Problem with hot water - indirect coil cylinder

[Ernie Bilko]

I have an oil fired boiler system with two circuits, both pumped. One drives the rads - which works fine, the other goes to the hot tank. Since September I've had no hot water from the tank. I fitted a new pump but no joy. What happens is this:

When the boiler has been running for a while the feed to the coil gets hot as expected but no matter how long the boiler runs, all day even, the return is always cold. If I switch off the pump I can hear the water "falling" back down almost like the pump has been pushing against a blockage.

So I drained the system and disconnected both ends of the coil and there is standing water at both ends. I would have assumed that the coil would have drained out via the bottom pipe when I emptied the system, is this correct?

Does this all seem like the coil is blocked inside the tank. If so can it be unblocked or is it time for a new tank.

House was built in 1995.

[The Natural Philosopher]

sounds blocked to me. Try blowing through it.

It may be scaled, in which case a flush with acid MIGHT get it sorted, or destroy the heat exchanger..but it sounds too new for this to be an issue.

[Ernie Bilko]

VERY soft water area. Never had so much as a hint of scale on anything in the house.

[BigWallop]

Have you noticed any automatic valves on the pipework? Some systems have a zone valve, even if gravity fed or sealed, to close off the circulation to particular parts of the loops. Have a look along the pipework for anything that looks like a shiny metal, rectangular box which is fitted on the pipes.

If it does have closure valves, then the one for your hot water system might be faulty and not opening to allow the water to flow through. The pump can push as much as it likes, the valve won't budge. Which is what you are describing.

Have another look and get back to us, please. Just to satisfy my curiosity. :-)

[Doctor Drivel]

Blow through it. Does the pump have a check valve? It should as two pump systems require one after each pump. Check that the valve is OK.

It will not be scale as it is on the primary side. It the system full of sludge?

[ARWadsworth]

It could be an air lock. The may be a bleed valve near the cylinder or you may need to put one in.

Adam

[Ernie Bilko]

Pump check valves are open. No motorised valve or anything else between the pump and the coil. Very simple system, two loops two pumps. Has worked perfectly for 13 years until now. When drained the water looked pretty gunky. I was wondering if it is sludge and as the system has been unused over the summer, it may have congealed and blocked it. Top of the feed pipe is the expansion path to the F&E cylinder so any air would go straing out. Also when I refilled I filled from the system drain point using a hose so the system filled from bottom to top. That should have shifted any trapped air.

Setting the pump speed to max has no effect either.

What concerned me most is that when I disconnected both ends of the coil there was still water at the top coil connection on the tank, i.e. the feed. If the coil was clear wouldn't the coil have drained down with some water maybe at the bottom right angle joint, i.e. the return.

Replacing the tank could be expensive as I have a Willis type immersion heater, which is fantastic and I want to keep even if I need a new tank.

[BigWallop]

The best way to find out exactly is to undo the coil and fish a drain spring through it. If it's clear of any blockage the spring will pop straight through. If it's blocked the spring should clear it out for you.

I do know, from my recent fluid dynamics learning (thanks Rod), that the coil in the tank works like the bend in a river, and causes a fast and slow running current as the water turns around the bends. The slow moving part on the inside of the bend can deposit silt against the inside edge which is not removed because the water is so slow moving. The silt build up over time creates a narrowing in the pipe which only reinforces the slowing of the water current even more, so more silt is deposited until the water is so restricted that it blocks itself off.

You say you drained the system and found it really dirty, so it might be the gunk that has built up in this part of the system, due to the effect described. Push a drain spring through the coil to find out for sure. The springs are cheap from the DIY sheds, and a fully re-usable for other things.

It the only other thing I can think of.

[Doctor Drivel]

Run a long wire through the coil, like a drain wire. Clear if necessary. Replace check valves. Insert a Meganclean filter on the boiler return, near to boiler. Insert an in-line gauze filter on the same pipe near the filter. Put in X-800 and stick to instructions. Drain and flush and clean out F&E tank. Fill and remove air. Insert X-100 into Magnaclean filter. Run for one day and clear filters. Run for a week and clear filters. Clear each week until clear. Clean filters every year. Pour in X-100 every 3.5 years. No need to drain.

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