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BLOCKED CYLINDER HEATING COIL

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OldDIYplumber

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We have a hot water cylinder problem. The hot water from the boiler (system boiler, gravity fed, C Plan) goes into the cylinder in the middle after the 2-way valve, but the temperature at exit at the bottom is always at least 20 degrees C lower. It at best warms the water in the cylinder, or, if the immersion heater is used, keeps it hot. We suspect a gungedbup, I.e. sclerosis, coil in the cylinder. Tried back and forward flushing from the top up tank, but as there are two bathroom radiators on the DHW hot water loop (the other pumped CH Loop is seperate), we reckon flushing bypasses the cylinder. What do people do if the coil gets clogged? Tried putting sludge remover down the top up tank, it hasnt cleared it.
 
If the coil is clogged solid, no amount of flushing or chemicals will unclog it and old be new cylinder time, but if you’re getting a 20 degree differential then it’s obviously something. Does the heating work ok?
 
1. As Undertrained has said, if the coil were completely blocked its new cylinder time. However, as you appear to be getting some heat there is a very good chance it isn't blocked.
2. Coil could be airlocked.
2a. If there is an air bleed valve / vent on the top coil connection make sure it is open. Purge air with the boiler off.
2b. If there is no air bleed valve gently slacken the nearest connection to the top coil entry into the cylinder. Try and avoid touching the actual connection on the cylinder as they can be relatively easy to damage, again leading to new cylinder time.
3. If the problem isn't an airlock, could be a partial blockage. I'd cut the connections to the top and bottom coil entry points, and run cleaner / de-scaler through until it runs well and relatively clear. Then remake the connections, purge air and try again.
4. The temperature at the lower coil will always be less than that at the upper. The coil heats the water in the cylinder, the hot water rises leaving cooler water at the bottom, which cools the return pipe.
 
Thanks to both of you. There is no upper bleed valve, hot water from the boiler goes straight from the Honeywell into the cylinder. The Honeywell is over 40C and pipe into cylinder 6" further on Is not much lower. It's what comes out at the bottom being so much lower that's odd. Unless, that 40 degrees on entry itself is too low as the boiler is set at high 70s at the moment. So, the blockage could be further back in the feed from the boiler I guess. Except, the towel radiators are very hot. I'll try slackening the nut at some point to vent any air.

It sounds like it will require at least a drain down of the cylinder induction coil if not the entire top floor if the top and bottom nuts to and from the cylinder coil need taking off to flush the coil. That's a bit risky for a DIY yes?
 
Your cylinder thermostat (what you’re calling the Honeywell) should be set to 60-65 degrees to kill off any bacteri so legionella can’t grow.
 
Last edited by a moderator:
By Honywell, I'm referring to the two way valve, not the cylinder thermostat. I've tested the thermostat with a multimeter, and that's set well above 70 too, seems to work fine too. Checked the honeywell valve spindle and it turns. The motor is OK too. The immersion heater in the cylinder, incidentally, works ffine too so we can use that for HW. The central heating also works fine. I suspect air, or more likely, sludge in the induction coil.
 
OldDIYplumber said:
We have a hot water cylinder problem. The hot water from the boiler (system boiler, gravity fed, C Plan) goes into the cylinder in the middle after the 2-way valve, but the temperature at exit at the bottom is always at least 20 degrees C lower. It at best warms the water in the cylinder, or, if the immersion heater is used, keeps it hot. We suspect a gungedbup, I.e. sclerosis, coil in the cylinder. Tried back and forward flushing from the top up tank, but as there are two bathroom radiators on the DHW hot water loop (the other pumped CH Loop is seperate), we reckon flushing bypasses the cylinder. What do people do if the coil gets clogged? Tried putting sludge remover down the top up tank, it hasnt cleared it.
Click to expand...
Have you tried (with CH loop off) shutting off one lockshield vave on each of the two bathroom rads to force all the water through the cylinder coil?, with HW heating on, if the flowrate is still very poor then the boiler will cycle rapidly.
 
John.g said:
Have you tried (with CH loop off) shutting off one lockshield vave on each of the two bathroom rads to force all the water through the cylinder coil?, with HW heating on, if the flowrate is still very poor then the boiler will cycle rapidly.
Click to expand...
I've turned both towel rads off (no lockshields that I can see oddly), before flushing, but even with the rads off, the piping is still there to prevent the back or forward flushing pressure being just on the coil. I guess that's a downside of having tool rads on the same circuit? The entire system hasn't been drained for 6 years, so my last resort will be adorn Inc. all rads hoping that will suck via vacuum any sludge out....with fresh water giving it a push. My fear is this is like blocked arteries, I.e. a plaque. I appreciate all of the comments, as I am otherwise stumped.
 
I mean without flushing and with HW only on (cylinder stat turned up) and the two bathroom rads shut off (by whatever means) does the boiler cycle rapidly as the boiler water can now only circulate through the cylinder coil. Or are the bathroom rads on one pipe (the flow, one pipe system)
 
I think the boiler cycles/shuts down more rapidly when the towel rads are off (understandably as less heat loss). Cylinder stat is already up at 85C,

There is so much good lagging on the return pipe to the boiler back from the coil out at the bottom of the cylinder and in the boiler room that I can't get a good temp reading, but guess I should bite the bullet and undo some lagging to check? I have a sensor there at the bottom of the cylinder so I can monitor degrees C 24/7 every 15 mins. It went bad when I had to drain the main tank to replace a shower which had no h/c isolation valves, but think it was on its way out for a while before. Old system.
 
I think the boiler cycles/shuts down more rapidly when the towel rads are off (understandably as less heat loss). Cylinder stat is already up at 85C, There is so much good lagging on the return pipe to the boiler back from the coil out at the bottom of the cylinder and in the boiler room that I can't get a good temp reading, but guess I should bite the bullet and undo some lagging to check? I have a sensor there at the bottom of the cylinder so I can monitor degrees C 24/7 every 15 mins. It went bad when I had to drain the main tank to replace a shower which had no h/c isolation valves, but think it was on its way out for a while before. Old system.
 

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