Wall vents for mvhr

[PeterW]

19 minutes ago, readiescards said:

Gosh I don't want to be going up a 3 storey ladder every few weeks to clear some fluff - might need to have a think how to clean the vents from inside 

It's another one of those "experience" things the manufacturer never tells you that makes this board so valuable ..!

I plan to put the inlet (with additional inline filter) on a north facing wall under the eaves and the exhaust into the roof surface using either a lead or plastic vent cowl.

Using an additional inline filter means I can get away with a bigger mesh at the intake just to stop large insects etc getting in, and the finer stuff can get caught at the next filter, hopefully reducing the amount of times the heat exchanger filters need swapping.

 

 

[Jeremy Harris]

1 hour ago, readiescards said:

Gosh I don't want to be going up a 3 storey ladder every few weeks to clear some fluff - might need to have a think how to clean the vents from inside 

I can usually clean mine with a soft broom, from ground level, as our eaves are quite low (around 2.6m above ground, IIRC).  It's not easy to get at the vents from inside with the ones we have, as the outer grille is hinged at the top and clips open from outside to clean it.

I think our problem is largely to do with the type of grill and it's location and orientation.  It's located in the narrow alley (around 2m wide) between the rear of our house and the retaining wall behind, on the North elevation, at the East end.  This has been a bit of a wind tunnel (until I fitted a fence behind the garage a few months ago), with the prevailing wind blowing stuff through that gap, and I suspect that may well have contributed to the fluff problem.  If I get the new duct cowls in place in the next week or two then I'll monitor them and report back on whether or not they are as susceptible to catching fluffy stuff.  The main problem seems to be fluffy weed seeds, like dandelion and Old Man's Beard, the latter grows in profusion in the hedges to the West, alongside the stream and ponds.

[Stones]

1 hour ago, PeterW said:

It's another one of those "experience" things the manufacturer never tells you that makes this board so valuable ..!

I plan to put the inlet (with additional inline filter) on a north facing wall under the eaves and the exhaust into the roof surface using either a lead or plastic vent cowl.

Using an additional inline filter means I can get away with a bigger mesh at the intake just to stop large insects etc getting in, and the finer stuff can get caught at the next filter, hopefully reducing the amount of times the heat exchanger filters need swapping.

We had under eaves air intake on our last house but one, and found little difference in terms of the mesh clogging compared to previous houses where the intake had been in the wall.

We used lead roof vents on our last house. They didn't clog at all but didn't have mesh, rather a series of holes punched through the lead roughly 5p piece sized. The result was that the internal filter ended up taking the strain but as this was easy to get at and pennies to replace, seemed to be a good way of dealing with the issue. Using a second inline filter seems a very sensible addition.

We are using plastic roof tile vents for our current build, and given the smaller size of the inlet holes (greater number, significantly smaller diameter - large mesh in effect) suspect there may be a degree of clogging. One thing I am considering is to arrange the ductwork such that I can easily swap over supply and exhaust to reverse the air flow and clear the roof vents that way.

 

[ProDave]

3 hours ago, readiescards said:

Gosh I don't want to be going up a 3 storey ladder every few weeks to clear some fluff - might need to have a think how to clean the vents from inside 

That's part of the reason for wanting to separate mine. They will be going at either end of the gable end wall above the garage. Later on, a car port will be built joining on the the garage wall. So the aim is to place them one each end of the gable wall, above the line of the car port, but close enough to each end that I can reach them without actually climbing on the roof of the car port.

[janedevon]

Guys just jumping in here rather than start another thread, is a mvhr unit a requirement? Om my plans my vent spec are, allow for heat recovery extract fans to bathrooms shower room cloakroom and utility plus extractor in kitchen, i was,under the impression we would use single units in required rooms but it seems a system is better? And expensive by the looks of things, so,would a unit just connect to the various rooms that needed ventilation as listed above?

Ill need to get this sorted asap as roofers start today more expense lol

 

[Jeremy Harris]

1 hour ago, janedevon said:

Guys just jumping in here rather than start another thread, is a mvhr unit a requirement? Om my plans my vent spec are, allow for heat recovery extract fans to bathrooms shower room cloakroom and utility plus extractor in kitchen, i was,under the impression we would use single units in required rooms but it seems a system is better? And expensive by the looks of things, so,would a unit just connect to the various rooms that needed ventilation as listed above?

Ill need to get this sorted asap as roofers start today more expense lol

 

No, it's not a requirement, but it is a big (can be VERY big) energy saving measure.  With a well-built and well-sealed house, with a reasonably good insulation standard, then it's pretty common to find that ventilation heat loss dominates, especially for larger builds, where insulation has less impact on energy use.  For our build the MVHR saves around 50 to 60% of the total heat loss, so our annual heating energy bills would be roughly two to three times greater without it.

Any saving from MVHR will vary a lot with the overall insulation level, the level of airtightness and the size of the house (for small houses insulation tends to dominate heat loss, for large houses ventilation tends to dominate). 

Unless the house is significantly better than the airtightness level in building regs, MVHR (even in a  single room) probably doesn't give any worthwhile saving, though, so the decision is quite tightly linked to your airtightness target.  For example, building regs allows a pretty draughty 10m³/m² floor area/hour, with a pressure difference during the air test of 50 Pa.  Our house is just under the passive house limit, at 1.22 m³/m²/hour, so makes MVHR worthwhile.  If the air leakage was more than about  5m³/m²/hour then the saving from fitting MVHR probably wouldn't be worth it, in terms of energy cost saving.  It'd probably still be worth it in terms of much improved air quality, though, and some fit MVHR just to gain that benefit alone.

[janedevon]

So please explain to me like im 6 lol, do,i have a unit in the loft space then trunking to all rooms specified on my plan, like bathroom utility etc ? Or does each required room need an air flow in and out? Im just thinking of how many vents are going to be needed on my outer skin?, roof and block work started today so i kind of need to get a plan into action

[Declan52]

You will have wet rooms where you want to remove air, kitchen bathrooms, utility rooms. Then you have rooms where you want the fresh air put back in, living room, hallways etc. The exact layout will be done by whoever you get your system from. 

For the outside its just an air vent which brings clean air in direct to the mhrv and a vent through which all the "dirty" air gets expelled from.  Once again depending on your system these 2 vents might be in a wall or through vent tile.

[janedevon]

Many thanks, i like the sound of tiles rather than walls ill look up some suppliers, internally is it just ducting hidden by a grill that gets activated with the light switch to pull the air out like a standard old school vent axia type thing?

[PeterW]

Its working constantly on low speed. When the system is activated in an extract room, either by switch or a sensor in the duct (humidity from a shower etc) then it activates the boost.

Is it a new build..? If so, getting a unit that is SAP Q listed makes life slightly easier. 

Start here for a good overview BPC

[janedevon]

Brilliant thanks had been reading that before,you posted and i see they do a free quote from plans so ill start there.

 

thankyou

[Declan52]

Quite a few have got there systems from bpc, me included and I also used vent tiles. Much easier than going through block work.

[Russdl]

My plan at the moment is to have the MHVR in a ground floor utility/plant room with the external vents at ground floor ceiling height (around 2.6m above external ground level). It sounds like that will be useful when it comes to cleaning the vents, but is that high enough from ground level to work efficiently?

 

[PeterW]

I've seen them at various heights - I'm sure someone on here said theirs were at 300mm above ground level..??

[ragg987]

I kept ours at approx 2m above internal and external floor level. Combination of reasons, including that our MVHR has upwards-facing supply and extract (i.e. I avoided having extra bends which increase resistance, pressure and hence fan speed would need to be higher for a given airflow), plus also did not want the external ducts too near ground level as that would increase ingress of dirt / leaves plus be tempting for little fingers.

Someone made a point that the they kept ducts low to prevent water flowing into the MVHR. Not sure how important this would be. We have a stainless steel cowl like the domed one posted earlier, I am hoping this will prevent water ingress.

[Jeremy Harris]

The instructions for our MVHR stipulated that the ducts through the walls should have a fall on them, sloping down towards the outside.  I assume that this was to ensure any water ingress or condensation drained outwards.  The plastic grills I have fitted at the moment also have two small water drain holes at the bottom, presumable to let any water that gets in, flow out again.

There was also mention of the need for the ducts connecting to outside to not have any dips in them; they had to slope outwards towards the external terminals and inwards and down to the MVHR condensate drain tray.  I'm guessing that this is to avoid getting a pool of condensation anywhere in those ducts.

[Russdl]

Thanks for the replies.

5 hours ago, ragg987 said:

I kept ours at approx 2m above internal and external floor level. Combination of reasons, including that our MVHR has upwards-facing supply and extract (i.e. I avoided having extra bends which increase resistance, pressure and hence fan speed would need to be higher for a given airflow), plus also did not want the external ducts too near ground level as that would increase ingress of dirt / leaves plus be tempting for little fingers.

Someone made a point that the they kept ducts low to prevent water flowing into the MVHR. Not sure how important this would be. We have a stainless steel cowl like the domed one posted earlier, I am hoping this will prevent water ingress.

ragg987, I presume your's has been up and running for a while and there are no issues with having the vents relatively low. If that's so, then it's good news for me, it saves yet another redesign on my part!

[ragg987]

Been running for 2 weeks so nothing conclusive yet. I am running it to promote drying internally e.g. plastering and screed, system is unbalanced and running at a higher fan-speed than I would once we move in. So far I see no issues with water ingress (there have been some big rains in that period, though no strong wind AND rain as you might expect in winter).

[ProDave]

So, I have ordered two stainless steel cowls, and a length of 150mm diameter rigid spiral ducting to go through the wall.  Of course it has not arrived yet, and I want to get on.

So daft question of the week:  Do I drill a 150mm diameter hole, and expect to be able to insert the 150mm duct when it comes?  Or does the "150mm" refer to something other than it's exterior diameter and do I need say a 160mm hole?

[Leaway]

for the BPC system roof vents is there any issue with installing the extract MVHR pipe - vent on a 22 deg slated roof?

[Russdl]

@ProDave Dragging this thread back to life. What was the answer?

 

On 15/07/2016 at 20:42, ProDave said:

So daft question of the week:  Do I drill a 150mm diameter hole, and expect to be able to insert the 150mm duct when it comes?  Or does the "150mm" refer to something other than it's exterior diameter and do I need say a 160mm hole?

 

[ProDave]

Yes 150mm was the size of the hole needed.  In the end I did not cut the holes until I had the vents though.