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Author Topic: How to link up wood boiler and combi boiler?  (Read 4706 times)
SimonArran1
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« on: October 02, 2012, 03:12:03 PM »

 We are doing a total refurb on a stone built cottage in the west of Scotland. It is in an area surrounded by blown deadwood.
To link up an oil combi boiler, with a wood boiler stove, such as a charnwood or stovax, would provide an attractive heating solution.
 I attached pdf link up booklet from stovax, please have a look at page 11, 13.
 
 Possibly also to leave provision for link up to a solar water heating circuit later, if funds permit.
 
 The house would be a second home and or letting property for the first 3 to 5 years and mainly in use in summer.
 Therefore the combi boiler would provide the background radiator heat etc in winter.
   
1)  My main concern, is that that the combi boiler should not waste energy heating a thermal store, when the house is empty.
     Probably only providing frost protection, via radiators.
2) When occupied in winter, the boiler stove could provide most hot water, and plenty of heat for radiators.

  As boiler stoves require a gravity/thermosyphon heat store and combi boilers do not require one, poses a few  technical hurdles. Such as ;

A) If  the indirect cylinder, or heat store, were warm enough to provide warm water to a sink and then more were demanded for a bath , then the combi boiler would have     to suppliment the demand.
  Can the water store be used as a  preheat to the combi boiler?  However, this is impractical if the temp difference is less than 20 deg c ? 
B) Using a neutraliser seems to cover the central heating side, however hot water is still obtained via an indirect hot water cylinder. This highlights my concern with dorment      heat loss, as in point 1). Maintaining a hot tank in an empty house seems silly.
C) Several manufacturers of boiler stoves provide 2 upper and 2 lower pipe take offs to their stoves. The simplest system is to take one pair upwards to the water cylinder 
   and pump the other pair to the radiators. This simple radiator circuit is compromised if the stove heats the combi boiler or vice versa, or are there one way valves etc            which can accomodate both working together?

   To sum up our quest then, is for a system which is basicly, a combi boiler in dorment 'sleep' mode. Providing radiator frost protection and instant hot water, without maintaining a constant hot water store.  The wood boiler stove, in autumn, winter and spring would then take over, unless it were required to be supplimented by the combi boiler, between burnings. Such as in the morning, if lots of hot water were required. If this were fully automatic, with a wall stat being overridden by the boiler stove, then I think it would be perfect.!

Has such a link up system been installed before, can anyone offer advice on how to achieve this ?
Or are there any system diagrams available, we can discuss with our heating engineer ?

Thanks in anticipation
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dhaslam
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« Reply #1 on: October 02, 2012, 03:41:55 PM »

A lot depends on how well the house  is insulated and how often it is occupied.  Older uninsulated houses will be freezing in the morning if you just  run radiators in the evening direct from the stove.  Even in  normally  insulated houses more heat is needed in the morning  so a heat  store  is still  very useful.   If the  house is empty you can still use the remaining store heat for several days to stop freezing etc.   If the  house is not used constantly it probably would make sense to  use the simple direct radiator circuit and  use gas in the mornings.   If you are living in the house for most of the winter and want to use  heat from the stove    then you should install some kind of buffer tank.    You need to bear in mind that in the evening  the stove may be heating water, heating radiators and topping up the buffer tank so you need a good stove.   Remember that multofuel stoves are rated with coal so they lose up to 40% when used with wood.   Also  Stovax idea of radiators  15 kW supplng 19 radiators  would be a bit optimistic, radiators tend to be closer to 2 kW average, they are usually slightly oversized for rooms and limited by thermostats.     

You can feed a combi boiler with warm  water  by using a mixer valve that is set to  maximum input temperature but it needs several valves and isn't cheap.   It is more relevant to solar because it is  usable all year round.  Navitron now have a  ready constructed one  in the solar ancilliaries section.

http://www.navitron.org.uk/product_detail.php?proID=360&catID=122

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Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
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