Mystery loss of hot water!

[swordfish227]

Help! my hot water has been getting colded and colder over the last 2 months.
The main hot feed to the hot water cylinder from the boiler is as it should be in regard to temperature, and the return to the boiler is cool to the touch. The tank is cold at the bottom ( as it should be) and only warm at the very top of the tank.
Subsequently the hot water feed to the shower and hot taps is now lukewarm at best!
The system has not been flushed for sediment or sludge for a good 10 years, so I got a qualified heating engineer to have a look at the system and we're still stuck for an answer!
Checked the pump and valve - all ok - opened the feed to the cylinder and no flow at all!
Drained the system down, discovered that the F&E tank wasn't draining down even after backfilling & redraining. The guy then cut the rising hot feed pipe above the tee (1) to the cylinder (purple pipe also system vent and fill from F&E) and removed a lot of oxide sediment.
Back filled the system from the mains through the drain cock, rads are hotter and the feed to the cylinder now flows but is still not heating the cylinder water!
Checked the flow through the heat exchanger (HE) myself = good flow not blocked.
2nd Engineer looked at it and closed off the rads and put the pump speed up to max to clear possible obstruction on return from the HE.
This increased the heat in the top of the tank but not significantly.
Drained it myself today and refiled with Sentinel to flush hopefully but the F&E had to be drained manually by draining from the compression joint (1) and I had to put the Sentinel in by disconnecting the return from the HE and pouring it in there.
Noted the the purple line below the cut (1) was still full of water even with the system drain cock open.
A bit long winded I know but I though best give you all the full story!
I suspect there is a lot more iron oxide somewherein the system but don't know if the 2 problems (F&E drain and no hot water) are related.
Anybody got a theory?
System drawing attached

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• swordfish227
• 22 Jan 2014
• 1

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[Armo74]

Do you have to bleed your rads frequently?
If your feed and expansion tank isn't draining you have a blockage in your cold feed pipe (the 15mm pipe coming from the bottom of the smaller tank of the two).
If there is a blockage the system pipework will not completely fill properly.

 

[picasso]

I see you have mains cold and low pressure hot at the shower, you could have cold water backfeeding into the hot water cylinder, does the overflow from cold water tank in the loft ever run ?

 

[swordfish227]

Do you have to bleed your rads frequently?
If your feed and expansion tank isn't draining you have a blockage in your cold feed pipe (the 15mm pipe coming from the bottom of the smaller tank of the two).
If there is a blockage the system pipework will not completely fill properly.

Hi Armo,
The spare room rad was the gathering point for air for the entire system but you could open the screw fully and get neither air or water sometimes!
Now after the drain/refills all upstairs rads can be fully bled.
Just wish I could pinpoint the blockage!

 

[swordfish227]

I see you have mains cold and low pressure hot at the shower, you could have cold water backfeeding into the hot water cylinder, does the overflow from cold water tank in the loft ever run ?

Hi Picasso,
No issues with overflow from the main tank, also been losing temp gradually but consistantly over the last 2 months probably.
I could close the cold feed valve from the main tank and see if that makes a difference?