Joists for HRV

Eoghan Barra

Hi there all,

Can anyone advise on the possible options regarding joists for first floor in a new-build that will have HRV?

I know that open web joists, or eco-joists, make sense in that the ducting can pass through without drilling/cutting out holes etc., but they seem to be incredibly more expensive than solid timber. Are there any other - especially more cost-effective - solutions?

Also, what are the dimensions (ie width) of the ducting likely to be (including insulation), as the joists will obviously have to be able to accommodate this.

Any pointers gratefully received.

about2build

Hi there
After a consult last week I am being advised to create a false ceiling on the ground floor and that the ducting will work through here. 130mm width needed. The ducting in the attic will go on top of the joists...standard joists fine provided you are not using the attic as a room!! Thats how I read it anyway oh and th einsulation goes on top of the ducting in cold roof. hope this helps and open to criticism:cool:

Eoghan Barra

Thanks a lot for that, about2build.

I don't think using solid timber joists with a false ceiling would work for me, as I'd end up loosing too much space, as well as not wanting to have to carry out 2 operations (ie 1 install joists 2 install false ceiling) where I hope to be able to get away with one.

The 130mm width seems very little. How does that break down in term of duct/insulation?

about2build

130mm is for the ducting alone and at the mid floor which doesn't require insulation as it is inside the envelope....the ducting in the attic requires the same depth of insulation as you have on the plane eg 300mm wool or whatever. Did you think of adding an extra block on ground floor wall to give the space you need or would it push roof height too high??

Eoghan Barra

Yes, an extra block would push roof height up above what planning permission allows.

I must admit it didn't occur to me that ducting in first floor didn't need insulation, but it absolutely stands to reason. Thanks for pointing that out. Do you know if 130mm is about average for ducting, or do dimensions vary widely according to the system and situation?

Carlow52

about2build wrote: »

130mm is for the ducting alone and at the mid floor which doesn't require insulation as it is inside the envelope....the ducting in the attic requires the same depth of insulation as you have on the plane eg 300mm wool or whatever. Did you think of adding an extra block on ground floor wall to give the space you need or would it push roof height too high??

make sure all the ducting in attic gets insulated: was in a TF today with 350 mm blown insulation in roof. One of the HRV pipes popped up above this for about a meter and then back down due to a clash of joists, the insulation on the 'exposed bit' was pathetic, I reckon there is more on a ribbed....:)

The 4 extra 90 bends will make for interesting flows

about2build

No that was the width required and it doesn't vary...seems to be exact enough...sure throw in the extra block and say nothing

refco

if you cant supply height to put the false ceiling in you could possibly run a bulk head (box around ducting) along the exterior wall this would mean using regular joists and only really affecting ceiling height where it wont be noticed

Eoghan Barra

Thanks guys for all your suggestions.

I'm not sure about boxing the ducting in for aesthetic reasons: the intake/outlet ports will be in central locations in rooms and so the boxing would have to run out from corners of external walls to room centres, which wouldn't look great.

The best solution would be one which allows the ducting to run through the joists somehow. Has anyone seen i-joists used in this way?

sas

Could some of them drop straight from the attic (via wardrobes, cupboards etc.) to the exact location where they are needed downstairs.

That's what we did for alot of ours.

Eoghan Barra

Hi sas,

That's a very simple solution and I thank you for it.

Unfortunately it's not so simple in my case, as the house I'm building is a dormer, so there won't be an attic to speak of, just a smallish triangular space running along under the ridge (where the ducting for upstairs will go).

However, I may have a kind of storage space along some of the 'knee' walls (is that the right term?) upstairs, so it might be possible to adapt your idea somehow. You've planted a seed...

creedp

Eoghan Barra wrote: »

Hi sas,

That's a very simple solution and I thank you for it.

Unfortunately it's not so simple in my case, as the house I'm building is a dormer, so there won't be an attic to speak of, just a smallish triangular space running along under the ridge (where the ducting for upstairs will go).

However, I may have a kind of storage space along some of the 'knee' walls (is that the right term?) upstairs, so it might be possible to adapt your idea somehow. You've planted a seed...

Another possibility is to use rectangular ducting in the downstairs ceiling as this means the false ceiling can be minimised. I know that the circular duct is supposed to be the most efficient option but a rectangular solid duct of same mm2 size may not reduce the efficiency dramatically

Carlow52

even with dormer windows there can be room in the attic space all along the eaves for ducting.

re the I joists, subject to loading and span, as the flooring when screwed down serves as the bridging, it is possible to leave out intermediate bridging
Proper full depth hangers at the end will eliminate the need for end bridging so happy days for hrv

Do-more

If it's a dormer then you should have no difficulty in running the ducts along the length of the house in the void behind the knee wall above the joists and then across the house where needed dropping down between the joists.

Have you decided on a HRV system yet? Your supplier should design a layout for you, sizing all the ducts correctly to take account of the pressure drops.

The HRV duct work should be the very first thing installed in the house so that it doesn't have to bend around plumbing, electrics etc.

Once installed and insulated any openings should be taped off so no dust and dirt enters the ducts during the remainder of construction work.

Eoghan Barra

Some great advice there, I'm delighted in the response to this!

Do-more: The knee wall will be an external block-built cavity wall, so I won't be able to put the ducting behind that. But I may well be creating some storage space in this area anyway; if so, that would hide the ducts.

I haven't decided yet who I'll be going with for hrv, I'm only up to dpc level. But I needed to make a decision on joist type/dimensions etc because I'll soon be building in the hangers to carry them. Reckon I'll just go ahead and put in ordinary timber joists, rather than anything more sophisticated. I had got a quote of €20/m + VAT for 300mm deep eco-joists, which frightened the pants off me, hadn't figured it would be anything like that. Nice to have found another less costly alternative!

Carlow52

The knee wall will be an external block-built cavity wall: Can you explain this pls?

Have u prices I joists as opposed to the fancier ecoli ones?

sas

Carlow52 wrote: »

The knee wall will be an external block-built cavity wall: Can you explain this pls?

I was wondering about this too.

I hazard a guess that it's a 1.5 storey building, hence the knee wall being part of the external wall.

fclauson

On the ducting - you could use the round semi rigid - its part Q listed - various vendors do it now looks like an octopus - singal central distribution point

Eoghan Barra

Sas has it right, it's a 1.5 story, which is all we could go for from the planners on our site.

We're trying to achieve passive (or close to) standard, though not interested in certification. Many of the choices on buildup have been influenced by stuff written by you guys yourselves and others on previous threads in this forum - I've found it to be a fantastic resource.

Carlow 52: I haven't priced i-joists - for running ducting through joists open-web are better because no cutting required, so the 'fancier ecoli ones' were my first port of call :rolleyes:.

fclauson: is the round semi rigid ducting you mention smooth inside, ie is there much drag created? How does the 'octopus' layout differ from other systems?

Carlow52

knee wall: the learning continues:)

In passing where will the hrv unit be located

re I joists what I had in mind was that they be used when the ducting was running // to joists, even when the ecos are used, getting long lengths of rigid is is impossible.

I had the chance recently to visit 3 TF houses that are almost at the end of first fix[ plumbing/(wiring: music-tv-alarm-comms-etc) heating/hrv.] I had never had that chance before.

It was perfectly clear that who ever 'designed' them gave no consideration to how all this stuff would work in practice.

despite eco joints there were more holes bored than in swiss cheese and the number of after thoughts...

One example was there was a a pair of insulated pipes running diagonally from one corner to another on top of the ground concrete floor.
AFAIK they were from the solar thermal
The lads were trying to fit 100 mm rigid insulation over this: cruel...

My point is put it all up in layer in CAD and see how it all fits

BryanF

whats eco about 'ecojoists'?

fclauson

Eoghan Barra wrote: »

fclauson: is the round semi rigid ducting you mention smooth inside, ie is there much drag created? How does the 'octopus' layout differ from other systems?

Yes smooth inside and being an octopus as opposed to a main trunk with branches has the advantage of reducing or eliminating cross talk (hearing what is happening in the next room via the ducting)

I I have spent considerable time looking at this

The semi ridgid duct has a number of benefits over its solid friends
a) easy to install
b) easy to clean
c) can easily be added to if you have to change the config or add more ventalation

Eoghan Barra

Hi guys,

Carlow 52, not actually sure where hrv unit will be located, where do you have yours?

Bryan F, there's nothing whatsoever eco about eco-joists as far as I can see. 'Eco' has become just another band wagon for jumping on by those who in all probability have zero real understanding of, or interest in for that matter, ecology.

fclauson, which companies (are you allowed to say?) do that type of hrv system?

fclauson

Eoghan Barra wrote: »

fclauson, which companies (are you allowed to say?) do that type of hrv system?

http://www.sap-appendixq.org.uk/searchresults.jsp?parentID=1&categoryID=21&companyID=&brandID=&productSearch=&Submit=Search

Eoghan Barra

Nice one, fclauson. I'll check them out.