DIY Doctor

No Hot water, but central heating working OK

Postby Steve Block » Tue Mar 20, 2007 10:25 am


I recently drained down the h/w system and refilled it with and added a cleaning agent, with the idea to remove sludge build-up etc.

I took time to ensure that all of the radiators were bleed, started at the top and worked down – not sure if that’s the correct way round. I made sure that the system was fully filled before turning the boiler on again.

However, now only the heating works OK. The radiators all got hot, but for some reason there is no hot water. It appears that the when I set the boiler to hot water only, the boiler starts-up OK, but after about 1 or 2 mins, the boiler stops and appears to be very hot. I’m wondering if it is stopping as a result of any over-heat safety feature ?

Could there be an air lock somewhere in the system that could be preventing the water circulating around the boiler, causing the boiler to start over – heating and cut-out. As mentioned before, the central heating appears to be working fine.

Any pointers would be appreciated ?

Best Regards
Steve Block
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Simply Build It

Postby TB » Thu Mar 22, 2007 7:41 am

I have the same problem, can anyone help.
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Postby winebasket » Wed Mar 28, 2007 6:15 am

I had a similar problem and it was caused by an air lock. Look at the pipe run on the hot water side (usually larger pipes than the central heating pipes) to see if they are higher at one point and drop down to a lower point. That is probably where the lock is. Forunately mine had compression joints on that part. I simply put a bowl underneath and loosened the elbow joint. I could hear the air rushing out. After a minute the hot water started circulating. :D Good luck.

If the high point is all soldered joints........ it is a slightly bigger job. I would drain the system, and put in a compression joint (and or a bleed valve) in at that point. Goods luck
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Postby peter the plumber » Wed Mar 28, 2007 4:52 pm

It sounds to me that your 2-way/3-way valve is jammed with sludge or just failed.

A 2 way valve does hot water or heat.

A 3 way does hot water or heat or both.

You need to have a pro look at it.
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Postby valpenguin » Mon Apr 02, 2007 2:09 am

Hi, I followed the advice of 'winebasket' and lo and behold hot water again. Thanks winebasket!! I owe you a drink!

To re-iterate, check the hot water cylinder. The 'higher' feed pipe-this comes [i]from[/i] the boiler needs to be loosened maybe a quarter to half a turn. The noise of the air escaping sounds a bit like water but as long as there is no dripping you're OK. After about 30 sec to 1 min you'll get a bit of dripping and it's about then to re-tighten the compression joint. I turned on the H/W at the control panel and within a minute this pipe into the tank was HOT, knew we'd cracked it then. It's a very easy process with nothing ventured, nothing gained so if you think this might work it can be your first port of call. You do need a BIG wrench though!

Hope this helps anyone.

Many thanks to winebasket and all who contribute....

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Postby Danny » Wed Apr 04, 2007 10:07 am

My central heating works but if I turn it on, both heating and water come on regardless of the setting on the programme. In other words when I programme only water both heating and water come on. If I turn the heating down by the thermostat in the hall it makes no difference. What can cause this? Could it be the three way valve? Danny
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Postby strevs » Wed Apr 11, 2007 9:08 pm

Yep....try moving the lever manually to see if it makes any difference..don't forget could be programmer has stopped sending correct instructions.
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Postby staffy » Mon Apr 16, 2007 3:46 pm

I have the same problem... when i turn on the hot water the heating comes on too and visa versa, the boiler also kicks in for a few minutes and then stops but a few minutes later kicks in again and this happens whenever I turn the boiler on.

Any help or suggestions would be greatly appreciated
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