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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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    • CommentAuthorGBP-Keith
    • CommentTimeNov 26th 2008
    Does anyone know what the average annual consumption of a whole house MVHR system might be in an airtight home. I'm guessing about 550kWh if on all the time and only consuming 70 watts. but real world data would be far better.

    skeptics of passivhaus suggest a few thousand kWh.
    • CommentAuthorjamesingram
    • CommentTimeNov 26th 2008 edited

    I was just looking at this , the bottom of the page has got some basic consumption info.
    cheers Jim
    I measured my MVHR usage at 274kWh per year. It's an ADM/Indux WH300. It's on all the time, mostly on low power with a boost when it gets steamy. From measuring input and output temperatures I estimate that the heat recovery of the whole system is about 50% efficient and the COP is about 15.
    • CommentTimeNov 27th 2008
    Declaration : I design, supply, (sometimes fit) and commission test HRV equipment from a number of manufacturers (for the best fit).
    70W is very high, but I have seen higher!

    Over the last few years I have collected data and used SAP /BS EN standards, Passiv Haus and IES to model HRV performance.

    Here is a guide:

    1) Do not rely on the manufacturers Specific Fan Power w/l/s as these apply under certain circumstances.

    2) Secondly do not rely on the manufacturers heat exchange data as these can be compromised.

    SAP follows the above two rules by de-rating ALL HRV systems:
    The electrical usage is increased by 40%-70% to allow for poor ducting (increased air resistance, 40% for rigid, 70% for flexible)
    Anoying if you have a test certificate and/or check that ducting has been installed correctly

    All heat exchange rates are reduced by at least 15% (or up to 30%) - under ALL circumstances
    Strangely enough in Passiv Haus, 2 x3 m lengths of exposed insualted ducting in a loft to external cowl will de-rate efficieny by 15%

    So SAP assumes that nobody is installing correctly and imposes sanctions meaning that there is no reason to try for mass market housing!
    This attitude WILL lead to bad systems!

    Note: Appendix Q tests for SAP, are in my view not valid as they deviate away from European Norms - If BRE had a problem with the standards they should have gone to the committees (my mistake - they would then be unable to set up a huge fee based testing service in the UK (not just BRE labs I should say)...

    So to measure performance:

    Take actual energy used meter readings on trickle and boost.
    As a rule of thumb use 5% on boost (as a worst case, many systems rarely need boosting, of course somebody will cite an example that proves otherwise)

    Total = 24*365*Watt reading

    So you have a total KWH for the year.

    Our current lowest measurement is 87 Kwh for a 78m2 "ECO Park Home" (at CFSH energy Level 3)
    Heat Exchanger efficiency - 75%, de-rated to 63%, running at 10W on trickle
    Air tightness -4 m3/h/m2 (still some sealing to do so will be 2 or lower on site)

    Compared to a q50=10, trickle ventilation standard design:

    Heat Loss parameter reduces from 43.55 w/c to 18.09 w/c

    To calculate the predicted total heat loss we need to know the accumated temperature difference.
    Because of passive / occupancy gains and wind sheltering, etc this will be different for ever home, but in this case:

    degree days = 1355 cd (this is fairly typical)

    Ventilation energy - standard = 43.55 x 1355 x 24 = 1416 kwh w/c x cd xh/d / 1000
    Ventilation energy - hrv = 18.09x 1355 x 24 = 588 kwh
    (Net heat exchange rate x 0.33 xx volume)

    Saving 827 Kwh

    COP (savings/cost in delivered energy) = 9.5 (or 950%)

    This is an underestimation of the benefit as the system is running 365, stricly speaking the above could be performed over just the heating season.

    In summer the HRV helps keep homes cool durring the day and by-passes can provide night time cooling.

    In terms of electircal efficiency the above is a good as it gets - EC motors (better than DC). Be warned that some systems have huge motors!

    The heat exchange efficiency comes out at 63%, not fantastic but it still does the job in a small unit

    To compare to other "systems"
    Say you have a heat pump rated at COP = 2.5 (maximum permitted under SAP)
    Outside temperature 1C (so the heat pump may avoid frost), inside 21C

    instantaneous heating by 20C

    HRV providing 0.35 ach (volume of air is 202m3)

    Energy recovered 293W (o.35x0.33x volumex63%x20C)
    energy used 10W
    COP = 29.3 (2930%)

    This is a strong indication that it is far far easier to save energy than it is to produce it...

    Data measurement example
    In our test home, an instantaneous measurement
    Outgoing temperature 24C (kitchen) / 20 Bathroom and WC
    Delivered air 18C (in a bedroom)
    Outside air temperature 0C

    (This system is a 90% recovery model and 38W fans (oversized for other reasons, plan to replace with 10-15 soon) in an air tight semi that had excessive mould growth)

    Heat recovery delivered at the grilles - over 80% (poor insulation in loft over ducting), recovering 436W for a COP of 11.4


    I have a problem - you can get grants for moving heat energy but not for the single most important reduction in heat loss (air tight and HRV) - something very wrong with this scenario (not to mention indoor air quality issues).

    (I will read later to check numbers)
    • CommentAuthorEd Davies
    • CommentTimeNov 27th 2008
    PaulT, interesting post, thanks.

    Heat Loss parameter reduces from 43.55 w/c to 18.09 w/c

    Is "w/c" W/°C, i.e., watts of total building heat loss per degrees centigrade of temperature difference between inside and outside?

    EC motors (better than DC)

    What's "EC"?

    People often say DC motors are much more efficient than AC but I don't understand why. (E.g., the Vales in their _The New Autonomous House_ book). Can you throw any light?
    Posted By: Ed DaviesWhat's "EC"?

    Electronically commutated.

    Posted By: Ed DaviesPeople often say DC motors are much more efficient than AC but I don't understand why. (E.g., the Vales in their _The New Autonomous House_ book). Can you throw any light?

    Small DC motors are more efficient but large motors (power greater than 1kW or so) there's little difference. The largest motors are always AC induction and are around 96-97% efficient. It's something to do with the arrangement of the windings but my 1st year electric motors course at University was 27 years ago and I'm having a hard time remembering ;)

    Paul in Montreal.
    • CommentAuthormark_s
    • CommentTimeNov 27th 2008
    thats very inteesting.

    Are there p[articular suppliers whch are 'good'/recommended or do I need to google and read?

    In a retrofit what would you think about using disused chimneys for ducting?
    • CommentAuthorGBP-Keith
    • CommentTimeNov 27th 2008
    Posted By: mark_sIn a retrofit what would you think about using disused chimneys for ducting?

    So long as you use the chimneys as chases for sealed new ducts (rather than as ducts) you'll be fine. When we retrofitted a forced-air GSHP heating/cooling system we used an old chimney as a chase for the main supply and return trunks to the upstairs floor. The ducts are insulated too to prevent surface condensation when in cooling mode.

    Paul in Montreal.
    • CommentAuthormark_s
    • CommentTimeNov 27th 2008
    Yes Paul, I meant as a route for the ducts - the prospect of making holes all over the place when we have a number of unused fireplaces attached to convinient inter floor pipes seemed sensible.
    • CommentAuthorJackyR
    • CommentTimeNov 27th 2008
    Paul, for the record, what are the problems with using the chimneys themselves as ducts?
    Posted By: JackyRPaul, for the record, what are the problems with using the chimneys themselves as ducts?

    It's hard to know what shape they're in - there may be leaks. Also, there may be tar/creosote/gunk on the inside of the chimney which will slowly offgas into your air stream. Might be OK for an exhaust run, but not for supply. Damp exhaust air may condense in the chimney leading to damage - the hot gases from a fire would dry out the chimney. Also, you may want to run both supply and exhaust ducts in the same chase - in which case you'd have to use ducts. Also, make sure the ducts are rigid, not flexible - the friction losses in flexible ducts are very high - reducing the effective diameter by around 50%. They're OK for the last bit of a run (to a register/vent) but not good for the main trunk runs.

    Paul in Montreal.
    • CommentTimeNov 28th 2008
    Worked example for a 100m3/ 2.4 average room height bungalow
    Assume q50 = 3 and assume n50 is the same (The worst level I can find for any other Northern developed standard - Belgium)
    system providing 0.35 ach
    1300 degree days
    How much energy is used:
    Building regs assume a specific fan power of 2w/l/s:
    Fan energy Per annum: 187Kwh
    De-rating by 40% : 238kwh
    De-rating by 70% :290 kwh

    So worst case is assumed to be 290kwh

    In reality : 350kwh would not be unreasonable (38W AC system) if a larger system was installed into this size of house without fan speed controls - not unreasonable

    (although I have seen 70W DC systems (613kwh) shoe-horned into this size of building)

    Savings - Energy recovered
    At 50% 1236 kwh
    60% 1483
    70% 1730
    80% 1977
    90% 2224

    In terms of delivered energy it turns out that HRV systems should give a good energy return under most circumstances in an air tight building.

    In my view the future should include mandatory commission testing of all systems to check air flow rates and power consumption - something that could be done by air tightness testers (ALL buildings to be tested - not doing so is the same as not checking if insulation has been installed)
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