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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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    My father-in-law in Hungary has just had an ASHP for DHW installed internally. I'm a bit baffled by this. Apparently the installer said it was the first one he has installed, so didn't provide much information. Has anybody heard of anything like this? I could see how it could work in a warm climate when there would be incidental cooling benefits, but not year round.

    This one seems to have been going in the UK for quite a while, as you suggest it's a bit niche as it typically gathers room heat to heat the DHW, but I think you can also duct in air from outside.

    Here's another
    • CommentAuthorOtterbank
    • CommentTimeJun 6th 2022
    We've had the Eco cent for 12 years now, obviously an older model. It’s never been any bother. 300l cylinder. Initially it drew air from outside but we changed it to draw from the utility room and the open plan kitchen. Still needs an outside stale vent which is joined into the heat recovery ducting. 3 back to back showers wouldn’t be an issue but any more and you would need to be in and out. When this one packs up we’ll certainly replace it with the newer model or similar depending on what the deals are like. Just now bottom tank temp is 45 top 51. Not running at the moment. Two showers at 8 o’clock this morning.
    • CommentAuthorcjard
    • CommentTimeJun 6th 2022
    I've an air to water heat pump installed outside that provides hot water and in the heating season heats the underfloor. I also have an office in the top of the building that has around 16 sqm of south facing roof glazing for 40sqm of floor space; suffice to say the top of the building in summer is rather warm. Having felt the cool of the air the outside ASHP produces I'm currently looking for a second, small unit to mount in the top of the building and run to DHW during the heat of the summer - if I'm going to pay for cold air I'm going to store the captured heat for use elsewhere!

    Would I install the main unit in the house? No chance- after extracting the heat from the house to heat the DHW I'd then have to pay to reheat the house - I'd only do it if I had spare heat arriving for free
    • CommentAuthorOtterbank
    • CommentTimeJun 6th 2022
    No heating in the utility room. Fridge,freezer and washing machine in there so thought it would be feasible to draw air from there and the other inlet is close to the cooker in the kitchen area. The ASHP on the tank is obviously a small unit and my reasoning was if it was drawing from the house it wouldn’t be running so long . An option would be to draw from an internal garage I guess but we don’t have one.
    ISTR some discussion about using it as an extractor fan from the shower, like an Exhaust Air Heat Pump. When the shower is running, there's a short-term need for a large flow of warm damp air from only the shower room, not the other rooms. This coincides with hot water being drawn out of the tank. Running the ASHP water heater at that time, will recycle the latent and sensible heats from the shower room air, back into the hot water tank, instead of dumping them outside like an extractor fan does, or spreading the sensible heat all round the house like a MHRV does.

    Enough air has to be supplied into the house to balance the air that is being extracted by the ASHP, but for most of the year that air supply could be warmer and drier than the exhaust from the ASHP, so overall there is a net heat gain to reheat the hot water tank.

    They haven't massively taken off in the UK yet, so maybe there is some catch - too expensive? Too unusual? Too noisy?
    • CommentAuthorcjard
    • CommentTimeJun 7th 2022
    > Fridge,freezer and washing machine in there

    Yeah, but in essence you thus have to comfortably assert that the waste heat from those appliances, less any lost into the fabric instead of going through the ASHP, less any inefficiencies the ASHP has in extracting it from the air, is capable of heating the house/DHW. I think it'd take a while for my fridge and freezer to heat my DHW tank but if I were looking to do so I wouldn't let them dump their heat into the air and then try and recover it; I'd fit a water cooling system to the fridge's radiator..

    Washing machine makes more sense, if you can reliably extract the heat from the outflowing 30/40/60 degree water, but at what.. 50 litres per cycle, say half of it is hot washing, the other half cold rinsing, so you'd need 10 lots of 60 degree washes a day and perfect recovery to heat a 250 litre tank to 60 degrees..

    It's just not enoug; ASHP need to be somewhere where considerable amounts of air (containing relatively miniscule amounts of free heat) is available.,.
    • CommentAuthorcjard
    • CommentTimeJun 7th 2022
    > will recycle the latent and sensible heats from the shower room air

    There isn't enough of it to make it worthwhile; the volume of air an ASHP shifts would completely refresh the air in a good size shower room in less than 20 seconds

    Forgetting the ASHP for a second, let's just have a magic device that collects all the steam that pours into the air while a hot shower is being taken - I think you'd be doing well to get more than a few tablespoons of water; that's about the amount of heat you'll be recovering

    All an ASHP as a bathroom extractor will do is put you in a massive draft and make you feel very, very cold
    It's a very small ASHP - an airflow somewhere about 50m³/h would do, so about the size of an extractor fan. It's much lower power than the ASHPs we typically have outside our homes for central heating, which do indeed shift huge volumes of air.

    Warm air contains a surprising amount of water vapour, about 20g (bit more than a tablespoon) in each m³ of air at 25⁰, but invisible because it is evaporated. The latent heat you can get out of it if you can condense it, is several times more than the sensible heat from the temperature of the air, same as how condensing boilers work.
    • CommentAuthorOtterbank
    • CommentTimeJun 9th 2022
    The unit just sits in a cover on top of the hot water cylinder. If memory serves me it is rated at 800 watt. There aren’t any massive draughts, and it isn’t any noisier than a fridge, it is in a small machine room the size of a WC. The unit is running at the moment and the temp is 19.9 in the open plan kitchen. No heating on and overcast outside. Before the energy hikes our all electric bill was just over £2 a day during the summer. We run a business from a workshop that is on the same meter. It works for us but as I’ve said before it’s just based on everyday use nothing more scientific than that.
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