Unvented hot water system

I am looking for advice on un-vented hot water systems.

I do not like them as they are very dependent on safety devices which might fail when they are needed to operate due to a fault elsewhere in the system.

But I have little option but to use unvented in the renovation of the 490 year old Grade 2 listed thatched cottage I have just bought. No space in the attic for a reasonable sized cold water tank and nothing structurally strong enough to support it.

The heat source will be a heat only modulating gas boiler, ( Baxi Solo ? )

It might be that I install two indirect tanks as the pipe runs between upstairs bath room and ground floor kitchen and shower room will be very long and the waste of water while waiting for hot water to arrive will be significant.

Any suggestions as to manufacturers.

Any things to be careful about.

Unvented supplies are fine, as the system fails 'open', not closed, so pressure won't build up.
With regard to your 'waste' hot water issue, fit secondary pipework, with a pump on a timer, to alleviate this problem. The hot water supply will be nearly instantaneous, reducing waste water.

http://www.kensaengineering.com/Lib.../Fact Sheet_Secondary_Hot_Water_Returns_1.pdf

No, other than the sky falling in on your head

Check there is enough water coming in.

Unventeds only need their safety devices when water gets boiling point, and in the case of a condensing boiler this is usually almost impossible in itself.

I would say a chip fryer or a motorcycle is infinitely more dangerous. Try and find any figures of injuries or fatalities from unventeds in the uk.....might take you a while.

This all the impressive when, in my experience, about 1in 8 haven't been installed properly.

I wouldn't worry about the safety devices, despite what one much-banned poster on here goes on about. If you really want to do without them, you could use a thermal store and still have mains pressure hot water. Those need a few pages to themselves though. I think one of the main reasons for NOT using them if you aren't going to be monitoring it yourself, is that you could have trouble finding anyone competent to do it for you.

The safety devices on an unvented are 2 x valves which lift if pressure is exceeded, and one which opens if a temperature is exceeded - 90 degrees or something I can't recall. Of course they never get that hot unless there's serious trouble and other thermostats have failed. I've not heard of problems with them. I've not had any trouble with the pressure valves to be alarming - one leaked after the system had been uncared for for several years. I think it cost about £34.
There is a pressure reducing valve, and there has been a baddie in the market, used by several cylinder package makers, but I expect it's history by now. ( It leaked a bit).

Use a Stainless steel cylinder, is about all the advice I'd press.
Watch the pressure reducing valve setting - some are 2.5 bar, some 3.5, some adjustable.

If you have an actual "Megaflo" made by Heatrae Sadia after which most of them are known (like Hoover...) then it'll have an internal expansion space, which is a pain. You can add an external one & use that instead, probably costing under £100 fitted.

More of a concern is the mains pressure you're dealing with in the first place, and its source resistance. All too often the pressure is reasonable, say around 3 bar, but you can only get a low flow. Like 1000volts with 2000 Ohms in series - you only get half an Amp! You can use accumulator (like big capacitors) or get the pipes all a good size, or store water like you have now in the loft and use a pump.

How many showers etc will you have?
What's the cold water flow from all mains cold taps on together, ie include the garden one(s).
And get a mains pressure tester for about £15 and monitor that , morning and evening when demand is highest and pressure lowest.
If you monitor pressure at say the outside tap and take say10 litres/minute from the kitchen tap, see what the pressure drops to. More than 2 bar is fair, under a bar pretty poor.

The pipes which give the resistance aren't just the ones in the house, but the one from the road as well. Old lead pipe is bad news, and may need replacing.

Look also at a floor standing storage combi such as a Viessmann Vitodens 222-F with weather compensation. It's got the cylinder built in.

Temperature and Pressure Relief valves are about to cease to be compulsory, because, as already stated, modern boilers just will not overheat a cylinder.

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yet to see that happen.can you back that claim up. the 222-f is a reasonably good unit utilising the 200w. but for various situations not recommended. id fit a heat only/system boiler to an unvented all day long, due to less parts going wrong. plus immersion back up. and the option to add another unvented for extensions etc.

Thanks for the comments. So far confidence is building.

Re-circulating the DHW is not an easy option as there are 2 foot thick hard stone walls to get through for each additional pipe.

There is only one place the boiler terminal can go to avoid the thatched roof and that location determines it has to be a very slim ( 16 inch max ) boiler.

The idea of relaxing the requirement for a pressure relef valve to be fitted fills me with dread.

If you have to break through a stone wall to install pipework then you'll probably find several pipes will pass through the finished hole.

And maybe even you.

Norcon said:

then you'll probably find several pipes will pass through the finished hole.

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That is why I am trying to avoid making another hole. An existing hole is just about large enough for the three pipes needed.

As to manufacturers, which haven't been mentioned much so far, I'd use an OSO Super S, IMHO the best out there. I don't quite see why you're so restricted on your boiler's physical size, maybe you could post a picture to demonstrate the problem? Flues can often have 45º or 90º bends installed in them to avoid tricky situations.

To keep the flue and the plume clear of thatch and obstructions fitting the boiler in place of those cupboards is ideal with the flu going out through the wall.

as the pipe runs between upstairs bath room and ground floor kitchen and shower room will be very long

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From the cylinder location (which ideally should be sited centrally) what distances have you?

There is a 19 meter run between the two furthest apart hot taps due to the room layout and routes having to avoid cutting new holes through stone walls.

Using a "local" tank means only 3 pipes have to run to the area ( fresh cold and the central heating feed and return )

Thanks for the OSO suggestion. The neat arrangement of connections on the top seems ideal from an installation point of view. I assume there are unlikely to be problems with air getting trapped in the inverted "U" bends.

mysteryman said:

Temperature and Pressure Relief valves are about to cease to be compulsory, because, as already stated, modern boilers just will not overheat a cylinder.

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Thats fine when its a condensing boiler.

But electric backup immersion heaters can stay on when they are wrongly wired or thermostats stick.

For the small cost of a T&P valve I would sleep easier at night with one!

Tony

instead of asking on here like most people do. who want answers from people who cant see the dimensions of the property involved. get someone to quote who can. you now mention thatched roof, prob granite walls etc. you must understand your a customer from hell at the mo. you want advice but give little info, yet you seem to know how a systems work. but ask advice as your not sure. unvented has been around for decades. but i suppose you've heard more horror stories apart from the one were a young toddler got killed in penzance due to a VENTED cylinder stat.

holty said:

you must understand your a customer from hell at the mo.

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I mentioned thatched roof in the original posting.

I am doing the work myself so I am not "a customer ".In 1981 my wife and I self built with our own hands an entire house so I am an experience DIY who recognises the help and advice from tradespeople and building control officers.

Yes I know how the system is designed to work. But that is not the same as how it actually works in practise and hence I am seeking qualified people's experience of the system components being installed and in use.

The horror story I witnessed was an un-vented industrial system where a pressure relief valve was clogged with lime scale and the temperature control system failed.

I am aware of the melted plastic cold water tank and that it was a normal vented heating system that overheated the cold water tank.