Problems with unvented hot water cylinders in freezing weather

If you have gas central heating and in very cold weather you find the heating is working but no hot water is coming out of the taps and cold water is only available to some of the ground floor taps then you need to get your hot water working.

First let me explain an unvented cylinder. This is a pressurised tank of water cylindrical in shape which is usually heated by your gas boiler. They sometimes also have an immersion coil as back up. If you have a combi boiler then you will be unlikely to have a cylinder tank unless you have solar.

I have already explained elsewhere about problems with condensing boilers. This is a possible solution if you have central heating but no hot water with an unvented pressurised cylinder only.

Cylinders were vented; as the water heated up it expanded and an expansion tank in the loft was connected to the cylinder so that water could expand into the tank. In addition to get proper water pressure it was necessary to mount the cylinder near the top of the house and have a cold water tank in the loft.

Unvented systems do not need these expansion tanks at all. They are connected to pressure vessels, which enable the hot water to expand, and the pressure of the water mains is in most places sufficient to give enough water pressure to enable you to have baths and showers without pumps. You can, if you have no tanks in the loft, also use any cold water tap for drinking as the cold water comes from the mains without passing through a tank.

Now if you can get heating but no hot water the reason will be a blockage along the cold water pipe that runs from the mains to the unvented cylinder. The reason for the blockage may be a bit of loose dirt, but this is extremely unlikely from mains, unless the water company has been doing work outside your home.

The most like reason is very cold weather and somewhere along the pipe leading to the unvented cylinder ice has formed. This is especially likely with modern condensing boilers, because they run very cool and if located in a boiler room of garage then very cold weather, such as we have had recently, may cause a pipe to freeze.

It is most likely to freeze overnight, when no one will be running any water.

You can unfreeze the pipe using hot water bottles once you locate the frozen bit, or simply leave until the weather improves and the pipe will defrost naturally. The good news is that these cold feed pipes usually do not burst when defrosted because the water tends to expand along the pipe rather than across it.

Once defrosted you should have no problem but it is as well, if this has happened once in extreme cold conditions to get some pipe insulation and wrap it round the cold feed pipe.

3 Responses

  1. Insulation, Insulation, Insulation, that is the key to preventing frozen pipes, if you have pipes that are outside the heated space of the house, loft, unheated rooms of a house these must be insulated, but it is a good idea to insulate as much of exposed pipework throughout your property, you should try and use as thick of wall insulation as you can fit, all cylinder cupboards should have all the pipe work insulated to comply with Building Regulation part L, this is not just to prevent freezing, but also to reduce heatloss

  2. I have this problem but only in my utility room – no running taps and the washing machine wont work. The rest of the house in find. I can’t find the pipes though to defrost them so it’s a waiting game.

  3. Hello,i get plenty of hot water but my under floor heating is quite useless compared to how it used to work,could it be one of my 2 port m valve’s? HELP,,,,,Cheer’s. Stef.

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