Unvented Cylinder Questions

Britannia1863

Hi All,

I have a few questions about how an Unvented Cylinder works. I'm replacing my boilermate thermal store with a Unvented Cylinder and replacing my boiler (in the kitchen) also.
1) Does the system boiler deliver the water to the cylinder at the same temperature it will come out of the taps at? Or does the cylinder heat it further? Or just keep it warm? 
2) Are the coil(s) powered by electric? I won't need gas to the third floor where the cylinder will be kept?
3) I understand that it has an immersion back up so if the gas boiler fails it can they heat up a full tank of cold water to the correct temperature (although this will cost more than using the water heated by the gas boiler!)

Sorry if any of these questions are stupid, I'm just getting my head around how it all works!

Reed_Richards

1)  The system boiler circulates water through a coil inside the cylinder.  The coil forms part of a closed loop so it does not mix with the water inside the cylinder that ultimately cones out the taps.  The water in the closed loop needs to be a bit hotter than the temperature you want the water inside the cylinder to get to.  There will be a thermostat on the side of the cylinder that tells your boiler when to come on and heat the hot water.  At least once a week you need to heat the water in the cylinder to 60 C to avoid Listeria.  Water that hot can scold you so ideally you will have a blending valve on the output from the cylinder that mixes in cold water to bring the temperature down.  Or alternatively you could have a thermostatic mixing valve on the input to every hot tap (although I got away without either for 20 years without incident).
2) You only need gas to the boiler but you need pipes to take water from the boiler to the cylinder and back.  Somewhere there will be a three-way valve or two two-way (on/off) valves that control whether hot water is directed to the tank and back or to the radiators and back.
3) Most cylinders will have an immersion heater as a back-up with its own thermostat.  Heating the water with the immersion heater will cost approximately 5 times as much as using the gas boiler.

PS What was wrong with your thermal store?  In principle you should be able to continue to use that.      

Britannia1863

My boiler is 16/17 years old and on its last legs. Rather than replace just that and then the boilermate go wrong. I thought I would go all out and upgrade the whole system the hopefully it last for a good few years to come! Plus maybe adding value to the property if I sell in the meantime. 👍

[Deleted User]

There is an added level of complexity associated with unvented cylinders that you need to think about. Firstly, the only installers that are permitted to work with invented systems are plumbers with a G3 annotation.

Secondly, the water to your taps has to come from a balancing valve that ensures that the pressure of the hot and cold supplies are the same (3 Bar).

Unvented cylinders also come with a number of built in safety systems. For example, 2 thermostats with a top limit pressure and temperature relief valve. If this valve operates, then the hot water expelled from the cylinder has to be routed through to the outside of the property.

Unvented systems work well and suit homes with a high demand for hot water and simultaneous showers but they do need to be installed by a properly trained installer with all the necessary changes to the plumbing system. I do not wish to frighten you but ‘Google’ for unvented cylinder explosion.

Reed_Richards

There isn't a lot to go wrong with a thermal store.  I guarantee that potential buyers may note the age of your gas boiler but they'll pay no attention whatsoever to your hot water cylinder provided it appears to be well-insulated.  I have never had a thermal store but I imagine if you turn on too many hot taps at the same time then the hot water may get a bit tepid.  That should not happen with an unvented cylinder.  But the unvented cylinder manufacturer will recommend you get it looked at once a year so ensure it remains safe.   

Reed_Richards

Interesting article here on the evolution of the Boilermate: http://www.gasapplianceguide.co.uk/Gledhill%20Boilermate%20explained.htm
If your model is too old then it it may have a lot of electronics that most plumbers will not know how to deal with.  So because it may be more than just a simple thermal store I take back what I said about not a lot to go wrong.

coffeehound

[Deleted User] said:

Secondly, the water to your taps has to come from a balancing valve that ensures that the pressure of the hot and cold supplies are the same (3 Bar).

When unvented tanks first came in, they used to fit a large capacity PRV and strainer at the mains incomer so all the water was supplied at the same pressure.  But now they just fit a smaller PRV at the supply to the cylinder, requiring the extra measures you describe.  Seems like a backward step to me. . 

[Deleted User] said:
 I do not wish to frighten you but ‘Google’ for unvented cylinder explosion.

Well, two people have been killed in Britain by ordinary vented DHW systems where the thermostat has failed 'on', so each system has its dangers I suppose.

[Deleted User]

coffeehound said:

[Deleted User] said:

Secondly, the water to your taps has to come from a balancing valve that ensures that the pressure of the hot and cold supplies are the same (3 Bar).

When unvented tanks first came in, they used to fit a large capacity PRV and strainer at the mains incomer so all the water was supplied at the same pressure.  But now they just fit a smaller PRV at the supply to the cylinder, requiring the extra measures you describe.  Seems like a backward step to me. 

Possibly, but as I know from past experience the lack of a balanced supply makes it impossible to run a thermostatically controlled shower which has no hot override.