MVHR Design for review and discussion

[ToughButterCup]

The process of designing an MVHR system for our build has been very long one. All the way from 'MVHR whassat'? to buying a design. 

What was involved?

• Knowing that a passiv haus needs an MVHR system, and why
• Finding an architect who is sympathetic to  passiv standard builds
• Reading about and visiting many passiv standard houses and one passiv haus conversion ( @VIPMan of this parish)
• Spending hours on the Internet reading posts, blogs, and taking advice
• Talking to several producers
• Working through PHPP and out the other end of it and 
• buying a design

We haven't bought the system yet. So before we do (or don't), I ask for your critical appraisal.

 

Here are the facts.

PHPP shows us needing 18.06 kWh per meter square per annum and a total requirement for 2090. kWh per annum

This post , takes us into more detail than PHPP, and bits of it were helpfully explained by @Alphonsox. The bottom line is that I know how much heat (and power) I'll need, and where I'm going to lose it. I also know how much 'spare' heat capacity I have.

 

I asked one company to design us the pipework, and specify the 'box'; Here's the plan: Supply is red, exhaust is blue

 

 

That makes more sense if you look at it in isometric projection;

 

Because of planning we had to squeeze the POSIs down to 202s. We have (effectively) no roof space. So supply and extract has to be effected through the under-floor space. To do that, we needed to double the pipe work (100s instead of 160s). That means that we are more likely to have a better 'mix' because we now have twice as many inlets and outlets than the original design which used the bigger diameter pipes.

 

We have decided to have electricity as our underfloor heating system. Downstairs  and the upstairs in the shower room only. The balance of the heat required is to be supplied by MVHR and insolation. Cooling (and pre-heat) is to be supplied by the MVHR. And that means a GENVEX Premium system with a micro heat pump (for pre and post heating and cooling).

PHPP reports that we have a 10% likelihood of overheating.

 

For all of that plus the installation and insulation of all the piping and commissioning £11K

There's a big bit of me which thinks that the installation of the pipes is DIY (MVHR piping has first call on the between the floors space). So reduce that price by £2500.

No hot water to be piped anywhere in the house. No hot water tank. Each sink (and bath) has a local water heater (SunAmp for the bath) No standing losses. Easy DIY.

 

Thoughts?

Ian

 

 

 

 

[Bitpipe]

Looks well thought out and by considering it now before frame design is locked then you stand half a chance of sticking to the plan.

 

So you will have no fouls or DHW/cold feed competing for space? If that's correct then you're simplifying life a lot. 

 

DIY is definitely doable if you have the time and are not blocking other trades from working. Plenty of previous posts on the different strategies and pitfalls - have some help and resist the temptation to trim duct pipe too soon (if the flex type) as we played with it quite a bit to get everything as flat as possible. Also trim your plenums prior to plastering.

 

Make sure you allow for the increased diameter of your insulated MVHR pipes to and from your unit - this can add an additional 50mm to the raw pipe diameter, definitely caught us out. 

 

You'll also need to manage the condensation from the unit - in this cold weather there will be surprising amount.

 

We also had a design (from BPC) but it did not survive contact with the enemy (the already installed frame) and by sheer luck and some pencil chewing I got all the necessary duct where it needed to go - at the cost of over ordering on the rigid stuff (still have 5 x 2m lengths of 180mm radial in garage) to give us some flexibility in where to put the distribution boxes. In fairness, BPC did warn us that this was the likely scenario!

 

Haven't comissioned it properly yet - just turned it on when we moved in really (actually a week or so after and the change in air quality was noticeable). Am planning to hire the air flow monitor thingy and do a proper job of balancing sometime soon.

 

That said, it does seem to be working fine - our unit is quite large (but then so is the house)  - a Sentinel Kinetic Plus, no active heating or cooling. We had a full house of 11 + cat + dog over xmas, lots of showers, cooking etc and no obvious issues.

 

Think on how you'll be activating boost. We had our sparky put a PIR in each bathroom and also hook the boost circuit to the light switch, so entry will put the MVHR in boost - (these both also activate our secondary pump for the DHW). Need to add a current detector to get the oven hood to do similar as our hood is not compatible with the dedicated switch in the MVHR. It does have an in-built humidity sensor so this is all belt & braces.

 

 

Edited January 3, 2017 by Bitpipe

[NSS]

I assume the ducting is Lindab spiral tube. This is what we used and did the install DIY without any issues.

[reddal]

Hi,

 

A few points :

• Is the ducting insulated? I think that is important if you want the MVHR to do cooling.
• Worry about potential noise from both the unit and the outlets - we initially had a lot of grief with this. Make sure you test this in any 'sensitive' areas like bedrooms - even what seems like a quiet hiss in a building site is a super annoying loud noise in the dead of night 6 months later when trying to sleep.
• Work out how you will clean out the ducts - which is necessary eventually - maybe immediately if you run the system for testing whilst building work is still going on.

- reddal

[joe90]

Noise is one of my worries and when I visited Jeremy he explained that he had his flow resisters/adjustment  in the manifold rather that at the plenum ( he used semi rigid) but I think somewhere on here it was suggested that flow adjustment was better done at the terminal . 

[le-cerveau]

I remember the post about adjusting the flow rate plenum/manifold and the fact the cleaning a inlet/exhaust plenum is easier that cleaning a manifold, however I was (personally) skeptical as it was a company stating this and they tend to push what they stock.  (I can't find the post at the moment).  I am skeptical as I am struggling to fins a company that can do the whole job for me as I require a flow rate far in excess of the domestic norms (building regs 475m²/h) and most can only cope with smaller systems and aren't prepared to look at doing the ducting and using a third party MVHR unit.  (I know I am a cynic).

In my mind wherever you have a flow restrictor you will generate noise (based on 20 years at sea living with forced ventilation) so if you do it at the plenum then that is where the noise is!

The issue is with this type of system (non manifold) you are limited to your ability to balance it, stand fast  iris restrictors in the pipework once on a single leg and I suspect they are relatively expensive.

Cleaning is also a issue, I know from experience on how disruptive it is onboard Ship when we have the ducts cleaned, so if it is designed to allow cleaning, then you can get at the manifold to clean it relieving the previous problem.

[joe90]

Yes to all the above, my manifold will be very accessible in the warm loft next to the MVHR unit so mine will be easy to clean by design. I am even tempted to build my own manifold if I can get the bits. 

[Alexphd1]

@le-cerveau  bpc ventilation are selling the larger mitsubishi units on their eBay page. I think 850 and 1000m3 units they are advertising. Very competitive price aswell. 

Edited January 3, 2017 by Alexphd1

[PeterW]

9 minutes ago, Alexphd1 said:

@le-cerveau  bpc ventilation are selling the larger mitsubishi units on their eBay page. I think 850 and 1000m3 units they are advertising. Very competitive price aswell. 

 

I saw those but what's odd about the commercial ones is they have no condensate drain as far as I can see 

[Alexphd1]

That's strange. Quoted at 86% efficient so not far off the smaller domestic ones.

[PeterW]

10 minutes ago, Alexphd1 said:

That's strange. Quoted at 86% efficient so not far off the smaller domestic ones.

 Makes interesting reading from the installation manual..

 

• When using the product where it is exposed to high temperatures and humidity (40˚C or higher, RH 80% or higher), or where fog occurs frequently, moisture is likely to condense in the core, and may result in condensation build up in the unit. The product should not be used under such conditions.
• Outdoor air may enter the Lossnay owing to the pressure difference between indoor and outdoor or external winds even when the product is not operated. It is recommended to install an Electrically operated damper to block the outdoor air.
• In a cold weather area, an area with strong external winds or where fog occurs frequently, cold outdoor air, external winds or fog may be introduced into the product when its operation is stopped. It is recommended to install an Electrically operated damper. l When using the product in an environment where there is a window, or opening near the outdoor louvre , where insects are likely to gather around the interior or exterior light , take note that small insects may intrude into the product.
• In a cold weather area, or others, dewing or freezing could occur on the main unit, where the duct is connected, or other sections, depending on the conditions of outdoor air and indoor temperature and moisture, even if they are within the range of operating conditions. Make sure to confirm the operating conditions and other precautions, and do not use the product if dewing or freezing is anticipated. *Example of dewing condition - Outdoor air: -5˚C or lower, dew-point temperature at installation place: 10˚C or higher (When the indoor temperature is 22˚C or higher with the relative humidity higher than 50%, or other)

Not quite sure how to read that last statement !

[ToughButterCup]

4 hours ago, Bitpipe said:

[...]

So you will have no fouls or DHW/cold feed competing for space? If that's correct then you're simplifying life a lot. 

[...]. Also trim your plenums prior to plastering.

[...]

Make sure you allow for the increased diameter of your insulated MVHR pipes to and from your unit - this can add an additional 50mm to the raw pipe diameter, definitely caught us out. 

You'll also need to manage the condensation from the unit - in this cold weather there will be surprising amount.

 

[...] Am planning to hire the air flow monitor thingy and do a proper job of balancing sometime soon.

 

[...]

Think on how you'll be activating boost. We had our sparky put a PIR in each bathroom and also hook the boost circuit to the light switch, so entry will put the MVHR in boost [...] Need to add a current detector to get the oven hood to do similar as our hood is not compatible with the dedicated switch in the MVHR. It does have an in-built humidity sensor so this is all belt & braces.

 

Hugh, (@Bitpipe) thanks for all that.

• Fouls, hadn't thought of it because I'm still in the individual sub-systems loop: Your nudge is especially welcome because it's long past time for me to start stitching the whole thing together in my head
• Condensation: sorted (direct to foul)
• Balancing: I'll DIY or get an expert  @VIPMan ( he doesn't know it yet)
• Just on with the sparky's spreadsheet and diagram, so especially welcome to be reminded - lots of threads elsewhere making that point.

4 hours ago, NSS said:

I assume the ducting is Lindab spiral tube. This is what we used and did the install DIY without any issues.

 

@NSS, I'll double check, thanks!

 

3 hours ago, reddal said:

• Is the ducting insulated? I think that is important if you want the MVHR to do cooling.
• Worry about potential noise from both the unit and the outlets - [...]
• Work out how you will clean out the ducts - [...]- 

 

@reddal

Ducting: Yes, it is; a good deal of thought has gone into that aspect

The Unit is in our Piggery so no fear of noise, from that. But ducts are ducts, in fact ducts will be ducts; and they are annoying things, so we'll be watching the internal smoothness and also how to clean the damn things. I'd clean forgotten about that.

 

What do you use? A massive loofa ? Baggywrinkle? The cat? Keep newts in there and get them to munch any dirt?

 

 

[Simplysimon]

as we have long haired cats, one of the things i've worried about is ouze and fluff in pipes, and had thought to put filters above extract point to enable cleaning and not worrying about pipes

simon

[ToughButterCup]

33 minutes ago, Simplysimon said:

[...] and had thought to put filters above extract point to enable cleaning and not worrying about pipes

 

@Simplysimon,  Sid, our long-haired feline killing machine, is also laid back and long haired, (Tune In Turn On Drop Out, man). And when he molts well, the local birds nests are full of his fur: we know, we've found it lining their nests.

 

He'll be out on his ear if he fouls up the MVHR......

[reddal]

2 hours ago, recoveringacademic said:

What do you use? A massive loofa ? Baggywrinkle? The cat? Keep newts in there and get them to munch any dirt?

 

We have never cleaned the ducts... god knows what is lurking in them now... Its not going to be easy to do so I've been burying my head in the sand over that one...

 

If I was doing it all again I'd make sure there was some more sensible way to access all the ducts to clean them. There are gizmos around that can help - but I doubt they will work for very long runs / twist and turns etc.

[jack]

Our cats ended up going to another home.  I got sick of being woken up every single night to let them in/out, since I'm the only one in the house who can be roused from sleep by noise.  I don't miss them at all, and I'm sure the surrounding wildlife is pleased they're gone too.

[joe90]

Our MVHR does not have a condensate drain either and when I bought it ( over a year ago on EBay) a discussion on this and another forum came to the conclusion it was not a problem!!!!!. ( now you got me worried). With regard cleaning, it has just occurred to me that I have an industrial waccuum vac in my woodwork shop, if at the manifold I made an adapter to fit individual semi rigid pipes I am sure a " blast" with this should remove any fluff etc.

[ProDave]

Surely condensation will only be an issue if it has a cooling function, OR if you have duct runs in a cold unheated loft space?
 

[Bitpipe]

14 minutes ago, ProDave said:

Surely condensation will only be an issue if it has a cooling function, OR if you have duct runs in a cold unheated loft space?
 

 

Our MVHR (which does not have active heating or cooling) has a condensate drain and it's running freely at present - all our ducting is within the heated envelope of the house and the steel duct runs to and from the exterior (the 'cold' side, in winter anyway) are insulated. I used the flexible insulated 180mm ducting to make the connections to the distribution boxes, more for ease than anything else and from there it's the standard 75mm semi rigid duct to plenums.

 

I had a small section of insulation to fit (on a 90 bend) and it was left uncovered for a few months - as soon as we had the first cold snap in October that bit of pipe was sopping wet on the outside so I sorted out the remaining insulation pronto. So that was moisture in the air inside the house condensing on a cold internal surface.

 

I'm guessing that the condensate inside the unit is caused by the (previously warm but now cool) damp air leaving the house, either condensing on the inside of the unit or within the exterior facing pipework - although I stand to be corrected

 

 

Edited January 3, 2017 by Bitpipe

[ProDave]

So on units like the Misubishi with no condensate drain, does is just pool in the bottom of the unit?

 

did you have to put a slight fall on all the ducts to lead to the mvhr unit and hence the drain?

 

I was intending to make the runs from the mvhr to the inlet and outlet vent run downhill, i.e any condensation would run out of the vents.
 

[PeterW]

35 minutes ago, ProDave said:

So on units like the Misubishi with no condensate drain, does is just pool in the bottom of the unit?

 

did you have to put a slight fall on all the ducts to lead to the mvhr unit and hence the drain?

 

I was intending to make the runs from the mvhr to the inlet and outlet vent run downhill, i.e any condensation would run out of the vents.
 

 

I'm assuming the same as you and I'm going to run the main inlet/outlet at a slope, possibly even put a 22mm drain in the bottom of each for safety and run that out onto the roof.

[joe90]

I thought the condensate drain was to collect any condensation given off by the heat exchanger ( hence it is inside the unit and collects via a metal tray below the exchanger) someone somewhere mentioned getting the unit level so that water would run to the lowest part of the tray and out of the appropriate pipe!!! Also in the dim and distant part of my brain I recall Mention of paper elements in the heat exchanger not needing a condensate drain.   HELP.  sorry Iain from digressing your thread.

[TerryE]

Yes, the heat exchanger cools the out-going air and this can drop it below dew point causing humidity to condense out.  The drain takes this away.

 

On another point by reddal above, I would question why you would need the pipework to be lagged (other than the main external inlet and outlet runs from the unit itself.  All of your internal pipework is within the warm environment, and as you are effectively operating the whole house as a single zone, it largely doesn't matter where you add or remove heat within the internal space, IMO. 

[ProDave]

Terry

 

It's not necessarily the case that all mvhr pipework is "within the warm environment"

 

I have recently wired a new house, where the mvhr unit, and a lot of the distribution. pipework is up in the cold, ventilated loft space. Without lagging that would be a condensation nightmare (it might still be so)

 

If I could give just ONE recommendation on modern house design, it would be DO NOT have an old fashioned, ventilated cold roof space where you rely on insulation at ceiling level. Even if you are not doing "room in roof" it is SO much better to have a warm roof design where the insulation is at eaves level. SO much easier to detail to get an air tight house etc etc.

 

This new house I just wired, the builder was so smug about how well insulated and sealed (he thought) the house is, but i still noticed if you remove a light switch on a windy day, you could feel the cold air coming out of the hole, no doubt leaking somehow from that cold loft space into the service void behind the plasterboard.

 

[readiescards]

I previously asked bpc about cleaning ducts and was told this was not longer needed with the antibacterial internal coating that is in their red pipes