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.

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.

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.

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.

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: 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?

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.

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