Replacing two hot water cylinders with one

[HRP123]

Also when I release water from the cold water inlet on the slow tank it feels warm to touch and is cold on the fast draining tank

 

[Johntheo5]

That system can't work with one reducing valve set to 3.5bar and the other to 3 bar, its like having two vented tanks with the level in one tank 5 meters higher than the other so it wll allways feed into the system, at the very least both pressure reducing valves should be set to the same pressure.
The reason that the slow cylinder expansion valve discharges warm water when you open it for the same reason, I don't think shutting the fast tank hot regulating gate valve actually shuts it off as it will probably still flow if its labelled from 1.0 upwards, you should get a isol valve fitted on the cold feed to both cold feeds, you can then set them up individually, properly, you need somebody suitably qualified to fix this for you.

 

[fixitflav]

Post #7 suggests that both have PRVs but maybe this is a reference to pressure relief valves, one pressure reducing valve would be the only logical way to ensure equal inlet pressures.

I meant pressure reducing valve

 

[fixitflav]

Also any advice on what sort of pump should be installed to get from the boiler which is in the garage up to the loft which is 2.5 floors up so lets say 6-7 metres
higher? The current pump is a Grundfos Alpha 15-50

Just noticed this. The level of the tanks in the loft does not affect the pump differential. The Alpha 15-50 should be OK, but if not it's not because of the level difference.

I've also noticed that the plumber fitted both diverter valves which are connected to the thermostats on each tank on the return to the boiler from the cylinder. Each cylinder has a diverter valve on its return, so one cylinder can be open. However this means that if its on the return to the boiler, the hot water is still circulating into the coil of both cylinders, even if just one cylinder is calling for heat?

I don't see that. If either cylinder stat is satisfied, the diverter valve should stop the primary flow. The controls must be such that when either cylinder stat calls, the boiler and pump run.

 

[fixitflav]

That system can't work with one reducing valve set to 3.5bar and the other to 3 bar, its like having two vented tanks with the level in one tank 5 meters higher than the other so it wll allways feed into the system, at the very least both pressure reducing valves should be set to the same pressure.
The reason that the slow cylinder expansion valve discharges warm water when you open it for the same reason, I don't think shutting the fast tank hot regulating gate valve actually shuts it off as it will probably still flow if its labelled from 1.0 upwards, you should get a isol valve fitted on the cold feed to both cold feeds, you can then set them up individually, properly, you need somebody suitably qualified to fix this for you.

Agreed it's odd to have pressure reducing valves with different settings. Unless there's a partial blockage somewhere, when both outlets are open the 3.5 bar tank will give most or all of the flow. But it doesn't explain why, if the "fast" tank is isolated, the other only gives a trickle.
Also > 3.5 bar is high for mains pressure in most areas. If it's lower here the pressure reducing valves will be wide open and not doing anything.

 

[Johntheo5]

Agreed it's odd to have pressure reducing valves with different settings. Unless there's a partial blockage somewhere, when both outlets are open the 3.5 bar tank will give most or all of the flow. But it doesn't explain why, if the "fast" tank is isolated, the other only gives a trickle.
Also > 3.5 bar is high for mains pressure in most areas. If it's lower here the pressure reducing valves will be wide open and not doing anything.

3 bar is a fairly standard setting for unvented cylinder PRVs I think, the PRV on the slow cylinder should be removed and checked out, a process of elimination should soon show where the main problem is with no flow.

I've often wondered why unvented cylinders which are pressure vessels never seem to have a pressure gauge.

 

[fixitflav]

3 bar is a fairly standard setting for unvented cylinder PRVs I think

I wouldn't know, but I'd like to think the installer checked the mains pressure is somewhat higher or it won't do anything.

I've often wondered why unvented cylinders which are pressure vessels never seem to have a pressure gauge.

Might be worth retrofitting to help solve the problem.

 

[HRP123]

Just noticed this. The level of the tanks in the loft does not affect the pump differential. The Alpha 15-50 should be OK, but if not it's not because of the level difference.


I don't see that. If either cylinder stat is satisfied, the diverter valve should stop the primary flow. The controls must be such that when either cylinder stat calls, the boiler and pump run.

So the diverter valve is connected to the cylinder return to the boiler, rather than the flow into the cylinder. Not sure why it has been done that way.

Noted re pumps, thanks.

 

[HRP123]

3 bar is a fairly standard setting for unvented cylinder PRVs I think, the PRV on the slow cylinder should be removed and checked out, a process of elimination should soon show where the main problem is with no flow.

I've often wondered why unvented cylinders which are pressure vessels never seem to have a pressure gauge.

I can discharge hot water from the slow cylinder, with either the red PRV from the cold loop going into the cylinder, or from the one which is connected to the hot water outlet from the cylinder - it discharges very hot water only.

I would have thought the red PRV from the cold loop should only discharge cold water, but it seems the hot water from this cylinder is heating up the cold inlet (probably because it is not discharging). Picture attached.

Also the hot water outlet from the cylinder feels warm all the way to where it meets the other cylinder and goes down into the house hot water loop, so water is in there.

 

[HRP123]

I've also noticed this Setter flow valve on the heating loop into the slow draining cylinder, not sure why it was installed and what it does exactly. From Google it seems to adjust the heating flow into the cylinder - but this isn't really an issue in the first place as the cylinder heats up fine.

 

[HRP123]

That system can't work with one reducing valve set to 3.5bar and the other to 3 bar, its like having two vented tanks with the level in one tank 5 meters higher than the other so it wll allways feed into the system, at the very least both pressure reducing valves should be set to the same pressure.
The reason that the slow cylinder expansion valve discharges warm water when you open it for the same reason, I don't think shutting the fast tank hot regulating gate valve actually shuts it off as it will probably still flow if its labelled from 1.0 upwards, you should get a isol valve fitted on the cold feed to both cold feeds, you can then set them up individually, properly, you need somebody suitably qualified to fix this for you.

So I can shut the cold water going into each cylinder with a turning type flow valve - but I don't know how to drain out the water from the slow cylinder

 

[Johntheo5]

Do you mean a gate valve with a handwheel when referring to the turning type flow valve?, if you can isolate this and isolate the regulating valve from the hot outlet then you don't have to drain down the cylinder to access/remove the pressure reducing valve, just open the expansion valve adjacent to it by twisting the top and when water stops flowing then you can deal with the pressure reducing valve or any/all valves on that section of line.

 

[HRP123]

Do you mean a gate valve with a handwheel when referring to the turning type flow valve?, if you can isolate this and isolate the regulating valve from the hot outlet then you don't have to drain down the cylinder to access/remove the pressure reducing valve, just open the expansion valve adjacent to it by twisting the top and when water stops flowing then you can deal with the pressure reducing valve or any/all valves on that section of line.

Yes I think thats the technical term a handwheel gate valve.

Again this morning as I am working from home, I closed the handwheel cold inlet to the fast cylinder and closed the valves on the hot water outlet to zero - and I left both fully open on the slow cylinder - no hot water was produced after a minute or so. The hot water outlets from the slow cylinder seemed hot but for some reason it is not going into the house.

I've now shut the cold water inlet to the slow cylinder but left the hot water valve fully open - in the hope that slowly over the next day or two some water should escape from this tank. If we have a shortage of hot water over the next couple of days, it would also suggest something is coming from the slow tank. Lets see!

 

[HRP123]

Just noticed this. The level of the tanks in the loft does not affect the pump differential. The Alpha 15-50 should be OK, but if not it's not because of the level difference.


I don't see that. If either cylinder stat is satisfied, the diverter valve should stop the primary flow. The controls must be such that when either cylinder stat calls, the boiler and pump run.

I lowered the temperature on the slow draining cylinder right down yesterday but it is still heating up, so in my view I think this confirms the diverter valve is on the hot return to the boiler, instead of the hot flow to the cylinder from the boiler. This means whenever the other cylinder is calling for heat, the other one will also heat up unnecssarily right?

 

[Johntheo5]

Try this, shut the cold and hot valves on the fast cylinder, shut the hot only on the slow cylinder, open the expansion valve (by twisting the top), cold water should start flowing out, if only hot/lukewarm then problem is on the cold feed, probably the pressure reducing valve.