the loft tank to the airing cupboard AND the expansion tank. There are three shut off valves on this braided connection hose. There is a water pressure meter attached to the expansion vessel.
Each winter, just before the central heating is switched on, the meter indicated water pressure, is between 0.6 to 0.8 bar. I open the three valves on the aforementioned braided flexible hose and wait until the meter shows 1.2 bar, turn off the valves and switch on the central heating. There is no need to repressurise the system until the next winter. This system has been in service for 14 years now.
All was well until 10 years ago. Until then it took at 5 - 8 minutes to pressurise to 1.2 bar. But since then, it took ever increasing lengths of time to do so.
At the start of the current central heating season this year, it took 3 hours to increase the pressure from the 0.6 bar to the 1.2 bar required.
And also, now the central heating has started to cut out after being on for a couple of hours. A new circulation pump was installed 3 years ago, and last year I had to again get a plumber over when the boiler kept starting and stopping. The plumber replaced the motor head of the two way central heating motorised valve, and that fixed the problem.

So my question is: what is the most likely explanation for the long time it now takes to pressurise, and second: is this also the cause of the central heating cutting off? Many thanks in advance!

 

[martygturner]

Do you bleed the rads whilst topping up and is the inhibitor refreshed. It sounds like air build up from rusting of the rads.

 

[Jackrae]

Sounds like your header tank feed to the boiler is either fully/partially blocked or the header tank inlet isn't refilling the tank

 

[siliconeseal]

@martygturner
Thank you for your fast reply. I do bleed the rads while pressurising. I have not added any inhibitor for the past 12 years, fearing that this would contaminate the overhead tank, which also feeds the bathroom cold water. There is no separate tank dedicated to the central heating, which would allow me to add inhibitor. I was present when the circulation pump was changed. The water which came out when the connectors were undone was slightly rust tinged. I forget to mention that the old rusty overhead tank was replaced with a plastic coffin tank about 4 years back.
@Jackrae
Thank you too for your fast reply. What would cause this blockage to occur? I looked into the loft tank and it was pristine, without any sludge, deposits or any other solids. And the pipework to the braided tube is all pvc. Are there any checks that I can make either into the pipework or the expansion tank to confirm blockage?

 

[Bod ...]

I quickly gone through this and not read everything.

You say there is no header tank. ?

Have you got a big red round ish shapped vessel either with the boiler or in the airing cupboard?

If not something is seriously wrong. Your heating system could well be scaled and rusted up. Best get an heating engine to take a look.

 

[polesapart]

You seem to be describing a sealed heating system with gravity (tank-fed) hot water.

Could you post pics of your airing cupboard showing your cylinder, filling loop and expansion vessel?

 

[Mottie]

You seem to be describing a sealed heating system with gravity (tank-fed) hot water.

That’s what I can’t understand but later on there is mention of a filling loop and a pressure gauge. Maybe a sealed heating system with vented hot water?

Then again, there's an expansion vessel next to the hot water cylinder….

 

[BlueLoo]

I am confused as well.

Given the head tank/sealed system, i wonder if the topping up is backflowing into the tank?

But then you'd blow the tank out surely?

Where is all the water going on top up?

Draw the system out, take a few picks and host them up.

 

[siliconeseal]

Gentlemen, I have attached a photo of the expansion vessel in the airing cupboard. It appears to have been fitted upside down, with the Schrader valve facing the cupboard floor. Apparently (from my Google searches on this matter), it doesn't matter which way up the vessel is, but do correct me if I'm wrong. I do know that it worked fine for the last 4 years). The overflow pipe extends outwards through the wall on the right of the photo. The photo is worth a thousand of my words, as the saying goes. I have three questions:

1) Airlock. Would draining the system
(involving bunging the outlets of the loft water tank and shutting off its ball valve, then attaching a garden hose to the outlet valve of the ground floor hall radiator, then opening the outlet valve, then opening the bleed valves of all the radiators, then waiting until no more water flows out of the other end of the hose),

followed by refilling the system
(involving, shutting all the rad bleed valves, then unbunging the loft tank outlets and freeing its ball valve, then going outside to the free end of the hose and allowing water to flow for ten minutes, then shutting the ground floor hall rad's outlet valve & decoupling the hose, then bleeding all the rads, ground floor first then 1st floor),

cure any airlock problem which could be impeding the repressurisation? If you have a better method then please give details.

2) Does the photo lead you to conclude that there is another more likely possible cause than an airlock?

3) With the expansion vessel upside down, what air-pressure should I aim for, after connecting a bicycle pump to the Schrader valve?

 

[BlueLoo]

Dunno.

I wonder if your top up line (full mains pressure) being so close the pressure relief line (not an overflow) that one overcomes the other hense the time.
Perhaps check the prv outlet when filling. If its flowing, there is your problem. You might try to try filling very slowly, not opening any fill valves full.

Or the prv could have failed. Unlikely if it does eventually hold pressure.

 

[siliconeseal]

BlueLoo said:
I wonder if your top up line (full mains pressure) being so close the pressure relief line (not an overflow) that one overcomes the other hense the time.
Perhaps check the prv outlet when filling. If its flowing, there is your problem. You might try to try filling very slowly, not opening any fill valves full.
The current expansion vessel was installed at the same time that the loft coffin tank was put in (that is 4 years ago). Everything worked fine until this winter. If there were a problem with top up line being close to pressure relief line then wouldn't it have shown up earlier?

 

[polesapart]

1 What a fugly way to install a filling loop! Not necessarily wrong but little effort by installer.

2 I was hoping to see a bit more detail on the valves at each end but these are facing away in the the photo
(and from the user).

3 To answer the original question: the weight of the hanging loop may have flattened the the internal rubber of the braided loop near the top connection.

 

[siliconeseal]

@polesapart
Thank you. I quite agree that the installation isn't a pretty sight - the installing plumber was a Checkatrade man, and came highly recommended. This same man, banged into the rusted steel loft tank, and managed to make a penny sized hole in it, water seeping down, wallpaper & carpets ruined etc. Had to get in another plumber to replace the tank with a plastic coffin. It just goes to show how unreliable Checkatrade recommendations can be!

The main problems with the installation are (1) Impossible to see what is the pressure setting on the red top PRV adjustment knob (on top of the expansion vessel) (2) have the expansion vessel upside down traps the water inside it, when draining the system (3) difficult to extricate the expansion vessel in order to empty it of water and pressurise the air content.
There doesn't appear to be a kink at the top of the braided hose. The grey flexible PVC hose is fixed to inverted 'L' shaped copper tubing with a cable tie. I pushed the cable tie up the copper tube, thereby raising the braided tube, 10 minutes ago. It made no difference to the shut down heating system.

 

[BlueLoo]

BlueLoo said:
I wonder if your top up line (full mains pressure) being so close the pressure relief line (not an overflow) that one overcomes the other hense the time.
Perhaps check the prv outlet when filling. If its flowing, there is your problem. You might try to try filling very slowly, not opening any fill valves full.
The current expansion vessel was installed at the same time that the loft coffin tank was put in (that is 4 years ago). Everything worked fine until this winter. If there were a problem with top up line being close to pressure relief line then wouldn't it have shown up earlier?

5-10 mins of mains water flow isnt normal to top up a pressurised ch system.

It should take seconds.
You should only be topping up a very limited amount of fluid or pressure loss.

So either, you have a massive leak (you don't because your pressure would drop instantly) or, what you think is happening when you top up, isn't. I can only think that the mains water is ****ing out of the PRV or back filling up this tank in the attic (past overflow?).

Or you have a ruptured expansion tank and water is going into the hw system or whatever.

 

[siliconeseal]

BlueLoo said:
So either, you have a massive leak (you don't because your pressure would drop instantly) or, what you think is happening when you top up, isn't. I can only think that the mains water is ****ing out of the PRV or back filling up this tank in the attic (past overflow?).

Or you have a ruptured expansion tank and water is going into the hw system or whatever.

I can only think that the mains water is ****ing out of the PRV or back filling up this tank in the attic (past overflow?).
I have not seen any water ****ing out of the PRV, so that is out.
Backfilling is possible. But how to check this and more important how to prevent this?

Or you have a ruptured expansion tank and water is going into the hw system or whatever.
I tapped the upper and lower parts of the expansion tank. As you would expect, the airfilled lower part sounded hollow while the upper part sounded full of water.
EDIT: After googling I found that backflow can be prevented with a check valve, like this one:

Flomasta Single Check Valve 15mm - Screwfix

Order online at Screwfix.com. Brass, steel and plastic construction. Suitable for water. FREE next day delivery available, free collection in 1 minute.

www.screwfix.com

Is this right?