Thoughts on my plans very much appreciated

[cjc88]

I started a DIY MVHR build as I wanted to make my house a bit more insulated, but already have condensation on windows and smells that linger for says, so it seemed like a sensible next step.

 

My attic is cold, but the only place I really have space for a unit, and figured I could insulate the chimneys at the same time as passing MVHR pipes down them (after putting in a lintel) - 3 chimneys front and 3 at the back for a 3 story mid-terrace house of ~140sqm. The chimneys are pretty warm now I have insulated them at the attic (no external walls), and added small breather vents into the house. I have mostly insulated the ducting in the chimney and will of course do the same in the attic once I have finalised manifold locations, and probably the MVHR itself given it's a cold attic.

 

There are some rooms I still need to work out the semi-rigid duct path (no chimney to use for several rooms), but Im starting to think about next steps and would love to hear your thoughts/concerns please?

 

1. Im planning to put the MVHR in the middle of the chimneys, and the semi-rigid ducting can all meet at some manifolds there quite easily. Do you think it would be best to wall mount with some brackets, or just put some wooden joists to support its base?
2. If I put the external vents through the chimney then there will be a lot of bends to deal with (around the roof supports and going down to the opening for the chimney before going up the chimney to some form of cowl). Would I need a drain point at the bottom of this ductwork, or is it generally ok so long as the ductwork is insulated?
3. My alternative is to ask a roofer to cut a hole in my roof and put the external vents directly above the MVHR (and could then totally remove chimney at some point - given I think I can see water marks on the chimney above the lintel it presumably needs some work anyway). Is this worthwhile do you think? (shorter pipework but more invasive and risky to interfere with the roof, given somebody in the past made the strange decision to add planks added on the back of the roof)
4. A Zehnder Q350 is an impossible squeeze through my roof hatch at present, but I may cut the hatch just to allow entry and then re-seal... Though definite effort! Alternative options with a width <510mm seem to sacrifice a fair bit.
5. The Zehnder needs 160mm vents, but if I used 200mm vents could I use flexi pipework to make the routing easier (due to the above), or would I need more like 250mm to compensate for all the ridges....? (the chimneys are about 240mm wide, but quite sooty, so definitely needs lining)

 

Any thoughts or feedback very much welcome, thanks in advance!

 

 

Edited 20 hours ago by cjc88

[Mr Punter]

Can you cut a much bigger hatch and trim out the ceiling joists?  A drop down hatch and a proper ladder will make regular inspection and maintenance easier.

 

I think the MVHR units normally have a condense trap that needs to be fed into a drain.

 

Was a PIV unit ruled out?  They are a simple install.

[cjc88]

Thanks for your reply, Ive fed a drain down the chimney too, so that side of things is sorted.

 

It was last years project to install a nice new insulated roof hatch with ladder, which has now been done, and only now do I realise the downside of it being so narrow... Seems a massive shame to cut it too much, but if the Q350 is the right model, then I can probably make it happen and make it look smart again. Once installed its easy to access the attic though...

 

I didnt really look much at a PIV, but I was keen on some of the side benefits to an MVHR (e.g. installing secondary glazing to cut out road noise and improve insulation (even 10y old sash windows leak a lot), but would need a moisture management ventillation process hence figured MVHR was a good choice).

[Iceverge]

13 hours ago, cjc88 said:

Im planning to put the MVHR in the middle of the chimneys, and the semi-rigid ducting can all meet at some manifolds there quite easily. Do you think it would be best to wall mount with some brackets, or just put some wooden joists to support its base?

 

Wall mount. Or make a raised floor for the MVHR oN rubber feet or springs or hang from the rafters. 

 

 

 

Otherwise the vibrations on the joists will annoy you. 

 

 

13 hours ago, cjc88 said:

I put the external vents through the chimney then there will be a lot of bends to deal with (around the roof supports and going down to the opening for the chimney before going up the chimney to some form of cowl). Would I need a drain point at the bottom of this ductwork, or is it generally ok so long as the ductwork is insulated?

 

No, exhaust condensation is taken care of by a drain under the heat exchanger inside the unit. 

 

13 hours ago, cjc88 said:

My alternative is to ask a roofer to cut a hole in my roof and put the external vents directly above the MVHR (and could then totally remove chimney at some point - given I think I can see water marks on the chimney above the lintel it presumably needs some work anyway). Is this worthwhile do you think? (shorter pipework but more invasive and risky to interfere with the roof, given somebody in the past made the strange decision to add planks added on the back of the roof)

 

Long term maybe better.

 

You could put MVHR cowls where made most sense for the system then ..

 

13 hours ago, cjc88 said:

A Zehnder Q350 is an impossible squeeze through my roof hatch at present, but I may cut the hatch just to allow entry and then re-seal... Though definite effort! Alternative options with a width <510mm seem to sacrifice a fair bit

 

Add another roof hatch somewhere else? A cheap one that you can just tape up to air seal once the unit is in place. 

 

Otherwise there's slimmer MVHR units available. 

13 hours ago, cjc88 said:

1. The Zehnder needs 160mm vents, but if I used 200mm vents could I use flexi pipework to make the routing easier (due to the above), or would I need more like 250mm to compensate for all the ridges....? (the chimneys are about 240mm wide, but quite sooty, so definitely needs lining)

 

 

Flexi pipework is woefully compromised. Stick to rigid if you can. 

 

One thing to consider is how your house will behave in a fire. 

 

If you have a fire now in a room with a fireplace is it possible that it could travel to the attic? 

[cjc88]

1 hour ago, Iceverge said:

Otherwise the vibrations on the joists will annoy you. 

Got it, thank you! 

 

1 hour ago, Iceverge said:

No, exhaust condensation is taken care of by a drain under the heat exchanger inside the unit. 

Is that true, even if the insulated pipe goes up and down a bit? (i.e. you dont need a condensate point at every low point or something horrible like that)

 

1 hour ago, Iceverge said:

Add another roof hatch somewhere else? A cheap one that you can just tape up to air seal once the unit is in place. 

As annoying as that would be, it is probably the least invasive if I can find a nice patch of ceiling to cut!

 

1 hour ago, Iceverge said:

If you have a fire now in a room with a fireplace is it possible that it could travel to the attic? 

Hmm, thats an interesting point... Though my house is broadly open plan on the ground floor, and with linked smoke alarms in every room, so its probably no different to the fire travelling up the stairs (which is the only exit route anyway). All foam used was fire foam, so at least it will only have the plastic semi-rigid duct to burn (and surrounded in rockwool)

Edited 5 hours ago by cjc88