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Thread: Extractor fan and/or heat recovery

  1. #11
    Automated Home Legend Paul_B's Avatar
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    I'm making an educated guess that dH/dT is relative humidity? The dH being change in absolute humidity. This nbeing a better measurement of comfort level for humans

    Paul

  2. #12
    Automated Home Legend chris_j_hunter's Avatar
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    measuring rate of change (of humidity, in this case, so dH/dt) would usually be a way of getting earlier warning of the rise of something (humidity, here) ... ie: the change would be noticed sooner - which would probably useful to get the fan going asap, and so minimise the rise ... measuring d2H/dt2 would give even more sensitivity, but need more accurate sensors, so probably not worth it ! That's as I understand things, anyway !

    Chris
    Last edited by chris_j_hunter; 25th June 2009 at 01:43 PM.

  3. #13
    Moderator Gumby's Avatar
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    dH/dt is the rate of change of relative humidity (based on the labelling in the graph in Cortex).
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  4. #14
    Moderator Gumby's Avatar
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    I've attached 3 Cortex plots of RH, dh/dt and Dewpoint from the same period of time for comparison.
    Attached Images Attached Images
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  5. #15
    Moderator Gumby's Avatar
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    I have an interest in humidity control due to historical problems with damp that I believe are down to condensation on/in cold solid (ie no cavity) stone walls during winter - despite all the nonsense of injected damp-proofing etc.

    I don't have whole house extraction but I do have a dehumidifier in the coldest place and a PIV (passive input ventilation) fan that can move air from the loft (preheated from solar gain in winter) into the house.

    At the moment I control the PIV fan within a temperature range, so that it doesn't blow hot air into the house when the loft is very hot and doesn't blow air in when it's really cold. The speed is stepped up as available air is warmer.

    I control the dehumidifier based on RH in the room it is in.

    So far it seems to have controlled damp issues very well through this winter.

    A more intricate control scheme that attempts to manage the dew point to be above the temperature of the walls might be possible, but I'm not sure that it's worth it - especially since I couldn't find much actual research on the behaviour of water in solid stone walls.
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  6. #16
    Automated Home Legend chris_j_hunter's Avatar
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    Interesting - how do you deal with the water the dehumidifier extracts, to stop it getting into the air again ? And drying washing ... good ventilation, even extraction, is often a very cost-effective way of reducing condensation, ideally with a heat-exchanger (extractor fans with them are available) to recover some of the heat or cool (as it were) ... ??

  7. #17
    Moderator Gumby's Avatar
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    The dehumidifier is a small portable one with a 10litre tank - about a week's worth, then I empty it

    If I had known then what I know now I would have investigated whole house ventilation systems - although I am not sure how beneficial they are in old houses with draughts and chimneys etc. So the PIV fan is a sort of half way house, but very easy to fit since it sits in the loft and has one duct onto the landing.

    The dehumidifier will release some heat as a side effect of condensing the water. I have not measured it's power consumption.

    As much as possible we line dry outside and the tumble vents outside. Biggest sources of condensation in our house is probably cooking (very difficult to vent the extractor), showering and breathing, not necessarily in that order.

    However ... this is starting to wander off-topic ...
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  8. #18
    Automated Home Guru JonS's Avatar
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    I use dH/dt for controlling bathroom fan and hence humidity. It is the rate of change. So slowing changing humidity e.g. wiht the seasons (summer lower in our house) have low dH/dt; conversely showers which suddenly make the air moist have a high dH/dt.

    I've done quite a lot of trialling and found that dH/dT of 15 for switch on and 0 for switch off works well most of the time. It does cycle a little if in the bath for a long soak. I have tried a dH/dT and fixed threshold together for on-off and found this gace erratic behaviour as the thresholds competed. I've also had going above negative values for switch off (ie getting less damp but more slowly) but these tend to let the fan run for longer than necessary which got annoying.

    I think the position of the sensor will have an impact on the values you choosee and a little experimentation may be needed to satisfy your tolerance of either cycling or long running.
    HTH
    JonS

  9. #19
    Automated Home Jr Member jcmiguel's Avatar
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    Default dH/dt

    I will try dH/dt this weekend and will give feedback
    Thanks
    Quote Originally Posted by JonS View Post
    I use dH/dt for controlling bathroom fan and hence humidity. It is the rate of change. So slowing changing humidity e.g. wiht the seasons (summer lower in our house) have low dH/dt; conversely showers which suddenly make the air moist have a high dH/dt.

    I've done quite a lot of trialling and found that dH/dT of 15 for switch on and 0 for switch off works well most of the time. It does cycle a little if in the bath for a long soak. I have tried a dH/dT and fixed threshold together for on-off and found this gace erratic behaviour as the thresholds competed. I've also had going above negative values for switch off (ie getting less damp but more slowly) but these tend to let the fan run for longer than necessary which got annoying.

    I think the position of the sensor will have an impact on the values you choosee and a little experimentation may be needed to satisfy your tolerance of either cycling or long running.
    HTH

  10. #20
    Automated Home Legend TimH's Avatar
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    I've just finished my ensuite renovation and Chris thought some of the details might be relevant, so here goes
    [I don't run Idratek but it may still be relevant]

    Photos here: http://picasaweb.google.com/TimE.Hawes

    I have and Aqualisa Quartz digital shower with the processor box hidden in the loft. The pump and mixer valve is all in this black box. The silver round thing in the shower has two low voltage buttons “on/off” & “boost” and a dial (potentiometer) to adjust the temperature. The signals go back to the processor box and adjusted in there.

    The extractor fan (single speed with timed overrun) is wired through a DHW cylinder thermostat clamped to the hot pipe; the idea being the fan comes on automagically when the shower is running. This needs a bit more work as it takes a quite for the heat in the pipe to activate the fan, and even longer for the pipe to cool down (and then turn the fan off) afterwards. The timed overrun on the fan is set to minimum, so about a minute, but pipe cool-down is more like 25minutes… Maybe the contact between the pipe & stat isn’t great, maybe I need a different type of thermostat, but ideally I’d have the fan come on when the pump starts and overrun for 5mins when the pump stops. I’d rather avoid a flow switch, so a bit more head scratching I think.

    With three speeds you could drive the first (lowest) from the light switch (or a PIR), i.e. if the light is on, then someone is in, so gently ventilate.
    2nd speed activated by the temperature of the hot pipe to the bath increasing in temperature (i.e. a dT/dt condition, change in temp with time).
    For the 3rd speed use a humidity sensor; if the air becomes too humid then ventilate the bathroom at max rate.

    To turn the fan off, you could consider a rate of change condition for the bath waste pipe (pipe goes hot *and* feed pipe is/was recently hot). If you have a separate shower, again monitor the waste pipe temperature but look for a "cold" temperature; the waste pipe will heat up while the shower is in use, but then cool down again once showering has stopped.

    What I've found is that there seems to be too much thermal mass in the hot water feed pipe to turn the fan off - it just takes too long to cool down.

    Hope those thoughts are useful.

    Tim.

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