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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    Hi all,

    I hope one of you smart chaps/ladies might be able to share an opinion.

    I am currently building a 120sqm single story house. It's not going to be passive but it will be very well insulated.

    We are having a UFH system throughout the whole house (stone floors except for bedrooms). We will be on mains gas and also have a woodburner stove in the living area.

    Here is the question.... is it worth me having a thermal store and a stove with a back boiler ?

    As we are on mains gas, I intend to have a combi boiler and as I see it, the options are either: combi & expansion vessel or combi, thermal store and back boiler.

    Other points to consider are: the stove is to be a feature so I don't want to go down the industrial back boiler route, I cannot install solar panels, I have a free supply of logs.

    Basically, I think that the combi/expansion would suffice but is it be worth the extra cost of getting the thermal store and back boiler so that I can make use of the free hot water created by the stove.

    I don't like the idea of sitting in front of a fire and thinking "this could be heating my hot water for free" and I also don't want to pay £1-£2k extra on plumbing just to have a thermal store that will take me 10 years of burning logs to get it to pay for it's self. Assuming that I will be in the house for 5 years, will the back boiler save me the cost of heating the hot water through the wood burner vs the combi boiler ?

    Any advice would be greatly appreciated.

    IMO if you are going to have a wood burner stove as a feature stove in the lounge and you have mains gas then forget the thermal store and go for the standard solution of combie boiler running UFH and DHW. Think about how often you will light the feature wood stove, never in the summer and probably only at weekends in the winter (depends on life style) if you go down the thermal store combined system you will feel guilty whenever the wood stove is not lit as it is not giving you your return on investment.

    Also if you are planning on selling the house in 5 years then there is a lot to be said for a standard heating system that all understand, the combie boiler doing CH and DHW with the feature wood stove in the lounge sounds great. The general public get frightened by what is seen as complicated heating systems.
    • CommentAuthorDarylP
    • CommentTimeJan 6th 2015

    If you have the wood for free, and the space to store/season it, then by all means invest in the hardware to utilise it as a major fuel source...
    But, if not, forget it!
    Use mains gas as your fuel source, and perhaps the WBS as a 'feature'.
    The extra/over to purchase/install the plumbing for a back-boiler/TS will buy you a lot of mains gas kWhs....

    Good luck:smile:
    • CommentAuthorBeau
    • CommentTimeJan 6th 2015
    Keep it simple and have the WBS as stand alone. Fitting a back boiler to a WBS drops the firebox temperature and in turn reduces the stoves efficiency. Some of the larger stoves cope better but with a 120m2 well insulated house I can't see you needing a large stove. Incidentally I have been surprised how well UFH distributes heat that our WBS kicks out. The hearth get surprisingly warm and in turn the UFH circulation distributes this heat quite well when the GSHP is not running. With a bit of planning you might be able to maximise this effect with extra UFH piping around the stove and possibly up the wall or alcove around the stove. Just a thought :smile:
    Thanks of the replies.
    I was hoping that the simple option was the best and it seems like you agree. I guess that if the back boiler was installed then I would feel that I had to use it to, even if I didn't necessarily want the WBS on.
    Beau, I like the idea of a bit of extra piping around the WBS as I might as well make use of the heat that's coming from it. It may not make a huge difference but its only going to be a few pence for a bit of extra pipework so def worth it.

    BTW, is it worth having the expansion vessel ? I was led to believe that it's a good idea to have one to prevent the boiler coming on every 2 minutes to warm the UFH. I was told that, as the vessel holds a decent amount of hot water, the UFH will take it from this and the boiler will only heat the vessel when it's temp drops (which should be less frequent than without it). Is this a long the right lines ?
    • CommentTimeJan 6th 2015
    Posted By: theloneboarderBTW, is it worth having the expansion vessel
    I am not a plumber, but isn't that what a buffer tank does rather than an expansion tank. An expansion tank if really just that, it allows for thermal expansion of the fluid.
    Yeah, I am sure you don't mean expansion vessel - that's simply what's used to make it run as a pressurised, not open-vented, system.
    • CommentAuthorCWatters
    • CommentTimeJan 6th 2015
    If your house will be very well insulated a combi boiler might be sized to meet your peak DHW demand rather then the heating demand. This might give you a boiler that is on the big side for the heating load. Make sure it's a fully modulating boiler (eg one that can turn down it's burner to a low level) or use a buffer tank.
    Sorry, I have confused myself reading too many forums. Yes I mean a buffer tank.
    Are they worth while ?
    • CommentTimeJan 6th 2015
    Just do the sums. Heat load for different external temperatures and boiler output, especially if there is a large modulating range, will lead you to the right buffer tank size.
    • CommentAuthorringi
    • CommentTimeJan 6th 2015
    One issue with a buffer tank is that a boiler will tend to operate on it's max output while rechargeing the buffer tank.

    However some combi boilers can have there max output for heating reduced, so making the system more stable.


    I would expect that if a boiler would modulate down to 3kw (e.g. Vokera Linea 1) and you use a TPI based thermostat you may get away without a buffer tank. Or just put a control system that stops the boiler running for 1 minute out of every 20 minutes, while keeping the UFH pump running.
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