Overflow from Cold Water Tank & Low Hot Water Pressure

[Cuore155]

Hi,
About a year ago I installed a new bathroom suite, so had to drain hotwater system. I assume since then (but only noticed about 6mths ago) that everytime the water is on (indirect Cylinder with backboiler) my cold water tank level rises about an inch and then overflows. The water coming back into the tank is warm, but appears to be coming back up the cold water feed to the cylinder and not the return pipe. It makes no difference what temperature the thermostat on the cylinder is set to (currently 55deg C ). A few weeks ago the cylinder developed a leak around the top seam and i had to replace. I had hoped that it was a problem with the old one which was causing the overflow, but it is still there. I have always had an issue with hotwater pressure in the house, however since changing the cylinder it takes 30mins to fill the bath (although I'm sure it was fine for the first few days), and the pressue seems to be a bit erratic, although has always been pretty poor. Have I got an airlock somewhere or could it be limescale buildup in the pipes causing a blockage, the old tank had about 3in's of the stuff at the bottom!! Please can someone give me an idea of what the problem might be and how to solve it, as I am at my wits end.
Thanks

 

[Bernie2]

First of all hot water will expand from the cylinder up the cylinder cold feed and into the stored water cistern. Although in theory it should expand up the vent pipe as well. The thing is water exerts pressure equally all round so it goes up the cold feed as well as the vent. It depends how hot the water gets and a back boiler if its solid fuel is virtually uncontrolled so it will get hot.

Its more understandable if you remember that a header tank on a central heating system has a vent but also the tanks other names is feed and expansion tank. And that is where the hot water from a central heating system goes. An indirect hot water cylinder works the same way. If it gets excessively hot though there is usually another problem somewhere.

Of course as you say the hot water vent could be blocked with lime scale and its backing up the cold feed which it would do, as lime scale forms quicker at raised temperatures. The thing is if you lower the heat to below 60C then you increase the chances of legionella.

You could check the pipes for lime scale. And somehow connect the main to the hot with a hose, when the hot is cool, and try and blow any airlocks out. Don't hold it on any longer than a few minutes as it should blow out of the vent and into the store tank. Which will probably overflow.

So turn all taps off connect hose from main to hot and blow out air. Usually works if there is an air lock. If the hose keeps on blowing off, unfortunately this could also be a sign of lime scale in the hot water pipework. And as there was 3" of scale in the old cylinder it seems fair to say it could be in the pipes as well.

Its a case of trying the easy bits first.

Perhaps a reason is that the water level is to high in the storage cistern/tank. But check the other stuff first.

Good Fortune!