Leaving mains valve into boiler system open

Hi, any opinions on the following would be appreciated.
We had a new condensing boiler installed about 8 months ago.
After all the work was complete and almost as an after-thought the guy who installed it told us we had insufficient water pressure for the system.
It requires 1.0 bar to operate and there was only a pressure of 0.8 bar in the mains. He boosted the pressure up within the system using a hand-pump and it worked great for about 4 months. Then the pressure in the system dropped below 1 bar and it cut out. We were getting work done on the kitchen at the time and that may have caused the drop in pressure so I just called the guy out and he pumped it up again.
After this I decided to keep a close eye on the pressure. It started off (in Sept) at 1.8 bar. I have checked it periodically since and at one stage it was down to 1.0 bar. If the heat is on all day it seems to go back up a little (currently 1.3) but the trend is obviously downward.
I have had the council out and they measured the pressure entering the house at 2.0 bar so the loss of pressure is obviously internal. I could have got a plumber to see if he could find the problem but seeing as the boiler was under warranty and is not operating as it’s supposed to I decided to get the installer back.
Anyway to cut a long story short he has now suggested leaving the mains valve into the boiler system open during the day so that when (if?) the mains pressure goes up it will boost the system. Is this a good idea? Something about just leaving the mains valve open doesn't sound like a good idea to me.
Thanks in advance and sorry for the longwinded question.

Had to read that a couple of times so I could fully understand what was happening.

Don't leave top up valve on boiler open.

Had you a pressurised system before this or did you have a small tank that fed radiators in the attic?

Where did the council test your pressure from?
Did you tell the installer that mains pressure was tested at 2 bar?

Have more questions than answer for now I'm afraid

Hi DavyJones,
Old system was fed rads from the attic tank.
The Council tested the pressure at the stop cock coming in front of the house.
They said that once the line goes from there onto our property its our responsibility which is fair enough.
I only had the council out after the new boiler was installed and I became aware that there was a problem with the pressure. I have said to the installer that the first thing he should have done was ensure there was adequate pressure for the system he was putting in but he's fobbed me off.
I think I will just have to get a plumber to try and discover where I'm losing pressure between the stopcock and the boiler?

Mains pressure and your heating shouldn't be in any way connected as such, .I am perplexed by your story.

your old system was an open vented system, that was filled by a tank.If you lost water it would automatically be refilled by this tank. You now have a sealed system, I.E you fill it once and it doesn't automatically fill its self. If you are losing pressure you may have tiny leak on system that was never noticed as old system topped itself up and there was no visible way of telling of system drop, EG pressure gauge clock..

I would advice you get another indepentant installer into have a look. From what I understand, you probably don't have a major problem.

Thanks for the advice.
Think I will get someone to try and locate a possible leak alright.
I suppose any closed system will lose pressure over time?

I found that happening in the last house i lived in , apparently micro bubbles appear on the inside of some of the older plastic pipes ( about 15 years ago ) that were used to plumb the house & the water pressure used to drop down like that. Once a month I used to barely open the filling loop and a small drop of water used to top it up but it still had to be done a good few times a year. All of the houses in our housing estate had the same problem

I think it could be something like that alright Bertie1. The only problem we have is our mains pressure is pretty poor so opening the filling loop doesn't seem to have an immediate effect on the system pressure. That’s why the installer has suggested leaving the mains valve open. Not convinced by his rationale though.
What I might do is leave the valve open for a day and see if it has an effect of the pressure. Won't leave it open permanently.

..there's another reason for not leaving it connected: by doing so, you are in danger of contaminating the mains (potable) water system, with the heating system water - bad idea.

Don't leave it open , there is fernox in the heating system which could go back into the drinking water ( and will poison you) should your water supply go off for any reason, secondly if you lost water supply , the whole heating system will drain out & you will not be able to turn your heatig on , thirdly if the pressure gets too much in your heating system ( because you let it open ) the pressure release valve will just start blowing off from the boiler ( or the over flow)
You only need about 1 bar to 1.5 bars in the system when its cold ( it can rise to about 3 when its hot.)

There is a small chance of cross contamination. The filling loop or automatic filling vavle will have a non return valve fitted, this will only allow water to go in one direction.

I would advice you speak to a different plumber.

Thanks lads. There is a non-return valve fitted so water should only be able to flow into and not out of the system. I will definately get another plumber in before I do anything however.

Slightly off topic but I read that topping up a sealed system can lead to the radiators rusting. Why is this not an issue with an open system as it tops up as it needs.

silvermints - it isn't any different, so that advice is......whatever.

What they're thinking of is the fact that new, it probably has an inhibitor in it, and, as you top up with plain water, you're diluting it. You'd want to be topping it up a lot, with a lot of water, for the effect to be measurable imho.......

...which reminds me of a friend's install some years ago...........UFH in two wings of a single storey building, two separate manifolds etc.

Eventually, the system ground to a halt, and the circulating pump was toast...........turns out the ballcock in the tank stuck, up, and didn't replenish the tank, system ran dry, and pump failed. Cleaned it, all was well, and replaced the pump. The question is: where did all the water in the tank go ? And, over a period of the 7 - 8 years.........just how much had it topped the system up by, unbeknownst to the owners ? Again...........where did it all go ?