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Unstable Central Heating - Rads not Getting hot, Flushing System Worked but Problem Returned

Postby o-daniboy » Thu Jan 16, 2020 12:52 am


We have a central heating problem that has been dragging on for months. Boiler unstable - temperature rising rapidly before plummeting. Normally rising to around 70 then falling to around 40. Sometimes it overheats to 100. Radiators get lukewarm at best. Boiler often needs to be reset with fault codes relating to pressure problems.

House is a 1930s 3 bed semi with a loft conversion and ground floor garden extension. 7 rads on ground floor, 4 on first and two at the top. Boiler (Gloworm) in cupboard on ground.

Plumber and his mate have probably spent 30 hours failing to fix it. They have fitted a new pump, taken floorboards up and replaced small sections of pipe. The system has been drained several times and cleaned with cleaning chemicals. A mag filter has been fitted above the boiler.

They thought they had fixed it a month or so ago after clearing a blocked pipe under the floor on the first floor close to the hot water tank. The system ran stable for a few days but then the problems returned.

Last week they flushed the radiators on the ground and first floors. Plenty of sludge came out. Again it seemed to have fixed the problem - system was running stable and rads were getting hot. Left it running constantly but after two days the problems returned - now highly unstable again, with only a few rads getting slightly warm.

We had a problem in the summer with ground floor rads getting warm when the hot water was on. Plumber put a shut off valve under the floorboards in the boiler cupboard to fix this.

Water often comes out of the overflow pipe on the side of the house. The loft extension appears to have been a DIY job. The radiators that were fitted look cheap. There is an additional pump sending hot water to the bathroom tap and shower in the loft extension bathroom - this has not worked for a year or so. I've pointed these things out to the plumber but he does not think they are related to the problem.

I wonder if sludge from the cheap top floor rads which weren't flushed has worked its way around the system and found its way to a weak point and blocked it again.

The plumber appears to be fresh out of ideas and has now emailed the boiler firm for advice. I wonder why he did not flush the top floor rads.

I'm running out of patience and money. Cost is now up to around £2000. Latest bill just for materials not labour (mag filter, copper tube, copper fittings and chemicals) came to £420 which strikes me as high.

Apologies for the length of the post, but I wanted to get all the details in. Any thoughts would be greatly appreciated.


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Simply Build It

Postby ericmark » Fri Jan 17, 2020 1:08 pm

I can see your problem, reminds me my plumber has not returned and replaced the inhibitor, this is a problem, flush the system and new inhibitor required.

I have had in sisters house faulty pipes where copper was blocking the pipes and I used a tyre inflater to blow out the rubbish, not really a safe method.

However in the main there are two problems, one new water has air in it, so when it gets hot, air is released so you get air locks, so common when system drained to have to refill with water inhibitor mix and bleed, then return within the week and bleed again.

In the main the lock shield valve is the problem either not set, or not set to what is required, the problem is often to get the flow required so the TRV has time to open/close also means a restriction at the valve, so any muck builds up at the lock shield valve.

You have a host of controls, pump speed, by-pass valve setting, lock shield valve setting, boiler output temperature, TRV settings, which all impact on each other. It took me a year to set mothers central heating, and if one person fiddles it can all go up the creak.

What I look at is you have had plumbers on site who seem not to have fixed the problem, so how do you expect to fix it on a forum? I would say some one in the house is fiddling. May be not realising what they have done is messing it all up, had it with mothers house, marker pen was cure.

Once set I would draw a line one TRV, and lock shield so if some one fiddled at least I could return it to setting that has worked. In my case it was my dad, I would carefully match the temperature of the Hall lock shield, TRV and wall thermostat so the radiator would turn on full after front door opened, then turn down before it hit the wall thermometer setting so last few degrees took some time and allowed rest of house to get warm, that hall radiator was key to whole house. The hall was what decided how long the boiler ran for, rest of house did not really matter where the TRV was set, as long as hall was correct.

So he would turn the wall thermostat up/down and whole system out of balance, cure was set stops on wall thermostat, so he could turn it down when he felt too hot and wanted to save a few pennies, but when he turned it up to stop, it was at exactly right place.
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