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Ventilation: MVHR, heatwaves, chiller?

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    My house in Cambridge had a low energy renovation around 15 years ago with triple glazing, increased thermal insulation, solar thermal system, thermal store and MVHR. The MVHR unit is a Vent-Axia Sentinel Kinentic BH.


    For most of the time since it was installed it has worked very well and I rarely needed to open any of the windows. However, during the recent Amber Heat Alert the house overheated badly. Possibly the high humidity made it worse than in earlier years or my age and, maybe medication, made me less resilient.


    My suspicion is that during an extended heatwave with tropical nights the MVHR is unable to maintain comfortable indoor temperatures. With or without a summer bypass, if the air temperature is over 35º C outside then the MVHR will import that air. Even if the air temperature drops lower overnight it can't catch up and the house continues to retain the heat, made worse by the high quality of thermal insulation.


    Some form of air cooling may be required! My back garden is nowhere near big enough for a ground-coupled heat exchanger so active cooling may be an option.


    I have seen an example where a reversible heat pump was connected to a water/air duct heat exchanger (eg. Zehnder ComfoPost). However, my house has a gas boiler.


    Has anyone else experienced similar problems or can suggest possible solutions?

    • CommentAuthorMike1
    • CommentTime1 day ago edited
     
    Posted By: john.connettWith or without a summer bypass, if the air temperature is over 35º C outside then the MVHR will import that air.
    That may depend on the sophistication of the MVHR unit. My Zehnder cools the incoming air down to a set temperature using the cooler outgoing air, if that's cooler than the incoming air. For the summer, I've adjusted the set temperature down to the minimum 12°C, rather than the default 22°C to take full advantage of any cooler outside air - check your MVHR manual as you may be able to do the same.

    Posted By: john.connettI have seen an example where a reversible heat pump was connected to a water/air duct heat exchanger (eg. Zehnder ComfoPost).
    Unfortunately air has a low specific heat capacity, so you need to move lots of air to make a big difference - more than an MVHR unit typically handles.

    Because I had no other viable option, I've installed a HomEvap adiabatic cooler before my MVHR but, even though I can achieve >1 air change per hour, the limited airflow means that it can only provide a few hundred Watts of cooling, even though it does a good job cooling the incoming air (yesterday afternoon from 30°C to 23°C within the MVHR unit). It's enough to somewhat slow the temperature increase in the rooms, but nowhere near enough to reverse it (well, maybe in a Passivhaus with an oversized MVHR unit it might, some of the time).

    Posted By: john.connettMy suspicion is that during an extended heatwave with tropical nights the MVHR is unable to maintain comfortable indoor temperatures.
    If the air is so hot that the building doesn't have chance to cool, as in an extended heatwave, that's a problem that can only be solved by active cooling - typically air conditioning or chilled underfloor central heating. I have the same problem as I have nowhere to install a heat pump.

    Ceiling-mounted fans are on my list for next year, once my new electrical system is up an running. They can make a room feel significantly cooler, even if the temperature is unchanged, and are well worth considering - much quieter, more effective and energy efficient than desk fans.
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      CommentAuthordjh
    • CommentTime1 day ago
     
    I agree with pretty much everything both John and Mike have said.

    The summer bypass on my MVHR automatically turns on whenever the outside air is cooler than the inside air and the inside air is warmer than a set point. It turns off if the outside air is warmer than inside.

    When the air doesn't cool much overnight, the inside does begin to heat up and there's not much the MVHR can do. Even if the summer bypass opens, there isn't a large enough temperature difference between outside and inside to provide much cooling effect.

    Thermal insulation is generally a good thing, since it slows heat gain when it's hot outside and heat loss when it's cold outside.

    Ceiling fans are a very good idea. Most houses in tropical areas had them before air-conditioning and many still have them. High ceilings also help.

    And yes, air-conditioning is a very effective technique.
  4.  
    As all said, a Sentinel MHRV doesn't move enough air to control the temperature (not what it's intended for).

    If the overnight or early morning outside air is cooler than inside, then open all the windows wide to get very large volumes of cooler air through and cool the fabric.

    If it isn't, then active cooling is required.

    Actually for the OP in Cambridge with health issues I would not hesitate to get AC, whether portable or fixed. It used to be looked down on as a damaging luxury (by folk who had no such issues with heating!) but this summer there have been record amounts of solar on the grid, so even if a home doesn't have own PV there is plenty of low carbon daytime electricity for AC.

    Might still want to avoid running it in the evening peak (if possible) for a couple more years, or get a home battery. Overcool the house during daylight hours to take it through the evening.
    • CommentAuthorMike1
    • CommentTime20 hours ago edited
     
    A couple of further comments on this:

    Posted By: john.connettMy back garden is nowhere near big enough for a ground-coupled heat exchanger
    A garden isn't necessary; I've seen a vertical borehole drilled through the floor of a plant room in a London home. It would, however, cost several time the cost of an air source heat pump / aircon unit.

    Posted By: Mike1I've installed a HomEvap adiabatic cooler before my MVHR but...
    Another 'but' is that you need to have insulated the air distribution ducts & accessories for this to stand a chance, or the cooled air may well have warmed again before it reaches the ceiling vents. I'm loosing 2 to 3°C on mine, but I haven't yet insulated the manifold nor the main duct from the MVHR unit.
    • CommentAuthorCerisy
    • CommentTime10 hours ago
     
    Our 16 year old Titon unit is fine for most of the year but the summer bypass function is useless in our set up. In these heat waves (we live in Normandy and recorded a temperature over 41 degrees!) I switch the MVHR unit off during the day with all the windows and upstairs shutters closed. When the outside temp in lower than in the house I’ll open windows to draw some cooler air through.

    However at our mid-70’s ages we struggle seriously with the heat and humidity so I had A/C installed for the main lounge and our bedroom. Not cheap but essential. We got through the last heatwave okay. Our son who lives nearby is going for ceiling fans - look a good option and way cheaper!!
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      CommentAuthordjh
    • CommentTime9 hours ago
     
    Posted By: CerisyOur 16 year old Titon unit is fine for most of the year but the summer bypass function is useless in our set up. In these heat waves (we live in Normandy and recorded a temperature over 41 degrees!) I switch the MVHR unit off during the day with all the windows and upstairs shutters closed. When the outside temp in lower than in the house I’ll open windows to draw some cooler air through.
    Summer bypass can't work when it is hotter outside than inside. In those circumstances the best an MVHR can do is to bring the heat exchanger back into the circuit which makes the supply air a little bit cooler than the air outside. Air con is as you say the only real solution, although fans do help somewhat.

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