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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorsnyggapa
    • CommentTimeOct 17th 2017
    So, looking at a MVHR system to retrofit to a 1900s terrace. There is a huge variety of systems available, but they all basically boil down to two fans in a box, a heat exchanger and a condensate drain for prices that dramtcally vary. It's a big refurbishment so budgets are at best wild estimates and we have already found out that everything is more expensive than initially planned...

    I am trying to do the maths on "cost efficiency" - not necessarily from a money saving point of view (although that is always nice) but overspending money will constrict us from spending it on more useful or efficient things.

    The property is about 140m2 , kitchen + 3 "wet rooms" , needs about 42 l/sec of ventilation according to the building regs. My gut feeling is that is about correct (having turned down my current small house to well below regs) this is about twice the volume of current house and of ventilation.

    Looking at units available, there is a huge gulf in the SPF, heat recovery efficiency and cost - and not always correlated.

    Doing the maths on SPF is easy - and to be honest it makes very little difference even over 10 years to the overall running cost between a SPF of 0.5 and 1.0 - 200 kWh or £30/year . What I am trying to quantify if the benefit of a better heat exchanger.

    I have tried to do some dodgy maths to work out what 42 litres/second or air actually does in heat loss terms and although I did spend one evening with a bottle of wine and came up with an answer that looked good at the time, I can't actually be sure where any of the numbers came from :)

    Does anyone have any rules of thumb as to how to do this calculation. My drunken calc included heating degree days (3557 at 21 degrees)and numbers for "l/second watts per degree c" but I have no idea what the latter is :) I'm looking for a fag packet idea of the heat loss from 42 litres/second of ventilation, which I guess I can then multiply by the theoretical efficiency to see the "saving" but I guess the heat loss depends on the difference in temps between inside and outside , as if it's the same temp inside and outside there is in effect no loss.

    Can anyone help clear my mind. Simplistically, I guess I am trying to work out if its justified to spend £400 on a 75% efficient unit or £1500 on a 94% efficient, or something in between!


    • CommentAuthortony
    • CommentTimeOct 17th 2017
    Optopn B for me
    • CommentTimeOct 18th 2017
    If you oversize a unit you get a better performance from the heat exchanger.

    So you need to work out the usual flow rate that your house needs in lt/sec, convert that to kg/day (1m^3 of air is about 1.25kg).
    Then you are into usual heat loss calculation.

    Mass x temperature difference x specific heat capacity = joules

    3,600,000 J = kWh
    • CommentTimeOct 18th 2017
    I would be checking features first:
    - automatic summer bypass
    - washable filters
    - washable heat exchanger
    - noise level
    - control options (humidity etc)
    - frost pre-heater
    - constant flow rate & automatic flow balancing (nice, not essential)
    - post-heater control (nice, not essential)
    - length of warranty

    I would expect better SPF and efficiencies to come out naturally.
    • CommentAuthorsnyggapa
    • CommentTimeOct 18th 2017
    Thanks. I do have a features spreadsheet already :) which rules out a number of options. And I got a set of technical docs from boulder which ruled out them on SPF grounds 8-}

    Current favorite is the brookvent aircycle 1.2 with bypass and humidity control , 0.67 SPF and 90% efficient in a K+3 configuration, 5 year warranty. To step up from that seems to be the vent axia sentinel kinetic plus at quite literally double the money. I am trying to work out if the payback on that is the "14th of never" :)
    • CommentAuthorsnyggapa
    • CommentTimeOct 18th 2017
    Thanks steamy. I think the bit I am struggling with is "temperature difference" - as during the summer the efficiency doesn't matter, the shoulder months not a great deal either, but I assume in winter the temperature difference is where you make a difference.

    I would also be making a crude assumption that a 90% efficient MVHR if internal is say 21 and external say -5 that the "loss" would only be 90% of -5-21 = 26 degree difference, so 2.6 degree "heat loss"

    so fag packet, would appreciate a sanity check:

    over my 45l/s = 160m3/hr = 3840 m3/day = 4800Kg/day

    4800kg * 26 (temp delta) * 1.005 (Kj/Kg) = 125,424Kj/day = 34 kWh/day heat loss through air extracted

    a 90% efficient unit would reduce that to 3.4kWh/day - but only if it was -5 outside

    The numbers feel too small :)

    • CommentAuthorgoodevans
    • CommentTimeOct 19th 2017 edited
    You have your whole season heating temperature difference of degree days / annum at 3557 (85368 degree hours)

    and flow at 42l/s

    - I use a ballpark figure of 1.2 watts per litre/s of air per degree C. (0.0012 KW per litre/s.K flow)

    heat loss with zero efficiency = 85368 * 42 * 0.0012 = 4302kWh / annum

    at 94% efficiency - 258 kWh/annum
    at 75% efficiency - 1075 kWh/annum

    at 5p/KWh price difference in cost = £40 per annum.
    • CommentTimeOct 19th 2017
    Posted By: snyggapaCurrent favorite is the brookvent aircycle 1.2 with bypass and humidity control , 0.67 SPF and 90% efficient in a K+3 configuration, 5 year warranty.

    Hmm, looking at the brochure:
    - automatic summer bypass - seems to have fixed temperatures - I definitely value the adjustable limits on my Brink.
    - washable filters - it says G2/G3 - I would definitely want to be able to fit a G4 or F7 if required
    - washable heat exchanger - ?
    - noise level - dunno
    - control options (humidity etc) - OK - but see below
    - frost pre-heater - it doesn't use a pre-heater - it stops air supply - not so good
    - constant flow rate & automatic flow balancing (nice, not essential) - all manual
    - post-heater control (nice, not essential) -no
    - length of warranty - OK

    I'm not sure whether the system is sensibly sized for your house. I would check the flow rates in the ducts and the pressure drops carefully.
    It only seems to have two speeds, which seems rather limiting. We use a very low trickle speed for when we are away, a normal speed and a boost speed. There's also another even higher speed which I suppose we might use if we ever have a house full at Xmas or similar.
    • CommentAuthorsnyggapa
    • CommentTimeOct 19th 2017
    Thanks djh

    yes, the controls on that unit are basic , noise I think is OK (24db at 42l/sec , 33db at max) but you may have a point about it being man enough due to pressure drops - the 42l/sec is stated at ~20pa of pressure which unless it's a different measure to other units published stats which often quote 100 or 160pa... Have asked re the washable exchanger, that's a good point - my vent axia gets clogged with fine dust and the odd stray that got past the filters so I can see the advantage.


    goodevans - thanks for the crosscheck - the numbers kind of line up with what I calculated in the beginning (but couldn't work out how I did it :) and with steamy's numbers (I guess the variability is due to the approx weight of a m3 of air)

    I am still surprised how little difference the efficiency makes then! My primary heat source would be gas so the cost difference would be roughly half that. I won't get a 75% efficient model, as wasted energy is still wasted energy, but it does mean I will lose less sleep over the difference between a 89% and a 92% as that amounts to a fiver a year...
    • CommentAuthorGreenfish
    • CommentTimeOct 19th 2017
    Perhaps a sily question, but is the point of having MVHR to save energy?

    I think it is air tightness that does the energy saving, and increases comfort by making the house much less drafty. But living in an airtight box is unhealthy, human generated pollutants such as moisture, CO2 and smells build up as could VOCs and other nasties from furnishings. Moisture is the biggest issue, from cooking, showers and just breathing, and with it comes condensation and mould (with unhealth spores). Ideally you vent as little as necessary (for health/comfort) with the best efficiency. A way to do that is MVHR - controlled ventilation with minimum heat loss and energy consumption.

    Seems to me that the key question is not marginal energy consumption differences (based on efficiencies that may well deteriorate over time as the fan wears?) but will the unit deliver sufficient and controllable ventilation. And for me controllable means good manual intervention as much as sensors etc. Also will it do it quietly and maintainably.
    • CommentAuthorgoodevans
    • CommentTimeOct 19th 2017 edited
    Yes I agree with Greenfish - MVHR should be installed as a result of making the property airtight.

    I think a leaky house with humidity problems would be better served using modern dMEV unit in the wet rooms such as this one http://www.envirovent.com/home-ventilation/products/bathroom-extract-fans/eco-dmev/

    Also much easier to retro fit
    • CommentTimeOct 20th 2017
    • CommentAuthorsnyggapa
    • CommentTimeOct 20th 2017 edited
    Good questions above. No, the point of the MVHR is not to save energy but to reduce energy loss in a house that needs mechanical ventilation to expel the products of modern life such as humidity - so I guess I am trying to find the best balance between energy efficiency and cost efficiency to do it.

    Although the place we are renovating is a 1900s terrace, the walls are 2 feet thick and the floor solid concrete (sadly presumably a later alteration) so I would expect that even before we start it's pretty air-tight apart from the builder induced bodges that have occurred over time and can be easily corrected. It also has a huge damp problem which may be due to previous owner's lifestyle and the aforementioned relative airtightness.

    Interesting and unexpected lessons from the calcs I have done is that SPF makes s much more significant difference that heat exchanger efficiency so I can look at that more carefully. I also have a new "favorite" unit as a potential purchase, a vent axia sentinel kinetic FH - looks like it's halfway between the basic BH model and the "plus" range in output which is perfect for what I want. The BH which I currently have in my house was marginal for this new place. and it ticks lot more of the "controllability" boxes which have rightly been identified as important.

    Currently looking at ducting and whether to go full rigid or semi-rigid radial. I'm looking at getting someone to to a sketchup model of the house so I can visualise pipe runs but 75mm semi rigid feels like the easy solution.

    >I think a leaky house with humidity problems would be better served using modern dMEV unit in the wet rooms such

    interesting viewpoint. Theoretical heat loss if I just blew the hot air out and let cold air in is 4302kWh/year so approx £130/year. This mull MVHR system is going to cost ~£3,000 to install (1200 for the unit, more for the ducting and the rest for redecoration and assumes that my time is free). From a "payback" point of view it doesn't work at all!
    • CommentTimeOct 21st 2017 edited
    Posted By: snyggapaTheoretical heat loss if I just blew the hot air out and let cold air in is 4302kWh/year so approx £130/year.
    What ACH is that based on?
    • CommentAuthorsnyggapa
    • CommentTimeOct 21st 2017
    thats as per goodevans above, 42l/second and 21degree HDD days

    approx 160m3/hr so roughly 0.24 ACH/hr if my dodgy maths are right (140m2 floor plan, 2.5m height average)
    • CommentAuthorBowman
    • CommentTimeNov 5th 2017
    I can recommend Brookvent.

    Without any kind of measurements I reckon the MHRV saves about 2degrees of heat and damp has been significantly improved.
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