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Green Building Bible, Fourth Edition
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    • CommentAuthorminisaurus
    • CommentTimeJul 19th 2021 edited
     
    Something I've recently calculated in my job and that I wanted to share - newer heat pumps are now so effective that it is usually more cost and energy efficient to use the energy (and moisture) in exhaust air for heating water in an accumulator tank than to use MVHR.

    With MVHR you recover max 70-85% of the heat energy in the air, and only for those times of the year when you want to recover that energy - i.e. when the outdoor temp is below 18 degrees or thereabouts.

    With a heat pump you recover that 70-85% heat energy and the compressor will boost that so that e.g. with 20 degrees exhaust air in a house you will get 35-45 degrees water to your accumulator tank. Some of the newer pumps even recover energy from the moisture in the exhaust air - I have seen 50 degrees water in bigger buildings (higher airflow rates).

    Connect solar energy and/or an air/water or groundsource heatpump to the same tank, and you'll have very cheap hot water year round, very cheap heating water in winter, plus good ventilation, less ducting, 1 less fan, no rotor exchanger to drive, 1 less filter.

    The accumulator tank needs to have a coil, i.e. not one of those chamber things (or whatever they're called).

    These tanks can also be heated with gas, so you can e.g. switch to an air/water pump later.

    Both CTC and Nibe offer such solutions for houses - very reliable.

    I can post some calculations if anyone wants to get into that level of detail :bigsmile:
    • CommentAuthorjms452
    • CommentTimeJul 19th 2021
     
    Am I correct in summarising exhaust air heat pump for hot water?
    • CommentAuthorminisaurus
    • CommentTimeJul 19th 2021
     
    Yes, and heating (radiator) water
    • CommentAuthorMike1
    • CommentTimeJul 19th 2021 edited
     
    Figure are always interesting, but no one has convinced me yet :)

    - The best MVHR units are independently tested as being able to recover >90% of the heat (see https://database.passivehouse.com/en/components/list/ventilation_small) and have done so for years

    - MVHR is also about good, controlled air quality all year round, not just heat recovery

    - MVHR also means no trickle vents, so no unnecessary external noise

    - High airflow rates tend to be noisy, unless you increase the duct size to compensate

    - Running costs may be low, but the capital costs I've seen have been very high

    - etc.

    BTW, Drexel & Weiss, Genvex, Viessmann, Effiziento (and no doubt others) also make similar combination units.
    • CommentAuthorminisaurus
    • CommentTimeJul 19th 2021
     
    Interesting link, thanks, and we'll have to agree to disagree on our preference as to what to do with all that energy :)

    Can you explain more when you say "independently tested"?

    E.g. the Paul novus 300 claims 93 % - the image appears to show a "cross-flow" heat exchanger - now it is I who is not convinced by figures :)

    In my work, we have around 800 properties - schools, elderly care and other care homes, we can check the trends for heat recovery in the SCADA system, I don't think I've ever seen 93 % on MVHR units (although this is often claimed by manufacterers), we do see high 80s, but then only with rotating heat exchangers. Cross-flow come in at around max 65 %

    - you don't have to have trickle vents with exhaust air, there are other ways to build.

    - flow rates for e.g. the Nibe can be as low as 20 l/s.

    - Costs - a CTC tank ÂŁ1500? CTC EcoVent i360F exhaust air unit ÂŁ750? Life expectancy 20 years for the pump, longer for the tank.
    • CommentAuthorMike1
    • CommentTimeJul 19th 2021 edited
     
    Posted By: minisaurusCan you explain more when you say "independently tested"?
    Independently tested by a laboratory approved by the PassivHaus Institute, following PHI protocols; scroll down to 'Building Services':
    https://passiv.de/en/03_certification/01_certification_components/02_certification_criteria/02_certification_criteria.htm

    If a company hasn't submitted their products for testing, I'd also doubt their figures (and wouldn't buy from them either).

    Posted By: minisaurusYou don't have to have trickle vents with exhaust air, there are other ways to build
    True, but they come with various challenges and aren't as energy efficient. There's one interesting thread here:
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16772

    Posted By: minisaurus we have around 800 properties - schools, elderly care and other care homes
    Then this list of larger capacity units will also be of interest - up to 15,000 mÂł/hr, with the best again over 90% efficient and using cross-flow / counterflow heat exchangers:
    https://database.passivehouse.com/en/components/list/ventilation_big

    Posted By: minisaurusflow rates for e.g. the Nibe can be as low as 20 l/s
    It would be interesting to calculate how much hot water can be obtained / hour at that flow rate.

    CTC EcoVent i360F exhaust air unit ÂŁ750
    That's cheap. Just looked up the Nibe F370 to check it wasn't my memory, and that seems to be retailing at over ÂŁ3K.
    •  
      CommentAuthordjh
    • CommentTimeJul 19th 2021
     
    Posted By: minisauruscross-flow / counterflow heat exchangers

    Cross flow heat exchangers and counter flow heat exchangers are different things. Counter flow is what is used in PHI -certified products and is a key factor in achieving the high efficencies. I don't know which image you are looking at, but you're reading it wrong. The spec quite clearly says counter flow.

    Are any of the MVHR in your properties PHI certified? Are any of the buildings?

    It's not as simple as extracting the heat from the exhaust port of a ventilation unit. If you do that, then you have to heat the incoming air that replaces it and surprise-surprise that takes exactly the same amount of energy you just extracted. So what's the point of using an active heat pump to do that when a passive MVHR unit can do the job?

    Heat pumps generally need to use higher rates of air processing. IMHO it's better to separate the functions. Indeed CTC seem to agree with me:

    "For maximum energy savings, install the CTC EcoVent i360F with one of Sweden’s best-selling air-to-water heat pumps: CTC EcoAir. With this combination, you save up to 30% more than you would with a traditional exhaust air heat pump."
    • CommentAuthorminisaurus
    • CommentTimeJul 19th 2021
     
    Thanks for those links and comment Mike1 (sorry I don't know how to do that blue quoting thing you do)

    I've had a quick look at the calculation and if I've understood correctly I can see how you'd be able to get over 90 % in a passive house. Although probably not in the buildings I work with :)

    The calculation used most where I work is (Temp after exchanger - outdoor temp) / (Exhaust air temp - outdoor temp) * 100

    Of the units in the list, we have Swegon Gold and various Fläkt units in our properties.

    djh - I'm in Sweden, and appear to have mistranslated, but yes, as you say, counter-flow. No passive house buildings where I work.

    djh - all incoming air has to be heated - with MVHR you recover let's say 85 % depending on the calculation you use. With a heat pump, you boost that 85 % say 2-2,5 times. So for the same electrical input as MVHR, you get more heat. Plus you get that heat all year round; you don't vent it out on the warmer days, as you do with MVHR.

    djh - the EcoVent is one example of exactly what I'm talking about! see my previous post :)
    •  
      CommentAuthordjh
    • CommentTimeJul 19th 2021
     
    minisaurus said: "The calculation used most where I work is (Temp after exchanger - outdoor temp) / (Exhaust air temp - outdoor temp) * 100"

    There's a good explanation of why that is a poor choice of formula at:

    https://www.heatspaceandlight.com/cheap-mvhr-expensive-heat-exchanger-efficiency/

    and also said: "djh - all incoming air has to be heated - with MVHR you recover let's say 85 % depending on the calculation you use. With a heat pump, you boost that 85 % say 2-2,5 times. So for the same electrical input as MVHR, you get more heat. Plus you get that heat all year round; you don't vent it out on the warmer days, as you do with MVHR."

    No, a heat pump uses a lot more electrical energy than an MVHR (assuming we're talking about efficient/PHI MVHR). And no, you don't boost the heat recovered 2-2.5 times. Heat pumps don't work like that - they can't magic heat out of nothing. If you want 2-2.5 times as much heat then you have to pump 2-2.5 times as much air and/or increase the temperature difference, plus you have to power the compressor etc. It all takes electrical power.

    I have nothing against heat pumps. I just happen to believe they have different requirements to ventilation systems and are best kept separate.
    • CommentAuthorminisaurus
    • CommentTimeJul 19th 2021
     
    The attached image demonstrates my point better. You have circa 20 degrees exhaust air (GT41). And 0.6 degrees after the heat exchanger (GT43) The pump is using 9kW (Tillförd effekt) and "generating" 23 kW (Avgiven effekt). The bottom of the image shows the hot water being sent to the acc tank - 50.1 degrees (GT21)

    The unit in this example is an IV Eco Heater

    This is why connecting heat pumps to your exhaust air is great!

    Yes, that other formula is also used here, the best is of course to measure both; I think my organisation may be working towards that. However, I really don't think, with a unit in a good or even reasonably condition, you can get so much air sucked in from your room that it significantly affects the result. Sorry, I'm not buying that argument :)
      Flödesbild_Poseidon_Heatpump_0_grader.png
  1.  
    EAHPs work by cooling the outgoing air so it is colder and drier than the incoming air, so more sensible and latent heat is recovered from the outgoing air than is required to heat the incoming air. Particularly in milder weather when the incoming air doesn't need much heating, so MHRVs cannot cool the outgoing air very cold. The surplus can be used to heat the house or the DHW. They haven't really caught on in UK because many houses turn out to need more heating than that in practice.

    Edit: the heat that is recovered from the outgoing air is boosted by the electrical energy of the compressor, but that shouldn't be 2.5x the heat recovered, more like 0.5x to 0.7x, which would give e.g. CoP = (1+0.5)/0.5= 3

    And those formulae in the link don't account for latent heat, or motor heat, and they assume that the incoming and outgoing airflows are exactly balanced, unlikely unless the house is hermetically airtight. Any air leaking in/out of the house will mess up both those formulae which maybe is what the author meant to say. The stuff about room air leaking into the MHRV is perhaps a misunderstanding, half the MHRV is pressurised above room pressure.
    • CommentAuthorJeff B
    • CommentTimeJul 19th 2021
     
    The CTC EcoVent i360F brochure states that "with the compressor located outside the building, you get a considerably quieter indoor climate". Yet the diagram shows the CTC unit inside the house - is that incorrect then?
    • CommentAuthorRobL
    • CommentTimeJul 19th 2021
     
    Well, the numbers make sense:
    799l/s on the diagram has a mass flow of 0.8kg/s, and with a 19degC temp change gives 15kW of power.
    Add this to the 9kW of electrical, and you roughly get the 23kW into dhw. There’s some subtlety with sensible/latent that I've ignored.

    But the numbers are huge, 800l/s would be 10ACH for our 4bed house, a gale! We have a vent axia sentinel kinetic plus mvhr, it uses 150W to give 1ACH, 80l/s. On average it takes 20W as its on auto (Co2 and humidity control).

    That 15kW of power is pulled from the air. But doesn’t that same 15kW need to be provided to warm up all the infiltrating incoming air somehow? How does that work?

    How would the control work? You might not want that ventilation when you need dhw, or vica-versa.
    •  
      CommentAuthordjh
    • CommentTimeJul 20th 2021 edited
     
    Posted By: minisaurusThe attached image demonstrates my point better.

    Sorry I'm not going to work through a diagram in Swedish to try to understand whatever point you're trying to make, especially since I know it is a specious point.

    Heat pumps work. They are a good idea. They (ASHP) work by taking an arbitrarily large volume of air at some temperature and expelling it at some lower temperature in order to produce a measured increase in temperature of a measured quantity of some fluid (either air or water).

    MVHR also works. They are a good idea. They work by taking a measured volume of air from habitable space and cooling it to approximately ambient external temperature, whilst heating the same volume of air from ambient to approximately internal temperature. The good ones do this very efficiently at very low power.

    Trying to combine the two is a step too far IMHO.
    • CommentAuthorMike1
    • CommentTimeJul 20th 2021 edited
     
    Posted By: minisaurusI don't know how to do that blue quoting thing you do
    Below the box where you type your message, select 'format comments as html' instead of as text.

    Posted By: minisaurusI can see how you'd be able to get over 90 % in a passive house. Although probably not in the buildings I work with
    You'll get exactly the same efficiency in any building maintained at the same temperature, whether a passive house or not - the MVHR unit is still exchanging the same volume of air with the same inside-outside temperature difference. You'll just use more energy heating the building. And MVHR 'performance outperforms natural ventilation by between 20 and 30% in terms of carbon emissions for all levels of airtightness'. See https://www.passivhaustrust.org.uk/guidance_detail.php?gId=46

    Posted By: djhCounter flow [heat exchanger] is what is used in PHI -certified products and is a key factor in achieving the high efficencies.
    For domestic systems, that's right. I'm pretty sure that some of the high-volume commercial units claim to use cross-flow, but that could be a mistranslation. I've not come across any PHI approved units that use other technologies.

    BTW, if you have rotating heat exchangers, minisaurus, they are a potential source of cross-contamination. Particularly problematic with Covid around.

    Posted By: RobLThat 15kW of power is pulled from the air. But doesn’t that same 15kW need to be provided to warm up all the infiltrating incoming air somehow?
    If you have to increase the airflow rate beyond the rate required to ventilate the building, in order to harvest enough energy to heat the water, then yes, you'll be heating air so that it can in turn heat the water. Perhaps not an issue in summer when DHW demand may be low and air temperatures high, but likely during the heating season - would have to do that maths.

    Posted By: djhTrying to combine the two is a step too far IMHO.
    Agreed - I've not yet seen a convincing argument for combining them...

    Posted By: minisaurusIn my work, we have around 800 properties - schools, elderly care and other care homes
    It's good that you're engaging with this forum - maybe you'll be able to use your position to make a real difference at work with some of these issues.
    • CommentAuthorSimonD
    • CommentTimeJul 20th 2021
     
    Posted By: Mike1 And MVHR 'performance outperforms natural ventilation by between 20 and 30% in terms of carbon emissions for all levels of airtightness'. See https://www.passivhaustrust.org.uk/guidance_detail.php?gId=46


    That's a highly questionable conclusion. The paper by the Passivhaus Trust relies on one paper from 2000 that comes to this conclusion. In addition, it's early proposition about the failings of natural ventilation in the paper are almost entirely incorrect. There's been plenty of research and real world studies since then that demonstrate different results. At the end of the day it depends on both the design of the building together with the design of the ventilation system. In my view this report represents a really poor and lazy standard of research by an organisation that really should know better!

    Posted By: djh

    Trying to combine the two is a step too far IMHO.


    Posted By: Mike1Agreed - I've not yet seen a convincing argument for combining them...


    Well, in John Cantor's book, Heat Pumps for the Home, he makes specific mention of heat pumps used to supplement heat recovery ventilation. This is suggested to be ideal in mopping up waste heat missed by the passive heat exchanger (especially in passivehaus) and these systems do provide useful net heat input of a COP around 3.

    In commercial settings, I'd imagine the system being an ideal solution to utilise waste heat, from both ventilation and water drainage, for instance.
    •  
      CommentAuthordjh
    • CommentTimeJul 20th 2021 edited
     
    Posted By: SimonDThat's a highly questionable conclusion. The paper by the Passivhaus Trust relies on one paper from 2000 that comes to this conclusion.

    I think you must have misread Palmer's report. He explains just above that quote how HE calculated those results last year, not in 2000. The 2000 work is one of those criticised for reaching incorrect conclusions and he has now updated to use modern carbon factors as well.
    • CommentAuthorSimonD
    • CommentTimeJul 20th 2021
     
    Posted By: djh
    Posted By: SimonDThat's a highly questionable conclusion. The paper by the Passivhaus Trust relies on one paper from 2000 that comes to this conclusion.

    I think you must have misread Palmer's report. He explains just above that quote how HE calculated those results last year, not in 2000. The 2000 work is one of those criticised for reaching incorrect conclusions and he has now updated to use modern carbon factors as well.


    No, I don't believe I have. Whilst it is acknowledged in the report that things have changed, in particular grid carbon intensity and MVHR efficiency, there is no wider referenced support for his conclusions. In addition, the conclusions appear to be based entirely on desktop analysis, not of real world performance data. Nor does it seem to expand upon what a natural ventilation strategy consists of.

    For example:

    'An MVHR system will ensure a reliable air change rate throughout the year, whereas this cannot be guaranteed by a natural ventilation strategy.'

    There is plenty of research to demonstrate that natural ventilation can and does provide reliable air change. And over the long term can present a more reliable and less costly, both in terms of capital outlay and running costs, ventilation option.

    'The results, shown in Figure 4, indicate that this change alone results in MVHR ventilation out-performing natural ventilation by a significantly larger margin than previously, at any infiltration level.'

    References to wider research to support this theoretical calculation?? None..

    Anyone who has read wider research on MVHR will be aware of a fairly wide array of issues experienced in real world testing of MVHR systems, all the way from poor indoor air quality, to unanticipated inefficiencies. E.g. that despite laboratory and manufacturer testing, the systems often fail to live up to the promises, including and in particular, efficiency figures. It's also fairly common knowledge that occupant behaviour and environmental preferences play a significant if not overriding role in the efficiency of the ventilation system.

    Unfortunately, the report also fails to consider the contexts within which MVHR is not an ideal option for occupants.

    'Natural ventilation' is also such a terribly poor term as it can mean anything from someone opening a window to drilling a large hole through a wall, to a properly designed ventilation system that works with the building within its context, utilising natural pressure differences across the house and which gives the occupant full local control of the atmosphere. So really, in the context of the report, it's reference to natural ventilation is totally meaningless! Hence why I suggest it's just poor and lazy :wink:
  2.  
    Posted By: RobLvent axia sentinel kinetic plus mvhr, it uses 150W to give... 80l/s.


    Let's imagine Rob's MHRV is 90% efficient.
    When the outside air is 0degC and the inside 20degC, it recovers 80l/s * 1.2g/l* 1J/gK*20deg * 90% = 1700W,

    so CoP = (1700+150)/150 = 12 if we define it the same as for a heatpump.

    That's a good CoP, better than any heatpump.

    When the outside air is 15degC, the MHRV recovers 400W, so its CoP = (400+150)/150 = 3.7

    At those conditions, an ASHP could do a little bit better than that, especially an A-A heatpump, but youd still need ventilation.

    So it depends on how much of the year is at 15 degC and how much is at 0degC. Maybe you should run the MHRV in cold weather and the ASHP in milder weather. In warm weather, turn them both off and open the window.

    Further complications are that much more ventilation airflow is needed when the air is mild and damp, than when it's icy and crisp and the MHRV's CoP is best.

    The MHRV's (CoP-1) is inverse of its fan power consumption, so that number is just as important as its efficiency.
    • CommentAuthorMike1
    • CommentTimeJul 20th 2021 edited
     
    Posted By: SimonDWell, in John Cantor's book, Heat Pumps for the Home, he makes specific mention of heat pumps used to supplement heat recovery ventilation. This is suggested to be ideal in mopping up waste heat missed by the passive heat exchanger (especially in passivehaus) and these systems do provide useful net heat input of a COP around 3.
    Exhaust air heat pumps do exist, but I've not yet found one that's compact and affordable. See also http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=11668

    Posted By: SimonDIn commercial settings, I'd imagine the system being an ideal solution to utilise waste heat, from both ventilation and water drainage, for instance.
    I'd imagine so too; It's often cooling that's the bigger problem in commercial settings.

    Posted By: SimonDThere is plenty of research to demonstrate that natural ventilation can and does provide reliable air change
    It can but, unless there are mechanical controls, over or under-ventilation is likely, which MVHR overcomes - hence the 'reliable' bit of that sentence. But yes, it does require that the system is correctly designed and installed, and that the occupants understand what it does, why it does it, and how to use & maintain it. Of course occupant behaviour is a key reason why 'natural' ventilation systems can have problems - blocking vents leading to poor indoor air quality, over-ventilation leading to wasted energy (a significant problem in Germany, where they apparently like to open the windows for an extended period every day), and the like.
    •  
      CommentAuthordjh
    • CommentTimeJul 20th 2021
     
    Posted By: SimonD
    Posted By: djh
    Posted By: SimonDThat's a highly questionable conclusion. The paper by the Passivhaus Trust relies on one paper from 2000 that comes to this conclusion.

    I think you must have misread Palmer's report. He explains just above that quote how HE calculated those results last year, not in 2000. The 2000 work is one of those criticised for reaching incorrect conclusions and he has now updated to use modern carbon factors as well.

    No, I don't believe I have.

    I don't see anything in your reply that even begins to address the specific point I made criticising a specific sentence in your post.

    Whilst it is acknowledged in the report that things have changed, in particular grid carbon intensity and MVHR efficiency, there is no wider referenced support for his conclusions. In addition, the conclusions appear to be based entirely on desktop analysis, not of real world performance data.

    I think it's quite clear that that is all the author tried to do. Incidentally, you missed also his critique of the previous papers' methodologies. I agree it would have been stronger with some independent experimental confirmation.

    Nor does it seem to expand upon what a natural ventilation strategy consists of.

    For example:

    'An MVHR system will ensure a reliable air change rate throughout the year, whereas this cannot be guaranteed by a natural ventilation strategy.'

    There is plenty of research to demonstrate that natural ventilation can and does provide reliable air change. And over the long term can present a more reliable and less costly, both in terms of capital outlay and running costs, ventilation option.

    I agree that 'natural ventilation' warrants better definition. I'm not aware of anything or anybody that claims that all natural ventilation methods provide reliable air changes, so I think your claim is unduly broad. Indeed I would expect that the most common natural ventilation methods - viz permanent or intermittent openings to the outside - could be said to never provide reliable air change, as long as reliability includes a requirement not to over-ventilate. Even building regs make it illegal to build a new house that relies entirely on natural ventilation, no matter how leaky it is.

    I think you're considering the report to be something more than it is. And a report that included a lot of the information you're looking for would undoubtedly be a better report, but that doesn't make the present version completely useless IMHO.
    • CommentAuthorSimonD
    • CommentTimeJul 21st 2021 edited
     
    Posted By: djh Incidentally, you missed also his critique of the previous papers' methodologies.


    No, I didn't miss that. I just didn't comment on it. I think I probably illustrated quite clearly that I think the report is methodologically flawed which call into question its conclusions.

    Posted By: djhI think you're considering the report to be something more than it is. And a report that included a lot of the information you're looking for would undoubtedly be a better report, but that doesn't make the present version completely useless IMHO.


    I don't think I am. The report is essentially a statement being made by a trusted organisation upon which people are likely to make significant and expensive decisions.

    I didn't say it was completely useless. I said it was lazy and poor and that the conclusions are highly questionable.

    Frankly, I think it comes across as MVHR propaganda.
  3.  
    A flaw with that paper and with many MHRVs seems to be an expectation to run at the same fixed flowrate all year round, irrespective of how humid the incoming air is, or how the house is being occupied.

    You need less ventilation in cold weather when the outside air is dry. Two people living in a big house need fewer ACH than their big visiting family would.

    The humidity/CO2 sensing controls are typically an expensive optional extra. Or, they switch between a choice of 2 or 3 flowrates and every room's ventilation goes onto boost anytime one of the bathrooms is used.

    If 'natural ventilation' involves windows and slots being opened, it will be poorly controlled, but at least the people naturally open windows less in cold weather and unoccupied rooms.

    Seems like over ventilation is inevitable.

    Maybe overventilation matters less if efficiency is high, but it's still wasted fan power. RobL mentioned his controller runs his MHRV at a fraction of its capacity on average and so saves 130W, which is over 1000kWh each year that others are wasting without such controls.

    It should be possible to design MHRV that ventilates each room by the correct amount depending on how it is being used and on the outside air humidity, but examples are proving hard to find.
    • CommentAuthorminisaurus
    • CommentTimeJul 21st 2021
     
    Posted By: WillInAberdeen
    It should be possible to design MHRV that ventilates each room by the correct amount depending on how it is being used and on the outside air humidity, but examples are proving hard to find.


    Lindivent have great systems for this - we have them in quite a few schools, but I don't think they do residential (?) I imagine you could put one of these https://www.lindinvent.com/products/sensors/htdt2500/ in your incoming air duct before your MVHR
    • CommentAuthorRobL
    • CommentTimeJul 21st 2021
     
    Our mvhr came with humidity control built in, I think that's pretty standard now. I added an analogue output 0-10V CO2 sensor, which needed a longer 3 wire cable adding to it, but is completely compatible with the vent axia sentinel kinetic (0v, signal, +24V power from the mvhr):

    https://www.digikey.co.uk/product-detail/en/amphenol-telaire/T8031/235-1413-ND/5774483

    The mvhr controls are expecting CO2 on a 0-10v line, and you can program in what level of CO2 you'd like to give 50% power or somesuch from memory.

    They talk about putting it in a duct(probably a good idea), but I put it in a room as it was easier. Before adding the CO2 measurement we found it hard to stop the mvhr stripping all the moisture out of the air in winter. Now the continuous flowrate is dropped to to tiny levels, and we rely on CO2 and RH to push it back up. 50% is the max it can go to without manual intervention - say for a cooking disaster.
    • CommentAuthorbhommels
    • CommentTimeJul 21st 2021
     
    Posted By: RobLOur mvhr came with humidity control built in, I think that's pretty standard now. I added an analogue output 0-10V CO2 sensor, which needed a longer 3 wire cable adding to it, but is completely compatible with the vent axia sentinel kinetic (0v, signal, +24V power from the mvhr):

    The mvhr controls are expecting CO2 on a 0-10v line, and you can program in what level of CO2 you'd like to give 50% power or somesuch from memory.

    This is a LOT cheaper than the manufacturer supplied CO2 sensor for my MVHR!

    The controller for my MVHR has a similar option for external sensors. I am working on driving this input port from an already existing monitoring system which combines humidity and CO2 data from distributed sensors. The only item missing is an outdoor temperature & humidity sensor to avoid boosting the MVHR when it can't bring humidity down due to outdoors conditions.
    • CommentAuthorMike1
    • CommentTimeJul 21st 2021
     
    • CommentAuthorbhommels
    • CommentTimeJul 21st 2021
     
    @Mike not quite the same as a proper sensor....
    For monitoring I got the Sensirion SCD-30. Probably the cheapest proper CO2 sensor. It does not have a 0-10V interface though, only I2C, UART and PWM. Nice for Arduinos and Raspberry Pis, but not for direct connection to an MVHR controller.
    • CommentAuthorminisaurus
    • CommentTimeJul 22nd 2021
     
    Okay, I'm going to be brave now, and show how we calculated the savings ... see attached :)

    I used variables 60 l/s constant exhaust air flow, 18 degree desired input air temp, 8 degrees average outdoor temp.

    MVHR - recovers 5860 kWh/year
    Exhaust air - "gains" 13633 kWh/year

    I haven't removed the effects of the input air fan for MVHR, or compressor for exhaust air as yet ...

    I don't seem able to attach 2 files with one post - I can attach the actual spreadsheet if anyone wants to play with it.
      Screenshot comparison mvhr exhaust air.png
    • CommentAuthorRobL
    • CommentTimeJul 22nd 2021
     
    You can't "gain" more heat from a heatpump than there is available "in" the air. If the heatpump drops the air temperature by the same amount as the MVHR (ie. DT = internal temp - external temp), then it's the same heat - COP doesn't come into it (ignoring latent heat). Heatpumps are great, but they're not magic. :sad:
    You could drop the temp to lower than external, but then what's the point, may as well use an A2W heatpump - it will probably be more efficient.

    So 25A should say P=qxpxcPxDTxeff. 25B will give about the same answer as 18B.
   
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