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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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    • CommentAuthorowlman
    • CommentTimeApr 20th 2020
    Posted By: djh
    Posted By: owlmanRegarding internal temperatures, generally mid winter 22C occasionally 23C

    Thanks. It was the heated air temperature I was really interested in. As it comes out of the grilles maybe, or just after the condenser.

    OK, I'll try and position a thermometer close to one supply grille and another near the indoor unit supply side and compare it to the digital controller in the room below.
    There will be some loss due to duct-work lengths but I'm pretty sure the internal stat on the indoor unit in the loft is located on the supply side,it would make sense anyway.
    • CommentAuthorJeff B
    • CommentTimeApr 20th 2020
    Owlman – many thanks for your detailed reply. So as I understand it you will eventually have two air-to-air ASHP systems, one supplying the rear of the house and the other the front of the house (currently heated, at least partially, by a biomass boiler). The overall floor area must be quite large then? What is the rated output (kW) of your current ASHP and what will be the output of the future one?

    Our place is about 180 sq.metres in total i.e. ground floor plus first floor. Being a dormer bungalow there is attic space behind the dwarf walls in the dormer part so access to install ceiling vents at the back of the bungalow to the lounge, kitchen and dining area would be straightforward (any external ASHP unit would have to be at the back of the property). To get the warm air supply to the front (for bathroom, bedroom and study) I would need to run trunking under the bedroom floors to the void on the opposite side. Grilles in the ceilings in the two upstairs bedrooms and shower room would be trickier as ducts would have to be run up behind the sloping ceilings into the small attic space below the roof apex.

    Certainly food for thought!
    • CommentAuthorowlman
    • CommentTimeApr 21st 2020 edited
    Hi Jeff,
    You're right with the current layout, the house is about 230+ M2 incl. an as yet un-finished upstairs double bedroom and large landing, currently just a glory hole. The log biomass boiler system which is still working fine is now almost 12 years old, and like any boiler they don't last forever. I decided therefore to jump before I was pushed, going for the ASHP replacement, ( BTW it has a design load of 7.1kW maybe a bit overkill, but there were other considerations with the system choice and price differences weren't that great ).
    I'll soldier on with the current heating layout you outlined as I have plenty of wood, so it's costing nothing to run.
    I envisage the front of house situation will eventually have the more usual wall hung units probably two or three on a split system as the upstairs layout doesn't lend itself to a ducted one. I am however, keeping an open mind with regard to heating this area.

    Your interior architecture will dictate if you can go ducted or not. The duct sizes are generally much bigger than MVHR and duct installation and associated insulation can be a ba.. ache as I discovered, which is probably why many retrofits don't go down that route. I chose rigid duct in 300 and 315mm, on reflection I think I could have got away with 250, 275 but narrowing too much increases velocity and potential for noise which I wanted to avoid, and my plan worked. I avoided flexi which would have saved time and effort but with inferior results IMO.
    Generally duct sizing and air distribution is a bit of a black art and you'll most likely end up studying, and studying, and measuring, and in the end making a best guess. I chose good fittings, and insulation, and high quality directional grilles/registers, with metal insulated plenums up above in the loft space. The return grilles have removable washable filters with the option to upgrade filter medium if needed, ( lots of choice there ).
    My indoor unit is about the size of a large suitcase which together with the incoming, and supply side, plenums make for quite a big chunk of kit. I hung it from the trusses on 10mm studding with rubber bushes, for anti-vib, and now quiet as a mouse.

    Any more questions just ask.

    • CommentTimeApr 21st 2020
    Posted By: Jeff BTo get the warm air supply to the front (for bathroom, bedroom and study) I would need to run trunking under the bedroom floors to the void on the opposite side.

    It might be worth considering extending the refrigerant pipes instead and using a multi-split unit.

    Grilles in the ceilings in the two upstairs bedrooms and shower room would be trickier as ducts would have to be run up behind the sloping ceilings into the small attic space below the roof apex.

    Grilles don't have to be in the ceiling. That's the conventional position in a cooling climate, but since we're mostly a heating climate a position nearer the floor may make more sense.
    • CommentAuthorJeff B
    • CommentTimeApr 21st 2020
    djh: thanks for your comments. I like the ideas of extending the refrigerant pipes and having grilles in the walls of the dormer bedrooms etc.
    • CommentAuthorJeff B
    • CommentTimeApr 21st 2020
    Owlman (Mike) – thanks for the further info. Wow, 300mm ducts, I had no idea they might be that big! That would be ok for the all the rooms at the back of the house but extending through to the front would not be possible. I think djh’s idea of extending the refrigerant pipework and having a second internal unit would be the way to go in such a case. Grilles in the walls of the dormer bedrooms (rather than in the ceilings) would be ideal.

    Sounds like you have undertaken quite a job there but it has obviously been worth all the hassle! 7.1kW is pretty good I think. What make of ASHP did you go for?

    The crunch decision time for us will be next autumn when the RHI payments end, so plenty of time to think about all the possible replacements for our wood pellet boiler. It’s a shame really, if only we knew we could rely on a good source of spare parts and maybe some technical support then I would not be considering anything else.

    However there is a longer term concern in that my wife could not manage the system if anything happened to me (I’m now 73 but thankfully reasonably fit and virus-free thus far!). Also if we are blessed with many more years together then we almost certainly would want to downsize and I don’t think having a very hands-on heating system based on wood pellets would be particularly attractive to potential house-buyers! I might be wrong – who knows in the next 5 to 10 years there may be huge interest, as attitudes to climate change could also change.
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