Cistern fed unvented hot water cylinder - Is it possible?

[Dodger3]

I'm about to get some upgrading work done to my hot water / central heating system. Currently the gas boiler is on the ground floor and hot water cylinder on the first floor. The central heating circuit is pressurised and unvented, the hot water cylinder is vented. It's a 3 storey Victorian house with high ceilings so there is ample pressure/flow from the 50 gallon water tank in the loft to handle the 3 bathrooms.

I want to move the boiler to the cellar, no problem there, and also move the hot water cylinder to the ground floor to the location where the boiler is at the moment.

The problem is that it would be practically impossible to run a vent pipe all the way from the top of the hot water cylinder to the tank in the loft 30ft above whilst maintaining an uphill gradient to the pipe.

To get around this is there any reason why I could not use an unvented hot water cylinder fed from the tank in the loft? I'd omit the pressure reducer from the cylinder but other components would stay the same. That way I would have equal hot and cold water pressures of about 1 bar on the ground floor and no worry about restricted flow rates from the mains.

Sounds like a good solution to me. Have I overlooked something?

 

[Breesey]

Ignoring all other issues:
What would you do with the unvented cylinder's discharge pipe?



Any way as long as you have a vent pipe that goes back to the cistern a small amount of horizontal isn't a problem. You'll need top get the required height above the cistern right though.

 

[Dodger3]

Ignoring all other issues:
What would you do with the unvented cylinder's discharge pipe?



Any way as long as you have a vent pipe that goes back to the cistern a small amount of horizontal isn't a problem. You'll need top get the required height above the cistern right though.

I'm not sure what the regulations call for regarding the discharge pipe but it could go through the wall straight to outside.

If the vent pipe were to follow the route of the rest of the plumbing there would be some long horizontal sections. There's no obvious alternative route for a vent and this presumably is the reason why the central heating circuit is already unvented.

 

[BigBurner]

Another way is keep the tundish and have the discharge pipe to outside if convenient. Remove the non return valve on the cold feed from the tank. Fit a n anti-vacuum valve on the draw-off right at the cylinder. Any high temperature greater than 90C, will discharge into the tundish and to outside. Any high pressure will work its way back to the cold tank via the cold feed with the anti-vacuum valve preventing an implosion of the cylinder. If the cold feed is blocked for any reason then the pressure side of the T&P valve will open to the discharge pipe.

Then you still do not have to take the vent pipe back to the loft, which is what you want.

If it is inconvenient to take a discharge pipe to outside, or spoils the look of the building, which many do, then do what I suggested above and take the discharge pipe back to the cold feed.

Have the expansion vessel charge pressure below head pressure from the cold tank, in your case less than 1 bar. Or have it at least equal to the pressure from the cold tank. Even with no restriction on the cold feed from the tank, the expansion vessel will still absorb pressure and not expand up the cold feed into the tank. On the cold feed have a section of larger bore pipe to accommodate any hot water that works its way back up the cold feed in normal operation, then no hot water in the cold tank.

 

[Dodger3]

Big Burner, I appreciate your advice, thanks very much.

Do you know, am I allowed to DIY this as it is not actually connected to mains water pressure?

 

[doitall]

I have never seen so much stupid, dangerous advice on an DIY forum ever.

And not a mod in sight

 

[doitall]

And it's WRas approved.

 

[BigBurner]

And it's WRas approved.

WRAS do not make the water regulations. Their web site says what they do.

I first did one of these before unvented cylinders were available. I contacted the water board and they said all I needed was an expansion vessel set to the right pressure and an anti vacuum valve and best fit two in case one failed. A high pressure relief valve was not necessary as excess pressure would move up the cold feed pipe and the anti vacuum valve would prevent an implosion. I always fitted a pressure relief valve in case, and in blocks where I could not get through the outside wall I would take the pressure relief discharge pipe into the cold feed with no valve between valve and tank.

I would still do one as they are cheaper to do than unvented cylinders. They are not pressurised so very legal as long as adequate pressure relief and anti vacuum valves are fitted, that is what the water board told me. I have read the regs and have never found anything that counters what I did.

These days combis are the preferred method, so little call for cylinders in flats, even large flats.

 

[Dodger3]

I have never seen so much stupid, dangerous advice on an DIY forum ever.

For my benefit at least, would you please elaborate.

 

[Dodger3]

And it's WRas approved.

You're referring to this?

http://www.wras.co.uk/PDF_Files/Scalding information note Jan 08.pdf

 

[BigBurner]

And it's WRas approved.

You're referring to this?

http://www.wras.co.uk/PDF_Files/Scalding information note Jan 08.pdf

That is why I said fit a fibreglass tank not a poly tank as they do not collapse when holding boiling water.

 

[doitall]

WRas approve the design

Ask your water supplier and building control if they will approve the installation.

Why not just fit an store

 

[BigBurner]

I have never seen so much stupid, dangerous advice on an DIY forum ever.

For my benefit at least, would you please elaborate.

For me as well. Because it is not an off the shelf packaged unit does not mean it is not safe or not legal.

 

[doitall]

I have never seen so much stupid, dangerous advice on an DIY forum ever.

For my benefit at least, would you please elaborate.

For me as well. Because it is not an off the shelf packaged unit does not mean it is not safe or not legal.

If it is not WRas approved, it has the potential to be, but hey ho its not my house.

 

[Dodger3]

doitall, How or why is BigBurner's design any more dangerous than a conventional vented hot water tank. Don't the immersion heaters in unvented cylinders have a safety cut out as well as a stat?