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Hot radiators but no hot water after draining system

Postby patrick » Tue Jan 29, 2008 2:03 pm

I think I have an open vented CH system with boiler and pump downstairs, hot-water cylinder in bathroom and 2 tanks in the loft.
My radiators keep sludging up and corroding so I have been intermittently flushing them out, and/or replacing them. 2 weeks ago I came home to a new water feature as one of the upstairs rads had burst. I drained the system and replaced that and another radiator and while I was at it removed and flushed all the other sludged radiators. I then flushed and refilled the system and bled the radiators as I have done before without a problem.
When I have done this before it has taken a few days on full heat for the hot water to work until what I have been told is an airlock clears in the cylinder (no bleed valve at top).
I went away on holiday hoping hot water would be there on my return but still no joy 2 weeks later. Also the hot water tap in the bathroom releases no water at all (all other taps work fine). The radiators are all working perfectly - hot house but no hot water.
Another concern is there is hot water flowing from what I assume is an overflow into the header tank and then out through an overflow onto the roof.
Any ideas how I can restore the hot water? how to get the tap flowing?
Why does the system keep sludging? why is it always overflowing?
Would a new system, combi boiler(??) be the long-term solution to all my problems.
Sorry for the essay but it's only by writing it out I've realised how many problems there are!
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Simply Build It

Postby Kenj » Wed Jan 30, 2008 6:21 pm

Hi Patrick,
I am not sure where to start ! However here goes.

Many cheaper descalers are next to useless and will not flush the system properly. Try Fernox DS-40. This will do a good job on cast iron boilers and Copper pipes.

Re the lack of hot water: You should have one or two motorised valves. These switch the water flow, ie - Hot Water only - Central Heating only - or both. You need to check that the motorised valve opens when you switch to hot water. You should hear it move. Also the pipe from it to the H/W tank should start to heat up. You can depending on the type, often manually open these balves. Let me know if you have one or two motorised valves.

What happens if you select only the Hot Water to be on?

The hot water overflowing into the expansion tank may be due to a motorised valve being shut when it should be open. Is there an Aerator near the H/W tank ?

As for the H/W tap in the bathroom. This is unrelated if you have an Indirect H/W system, which you have if you have a H/W cylinder. Try removing the tap or disconnecting pipe work further back. I suspect that you might have a blockage.

Good luck.
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