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MVHR Reasonable Flow Rates - are the regs OTT

Are the building regs requirements for ventilation in an air tight build a bit bonkers or have I got a problem with my MVHR system? Finally got the dust under control enough to try commissioning the MVHR system in my new build. Plumber hired a calibarted and certified anemometer and off we went.... System is a Polypipe Silavent HRX2 with semi ridgid ducting in a radial layout (designed by Polypipe based on the house plans, or at least they said it met the demand).

Part F requires based on floor area at least 0.3 x 217 = 65 l/s flow, we divided this based on room volume between kitchen, bathroom and shower room, opened the vents and cranked up the fan to deliver sufficient flow at the "limiting terminus" the bathroom farthest from the unit. Ran around adjusting the vents to get the proportioned balanced flow rates (totaling 65l/s extract and 65l/s supply) for each room. That was fine but it is incredibly noisey at that level, the house was thrubbing! No way would we be happy living with this on continually, it was also drafty anywhere near a vent. Of course we have turned it down to a quietly comfortable level, but what gives, are we going to have problems - stuffiness, condensation etc. - if we don't live with the required minimum 65l/s flow?

If the reg values are sensible then is the fan noisy because it is under powered for length of ducting and volume of house it is serving? How do I find out and what can I do now?

I am hoping it is venitlation over kill and in reality a more gentle flow will work. We have been working here, air sealed and windows closed in the rain, and haven't noticed any lack of ventilation at all but haven't used the shower either.

Looking at volume the 65l/s flow is 234 m3/h or 0.43 ACH or 138cfm. I went seeking information from other authorities. The whole house requirements in the US are for 7.5cfm per person and 3cfm per 100 sqft. There are 2 of us hence 79.6cfm = 37.4l/s would be requred over there, a flow I can achieve without a headache. The house would need 7 people in it for the 65l/s to be necessary, yet people are complaining that these new US regs are too much!

Anyone care to share experiences with their MVHR flow settings.

Comments

  • I believe that on closer inspection, the settings that my MVHR system was commissioned to were safely written down but only lasted a couple of days for the same reason.

    I roughly halved my fan outputs about 2 years ago - no noticable downside yet although I wish I could work out how to make the humidity stat a bit more sensitive or have an overrun on it as the shower room doesn't seem to clear down as much as I think it should after a shower. It's a small room at the end of the run and you can hear the unit (vent axia sentinel kinetic) wind up speed but it winds down pretty quickly once you have finished generating steam, it would be good if it overran for say five minutes.

    There are some parameters to play with but are (IMHO) poorly documented :)

    -Steve
  • Thanks Steve, good to know at least one other has found the regs air flow too much and adjusted the flow down without adverse consequences.

    Issues of sensor reliability etc. made me decide to just have simple boost switches by bathroom and kitchen and manually control when to start and stop. OK so we have to remember to turn it off, but being a control freek I am happier doing that than have things start or stop when I don't want it. Home automation - pa! If this does become arduous then a timer switch would be an easy mod, used to turning on a fan to shower or cook.

    Been thinking about ways to measure if the ventilation is a healthy level in practice (rather than worry about a satutory level set based on what?). As has been discussed elsewhere we aren't going to suffocate! Comfort is a subjective thing, smells are easy to detect, but I do want to avoid condensation problems which may not be so obvious until the damage is done. Life in a draft free house is going to be a new experience. A room humidity meter perhaps?

    Still love to know how other people have their MVHR set up, flows, noise etc.
  • We don't get condensation on the windows so I would assume it doesn't occur in places that are hard to see - but this is an assumption an not based upon knowledge

    The only time we get condensation on the windows is in the shower room just after you have used it, and on the outside! of the windows during some slightly odd weather patterns

    -Steve
  • MVHR off, use windows + doors to ventilate. Have CO2 monitor, well within limits. Some condensation on windows, suffer from too low rather than too high humidity, wet room generally left alone so as to lose moisture into house.

    Lose a bit of heat, big hole under kitchen units to allow air in from outside via suspended floor void.

    It's OK.
  • I turned both ours down to minimum possible speed.
    After that I put one of them (in an office) onto a timer. The other has a manual override that lets you select one of three speeds or off.

    The one on a timer, just on for office hours, seems to work fine and is plenty adequate.
    We occasionally turn off the other one completely. If you leave it off overnight then you can tell it's not on (something about the air - not exactly stale but I think it is the beginning of stuffiness. CO2 only gets up to about 800 though).

    Down-sides for us of running them all the time are:-

    Noise - in one room in particular you can hear it if everything is very quiet.
    Very dry air - especially in winter.
    Filters need cleaning more often (and one of them requires me to go up into a loft to change it)
  • So another vote for turning MVHR flow rate down once the test is done, that's reassuring. I have found a few other posts that support that view too.

    Dickster I have tried the uncontrolled drafty floor void approach for many years, going to do the air tightness thing now and save energy, but will still be opening windows when the weather allows.

    Anyone know what the 0.3 x area Part F rule is derived from, is there any research to support it? Once you get to 100sqm or more it is going to dominate the required min flow, yet the air pollutants are down to how many people are and what they are doing there not the space itself.

    I'm thinking now that the best thing would be to get both CO2 and room humidity meters and actually measure the air quality (smells I can do subjectively), adjusting the ventilation as the house and occupants need. I'm pretty sure it is not a 65l/s gale blowing constantly.
  • Posted By: GreenfishI'm thinking now that the best thing would be to get both CO2 and room humidity meters and actually measure the air quality (smells I can do subjectively), adjusting the ventilation as the house and occupants need. I'm pretty sure it is not a 65l/s gale blowing constantly.
    Despite any regs that exist I am of the view that I want my MVHR to be controlled by stats so its only called to switch on if the air quality is compromised.
  • Anyone know what the 0.3 x area Part F rule is derived from, is there any research to support it?

    It derives approx, from a requirement of 10l/s of fresh air per person to remove oudours, moisture, contaminants etc and to provide a minimum amount of cooling.

    If you look back to work by Ole Fanger et al, and foward through to recomendations from ASHRAE and CIBSE on ventilation rates for habitable spaces, then you'll find that 10l/s and the conversion to xxxl/m2 floor area based on probable occupation density hold true for most cases.

    Of course, specifically you may not need that flow rate - but as BR's are simply trying to secure safe habitable conditions for occupants, then 0.3l/s/m2 is not an unreasonable figure.

    Clearly, in your case, there is a certain lack of performance inthe design flow rate and the selected equipment - achieveing those kind of flows whilst maintaining reasonable acoustic criteria needs well engineered fans in correctly selected ductwork - ie velocity is generally the main problem.

    Suffice to say that your system can achieve BR compliance - if you choose to operate at a point less than that, then that's up to you

    Regards

    Barney
  • Posted By: snyggapaI no noticable downside yet although I wish I could work out how to make the humidity stat a bit more sensitive or have an overrun on it as the shower room doesn't seem to clear down as much as I think it should after a shower.
    Further to this, the vent-axia manuals downloadable online seem better than the older printed versions I have - and better explain how the humidity parameter works. You set it at a level above which you want the system to kick in - so I reduced the humidity from the previous value of 65 to 60 and it made a huge difference (in fact, too much) - I have since upped it to 62 which seems to be a good balance between overrun after a shower and excessive running.
  • Posted By: barneyClearly, in your case, there is a certain lack of performance in the design flow rate and the selected equipment - achieveing those kind of flows whilst maintaining reasonable acoustic criteria needs well engineered fans in correctly selected ductwork - ie velocity is generally the main problem.
    I wish I had approached an independent designer in the first place, but didn't know where to look. The issue of air velocity never came up with the fan vendors, neither did the Part F small print.

    Interestingly my noise problem, if I keeps the 65l/s flow and I doubt that I will with only 2 of us, seems to be the supply vents rather than the ducting or fan. If I remove the ceilng "diffuser/valve" and have unrestricted flow into the room the sound is much less. You have mentioned controlling the flow along the duct rather than at the ceiling, that seems a great idea but again not something any of the vendors mentioned. Is there something I could add to the ducting on a radial/semi-rigid system?
  • Independent advisor??

    Is there such a thing? I'd love to talk to them as currently getting quotes and designs from the manufacturers as I can't find any independent advisors.

    If anyone knows of any please send their details my way.
  • Posted By: calvinmiddlecurrently getting quotes and designs from the manufacturers as I can't find any independent advisors.
    Hope that the manufacturers are more responsive than they were to me this time last year, some took months to come back with a "design" (not much more than a price and a sketch of the possible duct routes). See if you can get them to give terminal flow rates and duct air speeds and static pressures to help you evaluate if the system will deliver what you need.
    Independent advisor??
    ... If anyone knows of any please send their details my way.
    Me too. Like to know if I can fix my poor design without too much pain.
  • The terminal diffusers (and I think in your case also the throttling valves incorporated ) offer by far the lions share of resistance in any branch duct (even if it is 75mm dia).

    That's the point at which you'll experience the greatest static pressure loss and where you'll get the greatest impact on velocity pressure and heance shearing and hence noise.

    Selction of good terminal diffusers and use of manifold (or at least upstream) volume control devices (usually iris dampers) will go a long way to reducing velocity noise at the terminal (and thus into the room)

    If it's acceptable to you when the MVHR is turned down to below the 0.3l/s/m2 criteria then I'd do that. BR requires that a system should comply - you don't have to operate it in a compliant mode, do you.



    Probably not what you want to hear at this point, but the more I look at the domestic MVHR market, the more I see a gang of charlatans at work, preying on well meaning, but not technically savvy self builders and the like. They seem to perate on a basis of "just bash out a box that will meet BR's" and let the buyer unravel all the ductworkwork problems based on thier so called design service

    It shouldn't be this difficult - I've been involved in HVAC design for several decades, and once you know what data you are looking at (eg fan volumes provided at free air discharge conditions rather than as a measured curve over every case from FAD to stagnation and with octave band minimum noise data over that curve) then it's enirely possible to come up with very high air volume or high velocity but near silent systems.

    Regards

    Barney
  • edited May 2014
    hi barney,

    > Selction of good terminal diffusers and use of manifold (or at least upstream) volume control devices (usually iris dampers) will go a long way to reducing velocity noise at the terminal (and thus into the room)

    could you point out some good terminal diffusers, manifolds, vcds which you would recommend (ideally for use in a radial semi-rigid ducting setup)?

    thanks,

    slip
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