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    We have a MVHR system which I have been concerned about for some time. Eventually after much discussion and a visit to the manufacturers on an exhibition stand they sent engineers round to check the system having never heard of such problems before,
    Their engineers could'nt find any fault and measured the temperatures at the MVHR unit as
    18c stale air out
    12.8c Air in to rooms
    8.6c Outside air temp
    Which they calculated the efficiency of the system at about 45%.

    The stated efficiency of this unit is 70% and when I challenged the manufacturer the reply was that even at 45% this was better than if you had trickle vents when the air would be coming in at 8.6c!
    Ive had little help from them and before I go back again I just want to be sure that I have a case.
    They did say that the inlet fan was running slightly quicker than the outlet and they would send a manual controller that I could fit - i have declined this offer

    Any comments would be appreciated
    • CommentAuthorDarylP
    • CommentTimeApr 28th 2015
    ... have you/they checked the filters, any clogging of the HX?
    45% does seem a bit low, but then that may be due to many factors?

    Which mfr/model is it?

    Good luck :wink:
    • CommentAuthorCWatters
    • CommentTimeApr 29th 2015
    Check the temperatures actually at the heat exchanger.
    Thanks for your answers
    The filters were cleaned before the engineers tested the system and they inspected the unit and only found that one fan was operating slightly faster than the other.
    The temperatures were effectively tested at the heat exchanger as they were taken at the spigots off the unit itself.The manufacturers accept that it is at only 45% but don't want to do anything about it.
    I feel they have broken the terms off the contract by not achieving 70% as claimed and am looking for some way of backing this up rather than just my opinion
    • CommentAuthorwoodgnome
    • CommentTimeApr 29th 2015
    Supplier and make of MVHR?
    Ask for a refund, Trading standards, demand withdrawal of product, write shed load of reviews - this isn't like the mpg for a car, even a 10% error would be cause for concern (for me). There is no-one who would buy a MVHR unit with 45% HR.
    • CommentAuthorCerisy
    • CommentTimeApr 29th 2015
    Slightly scary as we move slowly towards setting up our MVHR!
    • CommentAuthorEd Davies
    • CommentTimeApr 29th 2015
    I'd agree: money back or trading standards, their choice.

    They've inspected it and said it's OK so further speculation is a bit of a waste of time. Still, there's no possibility that some sort of summer bypass is open or partially open, is there?
    Is the unit located inside the house?

    I got well below the stated efficiency for my MVHR unit until I relocated it from the attic to inside the thermal envelope and physically blocked the leaky summer bypass valves. It now reaches the manufacturer's claimed figures. They sell the same model without a summer bypass and I suspect it's that one which their claimed figures were achieved on. Incidentally I wouldn't bother with a summer bypass or humidistat second time around.
    • CommentAuthorborpin
    • CommentTimeMay 3rd 2015
    I generally get about a 1 deg difference in inside/supply temps. Helios EC500 Pro.

    My problem is that I cannot find anyone to service the thing. One of the fans has a slight whine which I suspect is some dust that needs blowing out.

    Supplier, Allergy Plus have ceased trading - what a surprise. http://companycheck.co.uk/company/04203741 Watch out for Nev Boon - I find these folk keep turning up under different guises (similar issues with the roof supplier).
    • CommentAuthorRex
    • CommentTimeMay 3rd 2015
    Interesting to hear the Allergy Plus have gone under. Not surprised. I bought my Helios MVHR from them; they designed the system and although it works (don't know about the heat exchanging difference?) they were a complete PIA during the design stage.

    They planned the ducting to go through two steel beams which were clearly indicated on the drawings and no amount of photos or videos showing them the beams changed their design. Finally, Nev Boon came to visit and saw the problem!

    I have never measured the system efficiency but is it as simple as taking a temp reading from each side of the heat exchanger?
    • CommentAuthorEd Davies
    • CommentTimeMay 3rd 2015 edited
    Doing efficiency seriously should take the flow rates and humidities of the flows into account but to get a good approximation, if the flows are reasonably well balanced, just measuring the temperatures of the outdoor air, clean air being supplied to the rooms and dirty air being extracted from the rooms should be enough. You don't need the temperature of the exhaust air being chucked outside.

    • CommentTimeMay 3rd 2015
    Having just read this again and reading in the past about others making their own heat exchangers it occurred to me that the claimed best efficiency heat exchanger is the wire one found in the Viking hous breathing window. If this is the case why is no-one using them in available Whole house units?
    Patent issues...........maybe???
    • CommentTimeMay 3rd 2015 edited
    • CommentAuthorEd Davies
    • CommentTimeMay 3rd 2015
    100 m² house, 2.4 m room height, 0.4 AC/h, 12 °C inside to outside temperature difference [¹], 1.28 kg/m³, 1 kJ/(kg·K): each percentage point of efficiency is worth 4.1 W (or 0.36 kWh/(m²·y)). That's £4.67/year at 13p/kWh for direct electric heating, less for almost any other sort of heating.

    While 45% efficiency is seriously disappointing and needs tracking down and dealing with, chasing down the last few percentage points of efficiency in the exchanger is probably not the best use of time and money.

    [¹] Year round average for Wick, Highland - 4374 HDD to 20 °C.
    • CommentAuthorEd Davies
    • CommentTimeFeb 21st 2017
    Johnsanctuary just whispered a question to me. Here's the generic part of my answer which doesn't need to be whispered and might be of interest to others:

    Both the efficiency equation on my web page and the calculation of costs are ones I just made up from simple physics. Still the efficiency equation is the same as the first one given here (“Temperature Transfer Efficiency”), just using different symbols:


    As to the energy use, it's just volumetric rate of air change × density × specific heat capacity × temperature difference × time. Density and SHC can be got from all sorts of places on the web, e.g.:


    There are some important caveats to using just the temperatures to measure efficiency. In principle moisture in the air should be taken into account as well but if there's unaccounted-for condensation in the outgoing airflow that would increase the apparent efficiency so it's unlikely to be the problem here.

    Perhaps more relevantly, if the airflows are unbalanced so that the outgoing flow through the exchanger is smaller than the incoming (pressurising the house causing additional air to be blown out through leaks) then that will depress the apparent temperature efficiency: there just wouldn't be enough energy in the outgoing air through the exchanger to warm the incoming air as the energy in the air blown out of the leaks is not available.
    • CommentAuthorJohn Walsh
    • CommentTimeMar 11th 2017
    Another example of MVHR heat recovery efficiency (a Sentinel Kinetic B). Just looking at temperature, I'm using the SAP method to calculate efficiency: (supply-intake)/(extract-intake). The sensors are Sheepwalk's 'DS18B20 in a stainless steel tube' pushed into holes drilled in the ducting. The system takes pre-warmed air from a solar gain space (yes, I realise that isn't universally accepted as a useful thing to do). Ducting is to manifolds and then semi-rigid pipe (i.e. it’s easy to balance). I wasn't sure what to do when the unit's summer bypass operates as the calculated efficiency goes well over 100% (this is most days at the moment, but probably not today - location is N53 deg.). For now, I plumped for recording the efficiency as 100% if intake T is greater than extract. Perhaps it would be better to log it 'as is' and eventually do calcs for power gained (eventually, a RPi will control fan speed and so I can then log flow rate too).

    The fluctuations at midday yesterday are because intake temp was close to triggering summer bypass. I'm assuming the efficiency gets messy when all the temps are very close - at the moment it's 18.9 intake, 19.8 extract, 19.8 supply and 19.3 exhaust (all being logged).

    As the graph shows, the unit mostly runs at low 80s efficiency. However, at this time of year (unless we have a cold period) the intake is always in double figures (range is 24.8 to 14.1 over the last couple of days). I'm interested to see how the unit copes with large differences (e.g intake down to 5), but unfortunately that's unlikely to happen until next winter.

    Any suggestions for improvements welcome and is the efficiency as expected?
    • CommentAuthorEd Davies
    • CommentTimeMar 11th 2017
    John Walsh, thanks for your useful post. I assume the sensors are a recent installation and you don't have data for last winter. As you say, it'll be interesting to see what happens next winter.

    It's also nice that you confirm that the SAP method of calculating the temperature efficiency is the same as the one I was using and the one that engineeringtoolbox reference gives.

    As to efficiency calculations when the temperatures are close, my thought would be to simply discount those cases completely.

    My instinct with this sort of thing is to keep all the raw data (i.e., the temperatures) then process them later so it's easy to tweak your calculation method as experience is gained without creating data which is not comparable. I've been logging various temperatures, humidities, pressures, light levels, power use, etc, on a Raspberry since the beginning of November. As simple JSON text files it's 172 MB of data. At this rate it'll take about 16 years to fill the SD card.
    • CommentAuthorJohn Walsh
    • CommentTimeMar 11th 2017 edited
    Yes, it's a recent installation (Thursday afternoon). As you say, good to store the data for later use. Agreed re discounting data where T's are close (currently, I 18.8, S 20.1, Ext 20.2).

    Re using the SAP method, it seems more suited to what the effect of the MVHR is for the house, whereas the PH method is oriented to the unit's performance. As you might expect, it's also complicated and could be expressed as ...

    ((extract-exhaust)+(power/mass flow x SHC)/extract-intake) + 0.08 x humidity recovery

    Having said that, it does seem more rigorous (whereas SAP is perhaps more helpful for unit makers) and with no vent-axia units in the PH component database it perhaps shouldn't be surprising that the advertised 'up to 95%' claim for the Sentinel would seem to have to be taken literally, i.e. 95% might be a sweet spot it occasionally reaches.

    At some point, I hope to do some combining of T data and the efficiency calc (in graphs).
    • CommentAuthorEd Davies
    • CommentTimeMar 11th 2017
    I'm sure the PH method makes sense in context but just looking at that it's not obvious, is it?

    With the simple formula, the one just using the temperatures, it's worth emphasizing that it only really works if the flows are balanced.

    Also, to be accurate one should take the humidities of the air flows into account. If just done a calculation for an extreme case (100% RH indoors, 0% RH outdoors) and got an even more extreme answer than I was expecting so I'm going to sleep on it. Partly trying to see where that 0.08 in the PH formula comes from.

    Have you calibrated your one-wire probes against each other? I've got three on my desk which I've been logging every 30 seconds for a few months. Must get round to doing scatter plots of them against each other. They seem to be pretty consistent and I've been thinking of them as a bit of a reference temperature to calibrate other sensors against but read the other day that somebody had found them to vary significantly. Maybe it's a batch-to-batch thing.
    • CommentAuthorJohn Walsh
    • CommentTimeMar 12th 2017 edited
    Point taken about balanced flows - I've only recently got the system running and, for the time being, haven't done any more than turn it up to No. 11 and feel with my hand the draft/ suction on all the valves. Obviously, I need to do more and will - however, the house form is mostly a 15m square with the unit in the centre and with supply and extract via semi-flex manifolds (i.e. it's been designed to be 'balanced' from the off).

    With humidity, this is attenuated somewhat by taking intake from a solar gain space - glazed roof, masonry walls on three sides, 300mm concrete floor. While very much acknowledging that pre-heating is a little contentious, this conversation has brought the potential humidity benefit to light.

    I haven't methodically calibrated the T sensors, other than when setting things up and reading the network ID on the bench, I've done that in batches of 3 and always seen almost identical readings. But it's a good point and something to work on. I have 28 in use in 4 networks - one 11 sensor network has been running continuously since March 2013. Some of the sensors are inside, some in outbuildings (housed in camdenboss potting boxes where I haven't yet bothered to screw the lid on) and, so far, it's never missed a beat. As such, I'd thought they were pretty reliable things. Also, I'm wary that the low cost of these things (including the RPi) guarantees a bad press from 'professionals in the field'. As you say, though, it would be prudent to have some kind of calibrating and checking procedure.

    Details about the PH method can be had from www.passivehouse.com - go to the 'component database', select any MVHR unit and there's a ‘download certificate’ link (the certificates detail the formulas used).

    Edit- just seen that not all the PH certificates include '+ 0.08 x humidity recovery' in the 'efficiency criterion' formula. This one does ...

    • CommentAuthorEd Davies
    • CommentTimeMar 12th 2017
    Yep, there can always be a bit of snobbishness around this sort of thing. The place I read about the scatter of DS18 sensors was this blog: https://edwardmallon.wordpress.com/ A bit more extreme than most people's houses but interesting - it's on my list of blogs to read from end to end. I'm not sure of his background but would call him a very serious amateur for now. I couldn't immediately find the bit I read originally but https://edwardmallon.wordpress.com/2016/03/05/ds18b20-calibration-we-finally-nailed-it/ says:

    “I can also add that the sensors from different suppliers showed strong clustering in these groups, implying that production run biases would introduce an offset into group normalization methods (ie: without a reference thermometer to compare them to) unless you bought them from several sources.”

    Obviously he's looking to get absolute temperatures (so he can see long-term trends in cave water temperature as instruments are replaced, I suppose) whereas for our sort of thing relative temperatures are what matter more. Still, it's a bit of a flag that just because a couple of sensors from one batch match reasonably well you shouldn't assume too much when you swap in a replacement from another batch.
    • CommentAuthorborpin
    • CommentTimeMar 12th 2017
    Posted By: John WalshI'm using the SAP method to calculate efficiency: (supply-intake)/(extract-intake)
    I'm going to have a go at this. I'm able to collect the data from the Helios sensors although the outside temp on that is recorded at 13 right now and my weather station (not a million miles apart, shows 11.

    On Bypass temperature, I run mine much lower in winter (so never bypasses) and much higher in summer (so bypass kicks in earlier). I cannot quite get my head around the Helios logic, but it does what I want it to do so even on a warmish winter day I never bypass and even on a relatively cool summer day it does bypass, as this then helps to cool the house. YMMV.

    The other thing for me would be that when the heater kicks in, that will distort the figures.
    • CommentAuthortony
    • CommentTimeMar 12th 2017
    Re improvements, I would expect to see all the ducts insulated with the possible exception of the return from the rooms
    • CommentAuthorJohn Walsh
    • CommentTimeMar 12th 2017 edited
    Borpin: re the 2 deg difference between your weather station and the unit's sensor (presuming you do mean that you can read the unit's sensors? If yes, 'envy emoticon' - vent axia don't allow that kind of thing), I often see the same kind of difference between the solar gain T and the sensor I've installed close to the unit. On another thread, djh talks about heat from his post-heater dissipating before it reaches outlets, that you do get a lot of T leakage from ducts (see http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14971&page=1#Item_0 ). Anyway, be good to hear how your unit's efficiency compares with the manufacturer’s claims.

    On heat loss from ducts, Tony, the unit is within the heated envelope and so I can't see a reason to insulate the extract (as you say) or the supply. Of course, the exhaust will need insulating given it'll (hopefully - HR efficiency permitting) be down to around 10 or lower in mid-winter (the ducting is already insulated where it passes through the loft space). The intake I'm less sure about - if it picks up some heat from the house along the way to the unit then is that a problem?

    There are 2 intake routes. One from a solar gain space which will be used between around early October to say early June. Given logging from sensors in the space over the past 4 years, the temp will not go below around 5 - presuming that the air extraction rate won't be high enough to reduce the temp. The second route is from an east-facing open porch, which will be used to bring in cooler air in the summer. With this second intake route, insulation doesn't seem necessary - if there's temperature gain over the duct run then that's fine, that's what we want to happen. Hope that makes sense.
    • CommentTimeMar 12th 2017
    The exhaust and intake are insulated mainly to prevent condensation on the outside of them in the winter but also to reduce heat transfer both in winter and summer.

    Don't forget that it's best to have the intake and the exhaust terminals all facing the same way so they see roughly equal wind pressure. Otherwise it either makes your MVHR work hard or unbalances it, depending on how it is designed.
    • CommentAuthorEd Davies
    • CommentTimeMar 12th 2017
    Posted By: John WalshThe intake I'm less sure about - if it picks up some heat from the house along the way to the unit then is that a problem?
    Yes, it does matter because, if your heat exchanger is 80% efficient, then in effect 80% of the heat from the house which gets into the intake will be chucked out of the exhaust. In other words, the exhaust air will be slightly warmer than it otherwise needs to be and so will carry more energy out of the house.

    Consider the extreme case where the intake duct warms the intake air up to indoor temperatures before it reaches the exchanger. Then no heat would be exchanged and you'd have 100% ventilation heat losses (± humidity effects).
    • CommentAuthorJohn Walsh
    • CommentTimeMar 13th 2017
    All agreed on insulating the exhaust.

    I'm less sure how useful it is to be concerned with the intake in what is a hybrid/ experimental system. Equally, the idea of which way the terminals face is secondary to regard for the performance of the system in terms of providing heat in winter / coolth in summer - e.g. the exhaust is via a gable wall and the intakes are in an open porch (for cool air in summer) and a solar gain space (for pre-warmed air in the winter). So far, the intake shows around an average of 2 degs uplift - but calibration of the sensors needs sorting out. If this remains the average long term then insulating the winter intake becomes a condensation issue - again, we'll monitor that.

    Granted, for regular installations, there's no need for a discussion - just insulate intake and exhaust and make sure your terminals are positioned to help equalize pressure. For our experimental setup, what's perhaps more useful for consideration is the calculated efficiency of the mvhr unit (especially compared to expected efficiency), what's the logged effect for the house (in terms of warmth and in the summer coolth) and, perhaps the most important, the amount of heat we can bring in to the house via solar-warmed air.

    The latter has shown to be contentious on here perhaps due to an intuitive expectation that if we extract, say, 150m3 of air from a 20m3 space then we'll very quickly run out of solar warmed air. All of our pre-installation experiments show that isn't the case at all. Provisionally, I'd expect that we can extract 10ACH and extraction will have no effect on the air T. We'll collect the data and see - part of the reason for doing all this is to provide data which will help anyone else thinking of using pre-warmed air as part of their energy needs.
    • CommentAuthorEd Davies
    • CommentTimeMar 13th 2017
    Posted By: John WalshThe latter has shown to be contentious on here perhaps due to an intuitive expectation that if we extract, say, 150m3 of air from a 20m3 space then we'll very quickly run out of solar warmed air.
    Previous discussions of this sort which I remember have been about heating of loft spaces where it seems there's been a confusion between the temperature reached in the loft vs the rate at which heat is getting in.

    For a glazed solar heat collector it's just a matter of the area of the glazing and the amount of sunlight getting through it against the heat capacity of the air passed through. The internal volume is a secondary matter, at most.
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