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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorstones
    • CommentTimeJan 27th 2014
     
    As some of you will recall from previous threads, we have had an Exhaust Air Heat Pump (well known Swedish manufacturer) fitted to our home for the last 3 years, and thought it time to give an overview of the systems performance.

    Our unit provides all our DHW, heating and mechanical ventilation.

    Ventilation is however mechanical Extract Ventilation only (MEV) not MVHR in the normal sense.

    This actually increases our space heating requirement according to SAP 2009 as follows

    Current system - EAHP – 8074 kwh/yr
    Possible alternative system ASHP with MVHR for ventilation – 5079 kwh/yr

    One could be forgiven for accepting the increased space heating requirement if the heat pump actually met all of the DHW and space heating requirement at a CoP of 3 as SAP2009 indicates. The big problem is that it doesn’t because of the limited size of the heat pump compressor (650W).

    For us, with an annual DHW requirement of 5000kwh a year (including losses) this means that the heat pump only has capacity to provide DHW and space heating down until around 7C ambient. Below that, any shortfall is met through the use of immersion.

    In our case, we have used an average of 5180 kwh of immersion each year.

    So, to meet our total energy demand of 8074kwh heating and 5000 kwh DHW the heat pump uses about 3600kwh/yr for compressor operation and 5180kwh/yr immersion. This gives us a system CoP of just under 1.5

    Is it all bad – the system has good weather compensation control and maintains a very pleasant and even temperature around the house (19C) 24/7. The unit is the size of a large fridge and has simple enough controls, and can be described as fit and forget. Used for DHW only, in the same way as some of the other EAHP cylinder units, the system would work well.

    I for one have had enough. Having tried seeking redress from the installer (who still maintain it is a good system and meets our requirements at reasonable cost!) and taking legal advice (not enough evidence to succeed) I am currently investigating whether to cut my losses and install an ASHP and MVHR system. Based on known heating requirements and flow temperatures, I believe I could cut my current energy use by 50%.

    So, the moral of the story is buyer beware. These units whilst fine for DHW, cannot meet space heating requirements in addition to DHW. Many have been ripped out of social housing schemes in England. The continued insistence of the manufacturer that the units have a CoP of 3 is nonsense. The heat pump / compressor may have, but after taking back up immersion into account it most certainly does not. And the industry wonders why it has unhappy customers...
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 27th 2014
     
    Would a large unit work better for you? And may be a cheaper install option.
  1.  
    Posted By: SteamyTeaWould a large unit work better for you? And may be a cheaper install option.



    Surely a larger unit would need more airflow - wouldn't that lead to over-ventilation? IMHO that's one of the fatal flaws of EAHPs, unless the heat you take out is that provided by a reasonable ventilation rate (e.g probably enough for DHW but not for both DHW and heating unless the house as at passivhaus levels of demand).

    Paul in Montreal.
    • CommentAuthorstones
    • CommentTimeJan 27th 2014
     
    Paul,

    You have hit the nail on the head. A bigger unit would require more airflow, meaning over ventilation / bringing in more ambient air into the house which you then have to heat. 11 hours operation (including defrosts) to provide our DHW. Maximum compressor operation of 18-19 hours per day (rest of the time its defrosting)

    I currently ventilate at 145m3 per hour (360m3 house). I can increase the ventilation rate, which increases the CoP slightly but you just end up increasing the supply of ambient air which you then have to heat. I have tried various flow rates but 145m3 ventilation rate is optimum for our house. I suspect that even with a passive house type spec this unit would struggle and resort to immersion (albeit smaller amount than me)

    ST

    Cost wise these units were at the time of install cheaper than ASHP system's. ASHP prices have come down, but the cost of these units has remained static.
    •  
      CommentAuthordjh
    • CommentTimeJan 27th 2014
     
    Posted By: stonesI for one have had enough. Having tried seeking redress from the installer (who still maintain it is a good system and meets our requirements at reasonable cost!) and taking legal advice (not enough evidence to succeed) I am currently investigating whether to cut my losses and install an ASHP and MVHR system.

    If you just installed a smallish ASHP in addition to your existing heat pump, would that be enough? You could perhaps reduce the ventilation rate so there wouldn't be the same benefit from MVHR. But you'd need to find an ASHP with good low temperature performance.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 27th 2014
     
    Posted By: Paul in MontrealSurely a larger unit would need more airflow


    Posted By: stonesYou have hit the nail on the head. A bigger unit would require more airflow,
    Or could you use the same airflow but get a higher temperature out of it, purely thinking aloud here.

    Posted By: djhIf you just installed a smallish ASHP in addition to your existing heat pump
    Good idea, especially if you know the split between DHW and space heating. If your existing setup can do one or the other you can find the cheapest solution.
    • CommentAuthorstones
    • CommentTimeJan 27th 2014
     
    It is possible to link an ASHP to the EAHP, and indeed Chris P Bacon has previously advised that the 5kw Ecodan is actually marketed in Sweden for this purpose. I had thought about this and did in fact email Mitsubishi Ecodan Uk about it. The answer did not fill me with confidence it must be said - along the lines, your system is undersized so take it out and replace with one of ours - no offer to contact Sweden etc. Now while this doesn't in itself deter me, the lack of anyone really knowing how to do it in the UK does - could really see it being a recipe for disaster if connected and set up wrongly as installers and manufacturers would start blaming each other. Maybe I'm too cynical.

    By trying to keep the existing unit, I maintain the higher heat demand, all I do by linking up an ASHP is eliminate immersion. If I choose to replace, I both eliminate immersion and reduce heat demand by around 1/3. I would however need a new DHW cylinder and buffer and an MVHR unit (ducting already installed). While this is an extra cost, it would allow me to properly integrate my solar PV, which the current EAHP does not let me do.

    I have tried reducing ventilation rates, but we have found 145m3 to be the optimum level to prevent condensation, have a fresh environment. Much lower and the air is very stale and unpleasant - much like an airliner.

    I could of course keep the current EAHP for DHW only and operate it as a standalone cylinder type unit, but that still leaves me with the question of heating. I already have large low temp radiators which operate at low flow temps ( 36C flow at -5 ambient) so tend to discount A2A as to get an even temp throughout the house, I would have to install a distribution system (assuming current MVHR ducting is unsuitable as per other discussions). A small ASHP for heating only is possible, but I would still need to have an additional buffer tank and the MVHR if I am going to reduce the heating requirement.

    Switching from the EAHP to an MVHR and ASHP set up would by my calculations save at least ÂŁ500 pa in electricity. The potential funding from RHI as and when it comes on stream also has to be considered on top of that. It will in the end come down to payback. I'm currently getting quotes to see the costs involved and will take it from there.
  2.  
    The exhaust air heat pump will never be as good as MVHR because with the EAHP you have to run a compressor to move the heat around. Even as a DHW only unit it doesn't make sense because you're paying to move heat from your house (which you have already paid for) into your DHW. If you're going to pay to run a compressor, it makes a lot more sense to move heat from outside to inside your house/DHW tank. I would go the whole hog and replace the EAHP with MVHR and ASHP.

    David
    • CommentAuthorRobL
    • CommentTimeJan 27th 2014
     
    I can see how EAHP can heat DHW, but I really don't see how your could efficiently space heat with it, except when the fabric+DHW losses are much smaller than air losses. All the heat to the house has to come from the exhaust air - so the exhaust air must exit the EAHP veryvery cold in order to impart enough heat to combat fabric losses as well as air leakage.

    Just consider the air input temperature - to raise this from external to internal will rob the outgoing air of all it's energy down to the external temperature if the COP is high. Just like a good MVHR. Going further than this is a loosing game, you'd be better off with an ASHP as the temperature is now higher outside than the exhaust temp!

    Ie - I think a bigger unit would not help you.

    Can you estimate the fabric losses ? If I remember correctly, you do have a well insulated house. I'm thinking you might get a much better COP if you didn't use it for DHW as well, so the EAHP has to provide air+fabric losses. I would try this for a while, before you junk the unit!

    In fact, I think that to work well, the exhaust temp must be >external temp - which I think then equates to:

    elecenergy*(COP-1) = air energy exchange ie temp delta*airflow*spec heat capacity
    elecenergy = fabric losses + other losses(DHW)

    In other words, the fabric+DHW losses must be less than airflow_loss/(COP-1) for good effciency. Hence the idea of not using the unit for DHW.

    Does this make sense to anybody else :-)
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 27th 2014
     
    I agree that the exhaust temp should be lower than the external air temp. Does the unit run a risk of freezing when it is closer to 0°C and humid (typical winter).
    This is why I was wondering if you can get a lower exhaust temp with a bigger unit for the same mass air flow (145m^3)
  3.  
    Best case output for an EAHP is the external temp > exhaust temp and have all the moisture in the exhaust air condensed. A bigger unit won't make any difference for a given airflow, even changes in COP won't have that much effect as you're limited by the delta T between outside and the exhaust temperature. If you freeze the exhaust, you'll ice up the evaporator and will have to run defrost cycles. The exhaust temp must be lower than the external air temp otherwise the unit is cooling your house!

    Paul in Montreal
    • CommentAuthorEd Davies
    • CommentTimeJan 27th 2014
     
    A bigger airflow would be a net win over using the immersion, even if it “over-ventilates“ the house, as long as it doesn't cause draughts which make people uncomfortable and so turn the thermostat up.

    Still, building your house in the inlet airstream to your ASHP seems a bit weird to me.
  4.  
    Posted By: Ed DaviesA bigger airflow would be a net win over using the immersion, even if it “over-ventilates“ the house, as long as it doesn't cause draughts which make people uncomfortable and so turn the thermostat up.


    It would also make the house too dry in winter.

    Paul in Montreal.
    • CommentAuthorstones
    • CommentTimeJan 28th 2014
     
    The exhaust air temp is routinely below 0C (currently at -4) but as suggested, this does freeze up the evaporator which then needs defrosting. If you increase the airflow to reduce the amount of defrosting required (because this unit uses the warm air extracted from the house as the defrost mechanism)you increase the amount of ambient air you bring into the house thus heating demand increases even further. You take with one hand then have to give back with the other. We tried various ventilation rates to test this without success. We ended up using the same amount of energy overall (slightly less immersion/ slightly more compressor run time) at higher ventilation rates.

    Are EAHP's of any value? I've been racking my brains trying to think of locations where they would work. Anywhere where there is another free or waste source of heat - workshop perhaps burning wood offcuts to heat a workspace, large solar gain collector (only work when the sun was out), warmer costal locations (UK) perhaps, Southern Europe where ambient temps do not drop below 7C and the householder cannot fit an ASHP/air conditioner. Certainly for our climate they are a big no (at least in my experience).
    • CommentAuthorEd Davies
    • CommentTimeJan 28th 2014
     
    Posted By: Paul in MontrealIt would also make the house too dry in winter.
    Less of a problem in NW Europe than a more continental climate, of course.

    Posted By: stonesIf you increase the airflow to reduce the amount of defrosting required (because this unit uses the warm air extracted from the house as the defrost mechanism)you increase the amount of ambient air you bring into the house thus heating demand increases even further.
    Yes, and you use more electricity, of course. But if that electricity would otherwise have gone to the immersion you have a net win.

    Posted By: stonesAnywhere where there is another free or waste source of heat - workshop perhaps burning wood offcuts to heat a workspace, large solar gain collector…
    Yes, there's a chap on Skye who posts (or used to) on the Navitron forum who has the air coming into the house flow through his workshop somehow which acts as a solar pre-warmer via roof windows. For him one of these Austrian tank-top EAHPs seems to work well.
    • CommentAuthordaserra
    • CommentTimeJan 28th 2014
     
    A south facing conservatory would be great for this.
    • CommentAuthorEd Davies
    • CommentTimeJan 28th 2014
     
    Yep: http://edavies.me.uk/2012/01/house-sketch/

    MHRV will feed through the gable wall into the porch/greenhouse then out of the south roof of that. Lots of flexibility to experiment with things like that. Redrawing the porch/greenhouse in SketchUp at the moment a result of a slightly different design arising from discussions with my house designer. Planning app should go in this week or early next, I hope.
  5.  
    Posted By: Ed DaviesPlanning app should go in this week or early next, I hope.
    Best of luck.
    • CommentAuthordaserra
    • CommentTimeJan 28th 2014
     
    Love that design. Is that Sketchup as it looks as good as the Acad stuff?
    • CommentAuthorEd Davies
    • CommentTimeJan 28th 2014
     
    Thanks both.

    Yes, it's SketchUp. Now I'm used to it I really like it while accepting its limitations. I have used AutoCAD in the past (mostly programming plug-in type code for it) and, though I liked some aspects, wasn't ever really happy with it.
    • CommentAuthorstones
    • CommentTimeJan 28th 2014
     
    Well, for those who have a suitable source of heat they can utilise, I'm open to offers for my unit as it will be coming out. PM if interested!

    Bear in mind exhaust air heat pumps are specifically excluded from RHI as they are not classed by the EU as a renewable technology. In some ways that is a plus as install can be done by a plumber in half a day as opposed to the eye watering ÂŁ1600 + ÂŁ300 commissioning I was charged by the MCS accredited outfit that supplied it.
  6.  
    How are you going to fair out installing the MVHR with regard to ducting? Presumably you only have exhaust ducts installed at present or?
    • CommentAuthorstones
    • CommentTimeJan 28th 2014
     
    My unit currently does both the supply and extract of air, so I already have supply and extract ducting in place. Some minor modification required to the ducting where the MVHR will be sited, but it is easily accessed. It certainly would have been a headache if I didn't.
    • CommentAuthoran02ew
    • CommentTimeJul 7th 2021
     
    Has these Systems improved since the OP? Or indeed are the newer MVHR/ASHP combined unit worth recommendation?
    • CommentAuthortony
    • CommentTimeJul 7th 2021
     
    I have an exhaust air heat pump hardly used at all, 11 years but very happy with it. Genvex
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