MVHR with a lot! of steels + other dilemmas

[BartW]

Hi guys,

Our timber frame project is soon to go live, but the design involves a lot of lateral steels, which cannot be omitted.

 

I posted before, and someone mentioned notching of steels. This is something we are considering, but there likely to be a lot of those, and I worry this will weaken the steel by the time it has been drilled with a multitude of holes...

 

I spoke to a couple of MVHR design companies, and they all seem to be forthcoming, and willing to offer their design service (which of course is of some value), but what if they end up designing a multitude of steel penetrations that will simply not be practical...

 

Am I better off just waiting until the frame is up, and try to find best routes, and passages that I might be able to incorporate into other voids, for example above joinery, and other "design elements"?

 

Or, as an option would it be possible, or in any way practical to run round ducting, and change to flat oval over the steel sections (whilst removing a 2" packer above it), then change back to round? There are a few possible places where such thing may be achievable. It would involve me being on site, when the frame is going up, but out of all things, I think I would prefer it.

 

Also, interestingly the brief we have is:

- 180sq m GIA

- 510m3 of air because of double height ceiling above the living room opening

 

One company suggested Zehnder Q350.

Their calculations, and how they arrive at it below:

 

 

 

Funnily enough, another supplier suggested Q450, and yet another Q600! That is pretty much full spectrum of ranges. One could almost guess that any unit "will be ok"...

 

Also, one suggested a pre-heater, but we are in Oxfordshire, so frosty days are rather none.

 

Post heater? Is this to add to heating ability?

 

Enthalpy? I have been reading about it, and the more I do, the more confused I am.

 

ComfoCool sounds great, but one party is VERY sceptical about the benefit of that.

 

World of unknowns, as you can see...

 

HELP?

 

 

 

 

 

 

 

 

A brief idea of our plans and steels:

 

Removal of packing above steel as seen below?

 

 

 

Plans:

 

Showing ground floor, with double dotted line where steels in the floor zone above:

 

 

 

First floor plan (with steels clearly shown on the floor level of this floor, in the floor zone):

 

 

 

Second floor plan (with steels clearly shown on the floor level of this floor, in the floor zone):

 

[joth]

Definitely try and plan this before construction. Even if changes are needed onsite it's much better to have a scheme in mind before everything goes up.

 

Do you know where the MVHR itself will go?

Try and think about the problem in 3D. Steels block horizontal runs, but you can get a long way by running ducts horizontally on floor 2 and then dropping down a floor where needed (maybe running horizontally on floor 1 before dropping down again.) A loft installed MVHR has several downsides, but can make this sort of vertical access into very large open plan areas much more convenient.

 

If you have ComfoCool it requires the higher boost mode of the Q600 so that maybe why some installers upsized it

[Wumpus]

An option could be to put in a suspended ceiling up to 100mm below the metal web joists to take services. Depends on your ceiling heights and whether you can afford the space.

 

Have the posijoists been designed yet? You may need to go larger for the spans, perhaps 253mm, and then there might be space above or below the steel, between the webs.

Edited April 9, 2022 by Wumpus

[Mr Punter]

I have only done one of these.  It was a small house but still on the limit of what the unit (Vent Axia Sentinel Kinetic) could manage.  Maybe your place would work well with 2 units and fewer steelwork clashes?

 

Obviously the more restricted the ducting, the higher the pressure needed to push the air through, the harder the units need to work and the noisier the system.

 

You will need to agree any steel necessary penetrations with the SE and get these made during fabrication, rather than on site.

 

Do you have a single design / manufacture / install contract for the superstructure?

[HerbJ]

1 hour ago, joth said:

Definitely try and plan this before construction. Even if changes are needed onsite it's much better to have a scheme in mind before everything goes up.

+1 to this. You cannot afford to delay this preparatory design work work....

 

It must be possible for you to make a preliminary plan of the ducting lay-out and this will highlight the locations clashes where ducting clashes with your steel work.  I specified 100mm penetrations for ducts and 50mm penetrations for cables and pipes - see photo

 

In practice, there will not be that many penetrations required. Penetrations in steelwork need a SE review and may need stiffening detail, so it is important to have this work completed in the fabrication shop. It is not something that can be easily done at site!!

 

Use this information to instruct your TF/steelwork supplier to provide penetrations at these locations. 

 

Do the same thing for cable and plumbing layouts and arrange for penetrations for these services as well. 

[BartW]

Hi everyone, and thanks for the responses. 
 

 

a couple of answers and clarifications:

- yes the unit will be positioned within the warm loft

- TF supplier designs and provides steels with fabrication

- we did not specify comfocool, but someone did indeed offer q450 and someone else stretched it 600. Why?

- joists are pozi 254mm high

- should I design it myself, or pay someone to do it? 
- I was planning on drilling “minor” openings for cables later myself. Is that a bad idea? 
- my major worry is that if I get someone to design this system for me, they will just map the routing of the pipe where it occurs to be. Will I end up with drill patterns that will make the steels impossible to pass any engineering?

- can anyone spot a hand with regards to pre and post heater as well as enthalpy?

- is comfocool worth it?

- ceiling heights are currently set at 2.4m. I worry that adding any amount of dropped ceilings would jeopardise the feel of airiness in the rooms, plus we are running floor to ceiling glazing in most rooms. Not bad if this would create a hidden pelmet for curtain tracks and blinds

- how do I calculate the unit size? If Q350 flows 350m3, supposedly that is at a certain pressure? What pressure to aim for? And how do I calculate those?

 

many thanks again

 

bart

[HerbJ]

33 minutes ago, BartW said:

how do I calculate the unit size?

This contains a lot of valuable information that should help you.

 

 

You need to start separating your problem areas  and make some decisions to move forward and make progress.  I don't think you can sensibly keep all your options open until you have installed the TF and steels, without delays and additional costs

 

The specification of your MVHR unit may be separated from the routing/layout of the ducting, given that you seem to have decided on the unit's location and if you have decided to use flexible ducting. More design work and decisions are essential if you have decided to use rigid ducting, but also more important to start thinking about it before your install the TF and steels.

 

44 minutes ago, BartW said:

I was planning on drilling “minor” openings for cables later myself. Is that a bad idea? 

 Certainly is - don't do it! It may compromise the structural integrity of the steel work 

[Buzz]

42 minutes ago, BartW said:

my major worry is that if I get someone to design this system for me, they will just map the routing of the pipe where it occurs to be. Will I end up with drill patterns that will make the steels impossible to pass any engineering?

Hi @BartW im currently in discussion with BCP and they will do a basic design for free which in your case would not be of much use but for £100 they will do a detailed design, the questionnaire asks for all the details , joist depths and directions,  web joists and size,  and steels , with a lot of emphasis on steels placements and a caveat that basically says if you move the steels so it's different to the plan submitted don't blame us if your mvhr doesn't fit as we have planned it.

[JohnMo]

Generally the bigger the unit, the less it has to do, so is used at the low end of its performance curve, so for a given duty will be more quite.

 

Pre heating - we are in NE Scotland and not needed it so far this winter, I wouldn't bother.  Only really needed below around -5 (ish)

 

Post heating, unless your heating demand is below 10w/m2, there is not enough heat carried by the air to be effective.

 

Enthalpy, again would not bother, not really any benefit in our (UK) environment.  We don't get the long periods of sub zero temperature, which leads to air drying.  Which is where enthalpy comes into effect.

 

The other thing to consider is Coanda effect supply nozzles.  These can be used to throw air across a room and simplify pipe routes.

 

I used two units one small doing two bedrooms and two en suites and a large unit to do the rest.  Simplified the routing considerably.

[Bitpipe]

3 hours ago, HerbJ said:

+1 to this. You cannot afford to delay this preparatory design work work....

 

It must be possible for you to make a preliminary plan of the ducting lay-out and this will highlight the locations clashes where ducting clashes with your steel work.  I specified 100mm penetrations for ducts and 50mm penetrations for cables and pipes - see photo

 

In practice, there will not be that many penetrations required. Penetrations in steelwork need a SE review and may need stiffening detail, so it is important to have this work completed in the fabrication shop. It is not something that can be easily done at site!!

 

Use this information to instruct your TF/steelwork supplier to provide penetrations at these locations. 

 

Do the same thing for cable and plumbing layouts and arrange for penetrations for these services as well. 

 

Listen to @HerbJ. We built our houses at the same time with the same TF and many of the same trades.

 

I did not specify steel penetrations and only managed to get my MVHR ducts routed by the skin of my teeth and pure luck that a few gaps were available - even I needed to go the long way round for a few routes which was less than optimal.

 

Also had to use some very creative thinking on 110mm fouls and bath/shower wastes. Wider bore water pipes, especially insulated runs will need similar consideration.

 

Cables should be fine as you will likely have a small service void between the steel and plasterboard beneath.

[dpmiller]

...and remember that holes in steels need to be positioned wrt the pozijoists too so there could be a to-and-fro between the pozi designer and the SE (if they're different people)

 

I just asked our TF guys to design in a couple of holes in each steel and had no problems routing the services from that point on.

[Nickfromwales]

Designing 2 such projects atm, so have specified the steel penetrations centres to be at 400mm recurring so the posi joists can drop in retrospectively, manipulated to avoid these locations. The joists will be wiggled about to suit during installation, so best to discuss this with the joist designer as well. Remember that posi joists will be in joist hangers in most instances ( not all ) so you will lose 70mm of each 400mm centre void with the wing of the hangers, so that can mean only 2 ducts per joist void. 
Can be done with most at 400mm c’s and one or two strategic 600mm ‘service sections’ that has beefier joists allowed either side to compensate.

I never spec posis at 600mm centres tbh as most designers will fall to the default deflection allowances and beam count to give the best quote, often leaving deflection factored in at north of 12mm of movement allowed. NO THANKS! 👎. I like a structural floor not a reasonably priced trampoline.

Also good to remember that rectangular aka oval ducting is available too, so on one current MBC TF I’m bringing fresh for a bedroom and extract for that en-suite up in a ‘4x2’ stud wall with ‘oval’ ( which is rectangular-ish but what the hell ) to allow those two ducts to rise without boxing in in either of the rooms. 
Also(!) remember that you can get 225x25mm ducting, which can allow you to hide your MVHR ‘duct’ in a 35mm service batten space. Very handy when you have OCD like me lol. I cannot tolerate boxing in, and many of my clients marvel at the solutions I come up with to get away from that requirement. 

 

[Nickfromwales]

[Nickfromwales]

It’s not the depth of the ducting that’s the issue hiding stuff in tiny spaces, but more the depth necessary to fit the terminal on that takes the ( adjustable flow rate ) air valve, so as per the pics, if you go that route then you will need to take a multi tool to a regular 100mm vent cover and fix it on DIY style, plus you need to attenuate elsewhere for balance and commission. 
Nick @ CVC Systems works with us a lot to get these kinds of issues dispatched, so I use them for provisional design mostly,

 plus supply and fit when I’m too busy to do the jobs myself.

20 hours ago, BartW said:

how do I calculate the unit size? If Q350 flows 350m3, supposedly that is at a certain pressure? What pressure to aim for? And how do I calculate those?

You don’t ideally, and Nick ( CVC ) is PH certified so we both strive got the same outcome, taking WAY more than just airflow into account  

That job pictured above would have lived quite happily with a Brink Flair 325, but I paid extra to go with the 400 so the fan was less laboured, the duct connections were next size up, and the pressure required for the same airflow was significantly reduced. 
All that added together equalled a very happy client who stated that the MVHR was inaudible in bedrooms at night, and barely noticeable when boosting to deal with showers / cooking etc.

This is about a lot more than “what size unit”. It’s “why that size of unit” and understanding the pros cons and caveats so you’re making properly informed decisions.

One system is avoid is series ducted, I’m not a fan of that at all,, so I always go with radial as of all the systems I’ve specified or installed have performed exceptionally well ( so if it ain’t broken….. )   

[JohnMo]

One thing you have to consider when specifying an MVHR unit is the pressure drop from the duct runs and the outlet nozzles etc.  You need to see the MVHR fan curves (supply and extract) that way you can see exactly at what point on the flow/pressure curve you are at, when at normal speed and boost.  The higher the pressure drops the system causes the harder the fans have to work to overcome the system resistance. 

 

Once you know the above, you should be able to see where you sit on the noise curve for the MVHR fans. 

 

Also you need to consider duct noise generated by flow velocity, and the attenuation given by supply or extract nozzle and plenum chamber (distribution box).

 

All in all lots to consider, but generally oversizing MVHR units is the way to go, but not so big that min speed is over the duty point of the system.  Also ducts are better larger, as the longer the run the higher the resistance to flow (pressure drop).

 

So one reason you may have been specified different units, could be duct size and duct length, as shorter bigger ducts (or double/triple ducts) have less flow resistance and a smaller unit can more easily cope.

[BartW]

Thanks All,

 

I seem to be getting somewhere.

 

I have dug out an old BPC proposal based on our previous floor plans. Attached here for comment.

 

 

I also followed the link to the other thread and found some useful spreadsheets, one of which I edited to suit my circumstances. Attached here. It is becoming obvious that in order to keep remotely close to the 2m/s velocity rule, I will need 2 ducts to the kitchen.

 

I am also intrigued by the flat ducts. I could supposedly use that in all bedrooms and go vertically, and install outlet on the walls? Is that wise / advisable? It would certainly give more flexibility. But what about quality of those ducts? Solarcrest pretty much slated all other products, but theirs (which from their veeeery vague quote, and inability to answer what exactly it was for). I am only guessing it is in their best interest to sell the equipment, but I was unable to compare their quoted prices, which although I was that close to parting with my money, got me go off the boil completely...

 

Anyway, Airflex Pro is apparently the thing that Solarcrest swear by. Funnily enough, it can be bought from BPC too (and others I suspect). But it only comes as 75mm pipe. Is quietvent ok? it would allow more air / less speed. What about the ducting offered by Paul? Apparently "theirs" is the best. A lot of confusion here!

 

Please, talk to me again

BPC20-15284 BART WILCZYNSKI_Rev01-A3.pdf MVHR_Sizing.xlsx

[Adsibob]

We had a very similar issue as we converted a 1930s 3 bed into a modern 5 bed. LOTS of steels. I think something like 12 RSJs and 5 or 6 columns.

i agree with the comment above about separating it into problem areas. For example, you don’t need to do the ceiling everywhere to create a service void, you just need to do near the problematic steels. So for example our ceiling height in our bathroom is 2.45 but where our shower enclosure is we dropped this to about 2.3 to create a 15cm void above the shower enclosure. This works really well as it makes the showering enclosure, well more enclosed, and gives it a cosier feel whilst providing a route around a steel that was right next to it.

In other areas, we widened stud walls slightly to take the duct. I don’t think we drilled any steels.

[Nickfromwales]

Any runs which are long, almost always get jumped up to a double run with my installations. Particularly habitable / quiet rooms where bathrooms which provoke a boost session are away from, eg so they are not made ‘noisy’ by the boost velocity ( whereas with 1x duct they may well have otherwise become ).

Needs fully justifying  if that means a lot of rooms which then “require” 2x ducts if that means lots of extra hold through steels / beams etc. 
Well worth the additional effort and cost I’m bedrooms / snugs / studies etc, plus living rooms.

MVHR unit = bigger the better…..

59 minutes ago, JohnMo said:

but generally oversizing MVHR units is the way to go, but not so big that min speed is over the duty point of the system.  Also ducts are better larger, as the longer the run the higher the resistance to flow (pressure drop).

……deffo observing the minimum duty is a good point indeed, but with Brink the knee-jerk 4 speed setting can all be user defined to x percentage per increment, so not had any issues oversizing with Brink tbh. 

[Adsibob]

20 minutes ago, Nickfromwales said:

but with Brink the knee-jerk 4 speed setting can all be user defined to x percentage per increment, so not had any issues oversizing with Brink tbh. 

Good to know. I have a brink though it’s not been commissioned yet because house is still so dusty. Hoping to comission  and move in soon… just need my never ending build to end.

Edited April 10, 2022 by Adsibob