Omnibuswoman Posted March 25 Share Posted March 25 (edited) As well as doing the plant area we are starting to fit our MVHR system this week. The ducting has just arrived in 3m lengths of 160mm, 125mm and 100mm diameter. I’m now looking at our ceiling thinking how the chuff am I going to get these long runs of pipe at right angles to the run of the 400mm centre I-joists without cutting it all up into 400mm lengths… I have a sinking feeling that the answer is “you can’t”… I’m also concerned that the web depth is only 160mm, and the pipes are 160mm, which means undermining the structural integrity of the joists. So my questions are: Should I jettison the 160mm pipe and do it all in 125 and 100mm diameter pieces? how do I feed rigid pipe through the joists? how do I cut rigid metal ducting? should I go back to Green Build Store (who did our design) and ask them to amend the design? Does anyone have any good advice for me? thanks! Edited March 25 by Omnibuswoman Adding in a question Link to comment Share on other sites More sharing options...
Omnibuswoman Posted March 25 Author Share Posted March 25 Update: I just spoke to the builder who built the house who said: “We designed it based on semi flexible duct. The rigid stuff you have to install as you do the joists which we didn’t want…” … which is useful information, about three months too late 🤦♀️ What I’m astonished by is that the company who did the design & supply knew that the house was built, and still designed a system that we wouldn’t be able to install. Link to comment Share on other sites More sharing options...
Iceverge Posted March 25 Share Posted March 25 I don't think what your aiming for is actually possibly without having one million joints in the rigid ductwork. It'll have to go back. Semi rigid radial ducting is lightly your only option. Some companies are a little precious (read snobby) about this but so long as you put adequate silencers and size the ducts appropriately it's idiot proof. I would say 95% of DIYers use it. 1 1 Link to comment Share on other sites More sharing options...
Omnibuswoman Posted March 25 Author Share Posted March 25 (edited) I’ve just spoken to GBS designer who said that it’s “very easy” to cut the rigid pipes into pieces to fit into the joists, and “hundreds of happy customers” have found this “easy to fit”. I’ve asked him to have one of these “very happy” people to call me to explain to me just how easy it is… Edited March 25 by Omnibuswoman Typo 1 Link to comment Share on other sites More sharing options...
BotusBuild Posted March 25 Share Posted March 25 I've been cutting that rigid ducting recently - about 8-10 times so far with an angle grinder and cutting disc, with a couple more to do. Believe me, you do not want to go there. The people doing the design need to know the orientation of your joists, and what they are made of so they understand what is possible. Insist on NO RIGID DUCTING. I actually ended up telling BCP where the rigid ducting in our system could go, where the distribution boxes would go and how much stuff I needed. They still insisted on sending materials I know that I will not be requiring. If you need any plastic 125mm ducting, you know where we are 🙂 1 Link to comment Share on other sites More sharing options...
Omnibuswoman Posted March 25 Author Share Posted March 25 Thank you Botus… I actually gave you a little wave this morning as I drove past on my way to Topps Tiles. Despite their protestations about how easy it is, I’m just not up for learning to angle grind galvanised metal pipes. I lost a thumbnail on a vacuum cleaner at Christmas, so I know how easy it is to do oneself a nasty injury! They knew full well what and where our joists were when designing the system. I actually checked with the guy I was dealing with asking him the question about how we would get the pipes in, and asked shouldn’t we have flexible or semi-rigid pipes to make it easier. I regret accepting his assurances now!! I’m taking out my frustrations on the floor, chipping off bits of concrete. Very cathartic! Link to comment Share on other sites More sharing options...
Iceverge Posted March 25 Share Posted March 25 (edited) You can calculate and design it pretty easily. 75mm rigid ducting is ok for 2.5m/s flow speed so 28m3/HR. Assuming normal ceiling heights and a desired ACH of 0.3 then each pair of 75mm ducts will supply 37m2 floor area. So if you have a floor area of say 35m2 that'll be ok with one 75mm supply and one extract. If you have a floor area of 300m2. Then you'll need 8.1 pairs of supply's and extracts. Manifolds only come in 8 or 10. I would be happy with 8 in this instance as it's pretty close. Then sketch out on your plans one duct to each supply and extract room. Double up extracts for the kitchen and then for the dampest/furthest room until all the ducts are used up. Similar process on the supply side. Easy. Keep sketching until you can draw it without any ducts over lapping. That'll make it easy to install. As for materials, It's all much the same stuff. I would buy the cheapest. About £113/50m roll at a quick Google. Leave space for two of these between the mvhr unit and the manifolds. Install your supply plenums opposite the room door, usually over a window. Install the extracts close but not directly over the source of steam. Say 1-2m away and again opposite a door if possible. These ones are cheap and work fine. For the larger ducting to outside I like the rigid insulated stuff. Nice and tidy and doesn't leak as much heat as the metal. Finally site the MVHR unit inside the heated envelope, with short runs to outside for the inlet and exhaust, where it won't cause noise or vibration issues and can be serviced easily . I wouldn't skimp in the unit or buy something too obscure as you'll need parts in future. Aim to have one with a headline M3/HR of about your house volume. That way at normal rates the fans run at about 30-40%. Bargains can be had. This one is ok for about 120m² floor area. If you post your plans I'm sure we could collectively design it for you in about 15 mins. Edited March 25 by Iceverge 2 2 Link to comment Share on other sites More sharing options...
BotusBuild Posted March 25 Share Posted March 25 34 minutes ago, Omnibuswoman said: I drove past on my way to Topps Tiles 👋 34 minutes ago, Omnibuswoman said: chipping off bits of concrete. Very cathartic! 🤣 28 minutes ago, Iceverge said: Leave space for two of these between the mvhr unit and the manifolds. You can get flexible versions of those - I have these already fitted. Happy to come over and help design it. 1 Link to comment Share on other sites More sharing options...
Iceverge Posted March 25 Share Posted March 25 (edited) 4 minutes ago, BotusBuild said: You can get flexible versions of those - I have these already fitted Me too, without personal comparison I'm led to believe the rigid ones are better. I think it's worth the investment if you have good hearing. Edited March 25 by Iceverge 1 Link to comment Share on other sites More sharing options...
Russell griffiths Posted March 25 Share Posted March 25 (edited) Do yourself a really really big favour and send that back and swap it for flexible ducting. I don’t know what your design calls for but I would go 90mm rather than 75. Edited March 25 by Russell griffiths 1 1 Link to comment Share on other sites More sharing options...
Omnibuswoman Posted March 30 Author Share Posted March 30 Brief update: I decided to keep the rigid system as it had already been delivered, and have outsourced the cutting of the ducts to a local guy who is a skilled metalworker. GBS amended the design to make it simpler, and jettisoned any 160mm ducts crossing joists. It all feels more manageable now, and we’ve made a start. Below is the fruits of several hours’ labour yesterday. Link to comment Share on other sites More sharing options...
Sparrowhawk Posted March 30 Share Posted March 30 10 minutes ago, Omnibuswoman said: Brief update: I decided to keep the rigid system as it had already been delivered, and have outsourced the cutting of the ducts to a local guy who is a skilled metalworker. GBS amended the design to make it simpler, and jettisoned any 160mm ducts crossing joists. It all feels more manageable now, and we’ve made a start. Below is the fruits of several hours’ labour yesterday. How are you rejoining the ducts at each cut? An additional metal joiner with 2 O-rings? Link to comment Share on other sites More sharing options...
Sparrowhawk Posted March 30 Share Posted March 30 (edited) On 25/03/2024 at 16:46, Iceverge said: You can calculate and design it pretty easily. 75mm rigid ducting is ok for 2.5m/s flow speed so 28m3/HR. Assuming normal ceiling heights and a desired ACH of 0.3 then each pair of 75mm ducts will supply 37m2 floor area. So if you have a floor area of say 35m2 that'll be ok with one 75mm supply and one extract. If you have a floor area of 300m2. Then you'll need 8.1 pairs of supply's and extracts. Manifolds only come in 8 or 10. I would be happy with 8 in this instance as it's pretty close... @Iceverge that's the clearest explanation I've seen on the forum, thank you. I spent last weekend messing around with spreadsheets trying to calculate flow rates and resistance and reached the point of giving up and admitting I'll have to pay £1000 to get someone to design the system. Now, I'll give it another go... Edited March 30 by Sparrowhawk 1 Link to comment Share on other sites More sharing options...
Nickfromwales Posted March 30 Share Posted March 30 On 25/03/2024 at 15:41, Omnibuswoman said: Update: I just spoke to the builder who built the house who said: “We designed it based on semi flexible duct. The rigid stuff you have to install as you do the joists which we didn’t want…” … which is useful information, about three months too late 🤦♀️ What I’m astonished by is that the company who did the design & supply knew that the house was built, and still designed a system that we wouldn’t be able to install. Yup. Astonishing seems to be the norm. Have they also specified cross-talk and acoustic attenuators? Ah, slow connection, now showing the pics lol! Good for you, getting stuck in, and looking good so far. 1 Link to comment Share on other sites More sharing options...
Nickfromwales Posted March 30 Share Posted March 30 Also, is the mist system your choice or specified by BC? Link to comment Share on other sites More sharing options...
Omnibuswoman Posted March 30 Author Share Posted March 30 4 hours ago, Nickfromwales said: Also, is the mist system your choice or specified by BC? We proposed it in our BC application because we don’t have the required access for a fire appliance. I took your recommendation from a previous post to use the Welsh company you are familiar with. They’ve been great so far. Link to comment Share on other sites More sharing options...
Omnibuswoman Posted March 30 Author Share Posted March 30 7 hours ago, Sparrowhawk said: How are you rejoining the ducts at each cut? An additional metal joiner with 2 O-rings? For the long runs through the joists it’s about 10 400mm pieces, joined with a male/male gasket (right word?) like this: 1 Link to comment Share on other sites More sharing options...
HerbJ Posted March 30 Share Posted March 30 Looking good I can see that you have made significant penetrations in your I-joists. Holes may be cut in I -beams but only in accordance with manufacturer's guidelines/rules - see typical guidelines at https://elliotts.uk/ideas-and-advice/i-joist-faqs Have these penetrations been checked by your SE for each specific location to see if acceptable ? 1 Link to comment Share on other sites More sharing options...
JohnMo Posted March 31 Share Posted March 31 On 30/03/2024 at 08:25, Sparrowhawk said: @Iceverge that's the clearest explanation I've seen on the forum, thank you. I spent last weekend messing around with spreadsheets trying to calculate flow rates and resistance and reached the point of giving up and admitting I'll have to pay £1000 to get someone to design the system. Now, I'll give it another go... Only thing to watch is long lengths can have quite a pressure drop. Look on the internet for duct data sheets, most the good brands will have install guidance on max length and flow and how ducts you need. Makes it really simple. Coanda supply nozzles can allow you use much short lengths of duct, but depends on house layout if they are appropriate or not. The most difficult bit is the apportioning of flow in each room to get the correct balance between flow and extract. I found this useful 1 Link to comment Share on other sites More sharing options...
Sparrowhawk Posted March 31 Share Posted March 31 Thanks John. I am stuck on the PH bedroom guideline figures as this puts a lot of supply upstairs. Ours is a 4 bed house (2 live here, but assuming 4-5 person occupancy). If I use the PH bedroom supply rates, upstairs is well ventilated - but upstairs is almost the whole house min fresh air requirement for 5 people. Downstairs at 0.3ACH gets v little; bedrooms are 1ACH+. Did you use the bedroom figures of 20m3/hr and 40m3/hr? Link to comment Share on other sites More sharing options...
JohnMo Posted March 31 Share Posted March 31 50 minutes ago, Sparrowhawk said: stuck on the PH bedroom guideline I did our bedroom to passivhaus flow rates, the other bedrooms that are not normally occupied, so treated as single bedroom rates. 40 for ours and 20 for the others. Your wardrobe is likely to flow through the bedroom, so take away what ever goes in there away from your bedroom figure, so the total becomes 40. Increase your lounge figure, to 40 to 45. Is your dining part of the kitchen or a separate room? If part of the kitchen are you getting flow through that area via a doorway etc. Look at where the air goes in and the likely exit point, see if air will pass through areas so it provides the ventilation for free 1 Link to comment Share on other sites More sharing options...
Omnibuswoman Posted March 31 Author Share Posted March 31 On 30/03/2024 at 16:18, HerbJ said: Looking good I can see that you have made significant penetrations in your I-joists. Holes may be cut in I -beams but only in accordance with manufacturer's guidelines/rules - see typical guidelines at https://elliotts.uk/ideas-and-advice/i-joist-faqs Have these penetrations been checked by your SE for each specific location to see if acceptable ? I did approach the SE, whose reply was “you need to seek the manufacturer’s specification for joist penetrations. Fortunately, Steico provide detailed tables for this, so I’ve been able to work out the suitability of the route for each run of pipe. I would have preferred the SE to have done this, but hey ho! 1 Link to comment Share on other sites More sharing options...
Iceverge Posted March 31 Share Posted March 31 3 hours ago, Sparrowhawk said: Thanks John. I am stuck on the PH bedroom guideline figures as this puts a lot of supply upstairs. Ours is a 4 bed house (2 live here, but assuming 4-5 person occupancy). If I use the PH bedroom supply rates, upstairs is well ventilated - but upstairs is almost the whole house min fresh air requirement for 5 people. Downstairs at 0.3ACH gets v little; bedrooms are 1ACH+. Did you use the bedroom figures of 20m3/hr and 40m3/hr? It would be useful to see plans. I would say you should be ok with 2 X manifolds of 75mm x 6 at that floor area. Like @JohnMo says if air can move through a room then you won't need one in there, much like the hall and landing. Similarly a room what won't be occupied can make do with a vent in the wall to ensure no buildup of gasses. Link to comment Share on other sites More sharing options...
HerbJ Posted March 31 Share Posted March 31 45 minutes ago, Omnibuswoman said: I did approach the SE, whose reply was “you need to seek the manufacturer’s specification for joist penetrations. Fortunately, Steico provide detailed tables for this, so I’ve been able to work out the suitability of the route for each run of pipe. I would have preferred the SE to have done this, but hey ho! Well done! It's a critical and often overlooked detail. 1 Link to comment Share on other sites More sharing options...
Sparrowhawk Posted March 31 Share Posted March 31 2 hours ago, Iceverge said: It would be useful to see plans. I would say you should be ok with 2 X manifolds of 75mm x 6 at that floor area. Like @JohnMo says if air can move through a room then you won't need one in there, much like the hall and landing. Similarly a room what won't be occupied can make do with a vent in the wall to ensure no buildup of gasses. So I don't derail @Omnibuswoman's thread any further I've posted the plans and TMI in a new thread: 1 Link to comment Share on other sites More sharing options...
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