I can't isolate (turn off) hot water to the house

[Gary Lapidus]

I'm trying to change a leaky isolating valve in the 1st floor bathroom basin, but I can't drive the hot water pressure to zero, and therefore can't remove the valve.
I have a combi boiler in the vault, a hot water cylinder in the cupboard, and a water storage tank on the roof.
I closed the "Hot water to house" valve in the cupboard, and the hot water recycle pump valves on the cylinder. Nope, still water pressure on 1st floor bath.
I closed every valve on roof storage tank, including "to cylinder red", which I assume supplies pressure to the cylinder. Nope, still hot water pressure on 1st floor bath.
I closed the water-main supply valve and turned off the boiler. Nope, still hot water pressure on 1st floor bath.
I closed every visible valve in the entire system: everything. Cold water goes to zero, but still hot water pressure on 1st floor bath.
I have suspected that the plumbing in this house was not engineered correctly but this is disconcerting.
Any thoughts?

 

[terryplumb]

Are you sure your boiler is a combi ?? Show us some pics of your hot water cylinder ,together with the
"Hot water re cycle pump"

 

[Gary Lapidus]

The boiler has a loop that heats water in the cylinder. Maybe that’s not combi? I think the system has two motorised valves; one heat, one water.

 

[terryplumb]

Close off the cold water supply to the loft tank ( tie up the ball cock float ,if there is no isolator on the tanks supply line ) and run hot taps until loft tank is empty.
Or you can bung the loft tanks outlets ,and run all hot taps till they run dry.
Your boiler is not a combi.

 

[Hugh Jaleak]

As above, you haven't got a Combi, so first thing you need to do is open all the valves you've closed again. Hopefully it wont upset the system too much, but that depends on what's been fitted and where.

Your Hot Water is fed from the cistern (tank) in the roofspace, it's completely separate from the water being circulated by the boiler. Highly likely the Gate Valve feeding the cylinder is broken, so you could turn it all the way 'Off' and the 'Gate' inside the housing still remains stubbornly open. If you're confident enough to do so, go into the roofspace, and tie up the Ballvalve in the cistern supplying the cylinder, then drain the lot down until the hot taps run dry.

Repair your leaking valve and I'd replace the faulty gate valve on the cylinder supply pipe at the same time whilst its all empty.

 

[Gary Lapidus]

You guys are great!
Let me be sure I understand. The pipe/valve on roof labelled “to cylinder red”; is that the “fresh” water supply for domestic hot water, and also providing water pressure (height of water) for the hot water?
Is it the case that if I close that valve then the pressure of domestic hot water should go to zero throughout the house? I.e. if all was working properly?
Is that the valve that you believe is broken?

Why is closing the valve (in the cupboard, by the cylinder) labelled “hot water to house” not sufficient to zero the pressure? Even if the roof tank is pressurising the cylinder, due to broken gate valve at roof tank, I’d still expect closing the feed from the cylinder to house would zero the pressure. Are you thinking that gate valve is faulty as well?

 

[terryplumb]

I have no idea what valve you refer to ,but those gate valves ( round ,red handle) are notorious for not fully closing ,and if overtightened in an effort to close them fully, snap internally and will not open.
If you close your mains cold water stopcock where it enters the property ,the loft tank will no longer fill. You then turn on ALL hot water taps ,which will drain the loft tank and once drained ,the hot taps will cease to flow. It will take around ten minutes. This is an alternative to what I suggested earlier and you don't need to get into the loft !

 

[Hugh Jaleak]

Are you thinking that gate valve is faulty as well?

Entirely possible both valves are broken and wont shut off. Drain system as described, replace valve in supply to the cylinder whilst drained down, and you should be ok in future for shutting off the hot water supply when required.

Valves seize through lack of use, I purposely exercise the valve sin my house every 3-4 months, (turn them off and then on again), to try and prevent this happening, otherwise it's too late when you're in trouble and the supply wont shut off!