Unvented heating and hot water c/h system has hot water tank?

littlerock

My son and his family recently moved into their own home and last week the heating went off and they had to get out an engineer. They have a gas boiler and an unvented system which I thought meant that the water was heated in a heat exchanger in the boiler and was thus more or less instantaneous. (This is what we have.)

Seems not, he tells me that they do not have a cold water tank but they do have a hot water tank which is filled directly off the mains so there is no drop in pressure in the shower when someone turns on a kitchen tap. However there is an extra cylinder attached to the hot water tank with a diaphragm inside to absorb pressure change when the tank is hot. This diaphragm broke last week. So water was going through the overflow pipe instead. So the system failed.

I am having difficulty understanding how this works. Is this some sort of special hybrid system does anyone know? The previous owner did a lot of the work himself.

saverbuyer

littlerock wrote: »

My son and his family recently moved into their own home and last week the heating went off and they had to get out an engineer. They have a Worcester boiler and an unvented system which I thought meant that the water was heated in a heat exchanger in the boiler and was thus more or less instantaneous. (This is what we have.)

Seems not, he tells me that they do not have a cold water tank but they do have a hot water tank which is filled directly off the mains so there is no drop in pressure in the shower when someone turns on a kitchen tap. However there is an extra cylinder attached to the hot water tank with a diaphragm inside to absorb pressure change when the tank is hot. This diaphragm broke last week. So water was going through the overflow pipe instead. So the system failed.

I am having difficulty understanding how this works. Is this some sort of special hybrid system does anyone know? The previous owner did a lot of the work himself.

That's standard. The extra cylinder is the expansion vessel. It's to stop the whole thing blowing up. I have the same sort of system.

Alex1983

It’s a pressurised hot water tank fed by mains cold water so no need for cold storage tanks.

The boiler send heat to the airing cupboard where some valves direct the water to the radiators and/or to the hot water tank depending on what the programmer is asking for.

It’s just a modern version of the old fashion tanked system.

DUTR

littlerock wrote: »

My son and his family recently moved into their own home and last week the heating went off and they had to get out an engineer. They have a gas boiler and an unvented system which I thought meant that the water was heated in a heat exchanger in the boiler and was thus more or less instantaneous. (This is what we have.)

Seems not, he tells me that they do not have a cold water tank but they do have a hot water tank which is filled directly off the mains so there is no drop in pressure in the shower when someone turns on a kitchen tap. However there is an extra cylinder attached to the hot water tank with a diaphragm inside to absorb pressure change when the tank is hot. This diaphragm broke last week. So water was going through the overflow pipe instead. So the system failed.

I am having difficulty understanding how this works. Is this some sort of special hybrid system does anyone know? The previous owner did a lot of the work himself.

It's not a hybrid system (as in the previous owner bodged it) , in my eyes the unvented is the superior sytem to have where possible, instant hot water and good flow rate, that'll be the sytem I go for next.

BoxerfanUK

http://www.megaflo-unvented.co.uk/megaflo-description.php

fezster

DUTR wrote: »

It's not a hybrid system (as in the previous owner bodged it) , in my eyes the unvented is the superior sytem to have where possible, instant hot water and good flow rate, that'll be the sytem I go for next.

It is not instant hot water though (for example, like a combi boiler is). The hot water is stored and heated in a pressurised cylinder.

Agreed it's the superior system for good flow rates, provided your incoming mains is up to the job.

littlerock

I guess the reason I am struggling to understand the need for a hot tank is that we have not had a hot water tank for 20 years. We have a Worcester combi boiler which heats our large house, and provides constant hot water for the taps and the bath and shower.

What the specific advantages of having a hot water tank if you can have instant hot water in your taps bath and show whenever you turn them on? Just seems like more to go wrong.

fezster

It's all to do with flow rates. Even the largest combi you can find would struggle to supply enough hot water for two rain type showers. And most would struggle to supply even the most basic showers in the coldest days of winter (combis raise the temperature of the mains water and the colder it is, the less they can heat up).

An unvented cylinder gives you the benefits of mains supplied water but overcomes the requirement to heat it on demand. It can also be used with alternative heating sources such as an electric immersion heater or solar power. If your combi boiler breaks down, you have no hot water.

littlerock

OK well we do not have two rain showers in our house, and shower in turn when we use the shower. Also the water pressure is notoriously low in our area and that too would probably make it unrealistic to have a non vented system. In my brother's case, they have only one shower and two small children so I am not sure that they really need the pressure for two simultaneous showers and will not for several years.

We have never found the shower pressure a problem in our house though, and the ability to have successive hot showers without waiting for a hot water tank to heat up, has always been a plus. I have heard the argument about instant boilers being slow to heat the water in very cold weather but to be honest this has never been a problem, but then we do live in the south east of England (as does our son and family).

Alex1983

As fezster has said it’s to do with flow rates, any new build property with more then 1 bathroom will be designed with a unvented system, some can recover from completely cold in as little as 17mins so waiting for the tank to reheat is not a issue.

As has been said a combi will raise the temperature of the water by around 35/40c, mid winter you may only have 5c incoming water so you’d only get around 45c water at a decent flow rate this is a huge downfall of a combi boiler, a unvented will supply water at whatever temperature you want all year around.

In my opinion a system boiler and unvented system is the best system you can get.

EssexExile

littlerock wrote: »

..without waiting for a hot water tank to heat up

You don't wait, it's full of hot water, it's insulated so it stays hot, you use it, it's heats up again. Only my wife can use up a full tank in one shower but, as said previously, it's hot again within minutes.