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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentTimeMar 29th 2016
    It's true that the MVHR sees the extracts, but in our house the showers don't have any heating and they are on the north side of the building, whereas the living room, dining room and most bedrooms are on the south side with larger windows. So typically the living room and dining room are slightly warmer although sometimes the upstairs is warmer than the downstairs. So extract from the showers is no warmer than the rest of the house, and that's what the MVHR's extract temperature sensor confirms to me on the odd occasions I look at it (I have to navigate its menus to see it). I intend to fit some logger sensors to the MVHR in the next week or so to tell me more.
    Posted By: djhIt's true that the MVHR sees the extracts.

    >>Remember that a MVHR sees the temp in the bathroom and the kitchen, often these rooms are hotter then the rest of the house. A MVHR that does not have a summer "mode that the user sets, has very little data to work on....

    I suspect we might be overthinking this. I have a few extracts from rooms other than the kitchen and bathroom (wouldn't have been able to balance otherwise) so theres a bit of averaging.

    - Outside the heating season the only reason bathroom and kitchen are hotter is because of activity (showering or cooking) so it's only really air rather than fabric and since rooms are relatively small their air temperature will drop pretty quickly once the bypass activates.

    - generally its only going to activate in the evenings/overnight as (unless your house suffers a lot of solar gain) air temperature will be normally be higher outside than inside on a summers day.

    - The minimum outside temperature (14C by default on the Vent Axia) further limits the active period. Overnight temperatures above 14C puts it firmly in summer in the UK.
    • CommentAuthorringi
    • CommentTimeMar 30th 2016
    How does the MVHR know it is "Outside the heating season"..... Also most MVHR do not know the time of day.
    Posted By: ringiHow does the MVHR know it is "Outside the heating season"..... Also most MVHR do not know the time of day.

    The Vent Axia doesn't know it's outside the heating season but it only operates summer bypass if
    - inside temperature is > 25C [by default, but should be a temp greater than you heat to]
    - outside temperature is > 14C [Default but by coincidence my weather comp heating is set not to operate when outside temp is >13C]

    I've been thinking of tweaking that 13C down a bit as well - a house well insulated/airtight enough to have MVHR installed shouldn't be requiring heating when external temp is >14C (hell, this thread is about whether a heating system is necessary at all).
    • CommentTimeMar 30th 2016
    Posted By: ringiHow does the MVHR know it is "Outside the heating season"..... Also most MVHR do not know the time of day.

    I don't think most MVHR do know what time of year it is, or the time of day. Yours seems to be the only one that makes a guess as to time of year.

    Posted By: Simon Stillshould be a temp greater than you heat to

    But not necessarily when the heating is not being used.
    • CommentTimeApr 1st 2016
    Just came across this thread late but, I have no mains gas, bought a 5kw ASHP for £750 ( new) an MVHR for £360. ( new) will use the ASHP for UFH and pre heat DHW to about 35', immersion on E7 or inline modulating water heater to top up to " hot". Probably use a wet heater in the MVHR from the ASHP to give a little boost to air temp or reverse engineer to give cooling to MVHR and slab in hot weather. Virtually no servicing to the ASHP, no standing charge for gas, no servicing for gas boiler. Will consider PV if the funds allow and a decent enough payback now that Fits are so low. Also if I have E7 I may run the ASHP on this tarrif at night to heat slab and DHW ready for the day ahead at even lower cost.
    The essence or consensus that I have drawn from this discussion, given we are talking about air-tight houses, is that MVHR is only part of the solution. Something else is needed.

    Nobody wants to lose control of ambient temperature and there are a number of ways of establishing the ability to make adjustments up or down. No two bespoke houses are the same and there is no emergent catch-all solution (so far). Even those who might claim to have cracked the code (e.g. Viking House's "Fresh R") have not made the case sufficiently convincingly - at least for me.

    So, I will not be installing anything that a lay person would recognise as "central heating" but I will provide for the ability to adjust the ambient temperature in either direction. It will probably only amount to the equivalent cost of installing an old-fashioned domestic boiler (excluding pipes, radiators, tanks, infrastructure, etc.). It will do so much more with minimal reliability concerns and negligible maintenance costs.

    I will publish a blog in due course.

    Thanks to all
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