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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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    • CommentAuthorgoodevans
    • CommentTimeDec 7th 2018 edited
    I though I would share this study: "Distribution of Ventilation Air and Heat by Buoyancy Forces Inside Buildings" by Claes Blomqvist (https://www.diva-portal.org/smash/get/diva2:220990/FULLTEXT01.pdf) - It seems like a good study and relevant to the buildings we discuss here. I have not come across it in my browsings in the past couple of years so for some reason it may not have come to your attentions either.

    It is of particular interest to me because I wanted to know how much heat could get into our bedroom from a warm landing. Figure 5.4 gives the answer - approx 200 watts if there is a 2 degree difference between the landing and the bedrooms.

    With 2 bedrooms upstairs it means that 400 Watts of heat from downstairs can be transferred to upstairs by keeping the doors open. It does mean that the bedrooms will be cooler than the landing but cooler is desirable for us.

    Surprisingly, It seems that a horizontal hole (like a stairwell) does not move so much heat for the same size hole as a door (because the in and out air paths mix in a chaotic way) sloping stairs and ceiling improve this lack of order so I am hoping a 3x1m hole for the stairs will move as much air as a 2 No 2x0.7m door holes. It also means that the landing could be a couple of degrees colder than the ceiling of downstairs. Difficult to believe but there you go. The difficulty is that we wont have the benefit of generating plumes of warm air from a stove - UFH only for us.
    • CommentTimeDec 7th 2018
    Interesting link, thanks. It will take me a while to read it. I'm not surprised we haven't noticed it before with that obscure web address! How did you find it?
    • CommentAuthorgoodevans
    • CommentTimeDec 8th 2018
    I was googling around stacks, draughts, chimneys etc looking for the necessary formulae to do the calc myself - but most of the info is designed around chimney/stack effects which isn't really applicable for what I wanted. in the end I googled "air buoyancy flow rate" - and this popped up as the top link.

    What I like about this report is that it has actual experiments - some with full sized door mock ups - with details of the issues their measurements methods raised and how they were overcome. What I really like is that it looks like the report is in the public domain.
    • CommentAuthortony
    • CommentTimeDec 8th 2018
    In my house the MVHR seems to homogenously mix the air and all the ground and first floor rooms are at about the same temperature, the lounge can exceed the rest if we sit in their with the lights and tv on, even more with the shutters closed.
    When we did a well-insulated loft conversion we calculated that the new upstairs bedrooms would require only a few 100 Watts of heat, which I expected would be provided by heat rising from downstairs, and heat from people and electronics. It seemed to work ok in the evenings, but the rooms were too cold first thing in the mornings. I think this was because the bedroom doors were kept closed overnight, and downstairs also cooled down overnight.

    It wasn't worth the cost of running wet plumbing for so little heat, but fortunately we had included low power electric panel heaters as 'insurance', good quality ones are very cheap, with individual timers to come on and warm the bedroom before anyone had to drag themselves out of their warm beds.
    • CommentAuthorgoodevans
    • CommentTimeDec 8th 2018 edited

    I've looked in this forum and have noticed that unlike most buildings you do have an even temperature throughout the house with MVHR distribution. Are you still pumping heat under your house in the summer?

    I think your insulation values are a grade or two above mine and therefore the MVHR and natural air movement can shift the necessary heat around - and the level of heating for you is tiny - if any.

    As my build is not up to those standards. Upstairs, during a prolonged v. cold spell (avg temp -5 degC) - I calculate I could be loosing 800W on the first floor. I will post heat the MVHR supply to slab temperature (probably 2 degrees above room temp) so that the mvhr supplies each bedroom at about +16W instead of -24W upstairs (assuming MVHR efficiency at say 85% ish) Reducing for allowances for people and service room heat loss, I will be in deficit by around 500w. Which either needs to come from downstairs - or I can supplement with a electric heater.

    For normal cold winter periods at an average temp of +5 degC the total loss upstairs will be approx 500W - leaving a deficit of 200W - I don't think I could easily stop that getting upstairs from downstairs if upstairs is 2 degC cooler - So no problems (I hope).
    • CommentAuthorgoodevans
    • CommentTimeDec 8th 2018
    Will, was downstairs well insulated? Do you know how often the panel radiator has to come on during the winter?

    in my case the most I would loose downstairs overnight is 1 degC so the effect may not be so dramatic.
    Downstairs probably cooled 3deg overnight in winter. We did like to close the kitchen and lounge doors into the stairwell when we went to bed ( fire safety habits picked up in an earlier life) and kids are old enough that everyone closes their bedroom door. The downstairs ceiling (upstairs floor) was insulated from before the loft conversion. I didn't get round to measuring the electric heating before we sold the house, but was useful throughout winter and into early spring.
    • CommentTimeDec 8th 2018
    Posted By: goodevansI will post heat the MVHR supply to slab temperature (probably 2 degrees above room temp)

    Why not post heat to 50°C or so? What's special about slab temperature?
    • CommentTimeDec 8th 2018
    Many advantages in running your radiating surfaces at just a whisker above room air temp, if radiating area is large enough relative to heat demand.
    • CommentAuthortony
    • CommentTimeDec 8th 2018
    I am still pumping excess solar heat under my house, this year the basement fell to 19C at the end of Feb but started too warm up from then onwards.
    • CommentAuthorgoodevans
    • CommentTimeDec 8th 2018 edited
    If I run at slab temp - It is difficult to overheat rooms so I need no thermostats and I also get a one pump heating solution. With 100m2 floor area downstairs the surface temp need only be approx 1.5 degC above target temp in v.cold weather - that indicates an average flow/return temp of like 24/22 degC.

    I could "steal" heat from the HWT for the post heater (use it like a thermal store) - that would have water at the right temp and post heat just in the bedroom feeds - it would need pump, valves etc.

    Or I could Electrically post heat the bedrooms - perhaps just for a hour or so before the start of the day when necessary - simple and could be implemented post build if necessary. (and also runs silent - ideal for nights).
    • CommentTimeDec 8th 2018
    Posted By: goodevansIf I run at slab temp - It is difficult to overheat rooms so I need no thermostats

    I think you're missing that most of the heat put in by the post heater will go to heat the ducts and whatever surrounds them.

    Heat in bedrooms will vary with the number of occupants and doors closed or not etc. Conditions will change. You'll either need some control mechanism per room or else decide that precise temperature control is not necessary in which case a single temperature sensor will be fine.
    • CommentAuthortony
    • CommentTimeDec 8th 2018 edited
    I like the idea of nicking heat from the hot water to use in a post heater, I think that it has “mileage”
    Heat capacity of air is bit over 1kJ/m3/C. If you heat the MHRV air by +2deg above desired room temp, how many W can you supply into the bedroom, based on your MHRV flow rate?

    Edit: just read back up the thread, you mentioned 16W. Is it worthwhile plumbing in a post heater for that?
    • CommentAuthorgoodevans
    • CommentTimeDec 10th 2018
    Will, to be honest I'm not sure - it has an effect of more than that in reality as it reverses the effect of cooler air coming in - so perhaps equivalent to 32 watts per supply room, but still not much. I'm going to chew on it another day or so. At the moment I like the idea of steeling heat for the DHW to run small radiators upstairs.
    • CommentAuthorJeff B
    • CommentTimeDec 10th 2018 edited
    Every winter I put up PVC strip curtains at the top of the stairs in our dormer bungalow. We don't use the rooms in the dormer roof much and the curtains prevent heat rising up to the landing from downstairs and are quite effective in this.

    Also I have finally got around to making up a heat transfer system which takes warm air (at about 29 - 30C) from the ceiling of the storeroom where my thermal store lives and blowing it out at floor level just alongside my front door. It consists of a low wattage, very quiet, axial fan at the base of a length of 110mm diam waste water pipe along these lines:


    Although the the thermal store is pretty well insulated, there are still standing losses. I have lined the storeroom ceiling with 25mm Celotex boards which helps prevent the heat raising up to the room above in the dormer, hence it can get up to 30C at ceiling level.

    This provides a flow of warm air across the bottom of the front door and out into the hallway. It is quite effective as the radiator in the hall rarely comes on now!
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