Commissioning continuous mechanical extract ventilation or cmev in domestics

[sparkyork]

Hi all, I've tested a bit of heat recovery before so mev is much simpler, just started testing a plot today.
Theres 3no outlets;kitchen, down stairs wc and bathroom first floor, all leading to the loft where the mev unit sits. Was looking for around 7l/s at trickle mode at all outlets which worked fine, on boost struggling to get to 13 l/s in the kitchen, only managing 10.5 at full bore on the unit! 8l/s for wc and bathroom are no problem.
Gonna have a read of the instructions to see if I need to apply correction factors for the cowl on the tester and the ceiling cowls aswell.

I'll check flexi connections to the mev unit for any crushing, would much prefer to use rigid flexi ducting for the mev connection. Gonna have another play with the ceiling valves and try and balance it all out, as in theory I'm getting the overall flow rates but just need more at the kitchen, I'm hoping It doesn't develop into a black art getting it all balanced haha

if anyone's got any tips or suggestions then I'm all for making life easier !

 

[Pete999]

What does the design drawing say you need at each outlet?

 

[sparkyork]

Kitchen, wc and bathroom are good with 7 l/s trickle, on boost I need 13 l/s for kitchen and 8 l /s for wc and bath.
will start again In the morning and throttle top floor outlet to try and get more in kitchen. Gonna be having to do this in every plot so wanna make it as efficient as possible

 

[marc8]

Kitchen, wc and bathroom are good with 7 l/s trickle, on boost I need 13 l/s for kitchen and 8 l /s for wc and bath.
will start again In the morning and throttle top floor outlet to try and get more in kitchen. Gonna be having to do this in every plot so wanna make it as efficient as possible

hi Sparkyork
could be you have to much system pressure this is causing your problem on the boost, you may have overcome this on normal running speed by upping the normal rate run, in other words the low speed is being run faster to get the 7.1ls in areas.
when it comes to the boost you can't achieve what you require.
I had this on a heat recovery unit myself, very lucky i managed to change configuration, Speaking with technical about my problem was to reduce system pressure, how the F...? after painting ect & all finished.
as they mentioned each bend (not a sharp bend) adds 3 meters of pipe, using flexi duct has ribs that cause system pressure,
not sure if you know or not but will try to explain for anyone who wishes to know.

Air travels down a tube along the edge, say you have an obstacle within the tube ( a rib from the flexi duct) the air hits the obstruction causing system pressure, the air as it hits the obstruction then rolls back on itself.
With any obstruction you need to increase the fan force to push the air past the obstruction, also same with a bend you need to push round the bend as air will not just float past it needs to be pushed.

i had used 6" insulated flexi duct on top of the heat recovery unit to outside & each manifold, but luckily I was in a loft so I managed to reduce system pressure by changing to the foam solid pipe & forcing it down the roof to the outside & ripping out the flexi duct. This reduced my system pressure.
I was also told by the Manufacturer any change from the drawing should have been calculated & agreed the length of run not changed without, I explained that customer asked for change as they did not want boxing in the study. but was lucky I managed to change ducting. hope this might help

 

[marc8]

Hi Sparkyork
did you install or others? if you installed what was your duct run I could look up if you know the meters used.
also have you got the right amount of ventilation in the building when testing, if not open the front door or windows whilst testing. this may increase your rate as you may be testing in a vacuum, have you checked undercut on the doors & trickle vents are open?

 

[sparkyork]

Thanks for your reply Marc,
I've installed a good number of mevs on this particular site. The particular plot in question has probably ten 5" lengths of ducting at 1.5m lengths and that particular run to the kitchen also has around 6 elbows.
I've not had chance to get back in today, but found at that I hadn't set the factor for the air box aswell.
I need to find out if it's specked for having trickle vents open etc. I also believe I should be initiating the test with mev on boost mode with furthest run valve open and others shut to make the "kitchen" the index outlet, then achieve the required results that way! Hopefully with the meter been used without the correction factor and possibly the wrong adapter for the ceiling, been using the cone but read that the 300mm square box is better.
all ducting joints have been siliconed and alimuium foil taped and flexi to the mev, I don't like these flexis! I've picked up some rigid flexi 5"ducting so gonna use this for the mev connection and tie rap insulation round it! Might not look glamourous but gotta be better than flexi