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Author Topic: DIY MVHR  (Read 654 times)
Stig
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« on: August 31, 2020, 09:44:47 AM »

I'd been idly thinking about doing this for a while so when lockdown happened it seemed an ideal project to keep me occupied.  My windows don't have any trickle vents and although the house is by no means airtight (gas heating drawing air from the room with airbrick in the kitchen) there is no ventilation upstairs without opening a window, also no extractor fan in the bathroom.  After a bit of research online I came across this design from an old magazine:
https://www.builditsolar.com/Experimental/DIYHRV/DIYHRV.htm

but decided that a counter-flow heat exchanger would be more efficient than cross-flow, there was also the consideration of the size & shape and whether it would fit through the loft hatch!  After many back of envelope designs I came up with this:
« Last Edit: August 31, 2020, 09:47:01 AM by Stig » Logged
Stig
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« Reply #1 on: August 31, 2020, 09:54:40 AM »

So, having found a supplier that was still operating during lockdown I ordered 10 sheets of polycarbonate greenhouse panels and a tube of suitable sealant (I used two in the end) and got to cutting:


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Stig
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« Reply #2 on: August 31, 2020, 10:00:02 AM »

The strips are used to make the dogleg channel between the sheets:


and seal the edges as per the original design in the magazine.  After much sticking and sealing - and that was a lot of work, I broke my (metal) sealant gun with the effort - I had the heat exchanger core:


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Stig
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« Reply #3 on: August 31, 2020, 10:11:35 AM »

I then built the box to contain it, with the required partitions and suitable bracing.  The magazine design used marine ply but as I couldn't just go out and get that (lockdown) and I was concerned about the weight I used some of the remaining polycarbonate.



I used a couple of high performance computer fans (12V) so I could control the speed with a home-made PWM circuit.  Nowhere near as powerful as domestic extractor fans but much quieter and I didn't fancy trying to make a speed controller for a 240V induction motor.



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Stig
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« Reply #4 on: August 31, 2020, 10:25:30 AM »

Got it installed yesterday with extract from a ceiling vent in the bathroom and supply to the main bedroom.  You can't see those in the photo as they're buried under the loft insulation.  Fresh air comes from the soffit vents (I fitted insect mesh so I don't have to clean the filters so often) and exhaust goes up to a ridge tile vent.



I've also put some insulation over it in the form of bubble wrap and sections of a hot water tank jacket.  What you also can't see is the condensate drain tube as that's also under the loft insulation.

I ran it for the first time last night as it's been getting a bit chilly to have windows (or the loft hatch) open all night recently and all seemed to go well.  Air wasn't at all stuffy and fans were inaudible at about 30% power, yet to see how well it clears humidity and smells from the bathroom but at full power you can't really feel much of a breeze from the vents so I don't think the boost timer is going to do what I'd hoped.  The fans are supposedly high flow even with some resistance but a few metres of ducting plus a filter plus the heat exchanger obviously cuts the flow a bit.

« Last Edit: August 31, 2020, 11:11:07 AM by Stig » Logged
pantsmachine
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« Reply #5 on: August 31, 2020, 10:57:17 AM »

Sounds great, I wish my phone showed photos at the moment. I look forward to seeing this when home.
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« Reply #6 on: August 31, 2020, 12:04:54 PM »

Well done Stig. Would it be less efficient without the dog-leg dividers?
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Countrypaul
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« Reply #7 on: August 31, 2020, 12:26:14 PM »

Looks good, I couldn't realy be sure if I could feel a draft through our MVHR system on low setting, but if I put a piece of paper over the inlet in the bathrooms it would stay put, whereas if the system was off it would just fall.

I was warned not to use flexible pipe on any sharp bends or for any more than a meter length as it supposedly creates turbulence and restricts the flow (don't know how significant that is). The semi-flexible has an extra layer on the inside that is much smoother - but then that is narrower (70mm e.d.) and probably sued in longer lengths since it goes to each room from a central location (some runs as 15m).

If you attic is warmer than outside beware of getting condensation on the inlet pipe especially  during still frosty periods. Don't know whether that can be a real problem or not, but again I was warned about that however ours is within the thermal envelope right upto going through the wall, so would be much more prone. As a result our pipes to theoutside are wrapped with polystyrene (about 1") and then rockwool above and below (150mm pipe runs between the joists). 
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todthedog
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« Reply #8 on: August 31, 2020, 01:10:28 PM »

Cracking Stig genuflect
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Kidwelly South Wales
Stig
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« Reply #9 on: August 31, 2020, 04:13:03 PM »

Well done Stig. Would it be less efficient without the dog-leg dividers?

That was my thinking, I was trying to get the longest counter-flow path I could and I needed to put in a central support to stop the stack sagging anyway.  From what I've read a cross-flow design might have 60% efficiency, I'm hoping this is around 90% but I'm not nerdy enough to try to measure it!

Also, forgot to mention that each fan only takes 1W at full speed so very lower power draw.
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pantsmachine
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« Reply #10 on: September 03, 2020, 12:12:57 PM »

Wow, That's a truly epic bit of DIY. Deeply impressed and looking forward to hearing on its performance,  genuflect
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8.045kWh PV system with Solar edge
7.2kWh Pylon tech battery storage
18kWh Heater Storage
Solar I boost charging 12kWh 210 ltr OSO system tank
Deep insulation, air leak controlled home
Zoned & Balanced CH wet system
Hive 2
Low energy bulbs
24 kW Leaf
Masanobu Fukuoka garden & wooden hot tu
desperate
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Backache stuff!!


WWW
« Reply #11 on: September 12, 2020, 04:09:33 PM »

Brilliant stuff, that's a very worthy project to take on, energy savings and good air quality without spending a load of wonga on hi-tech kit.

Don't forget the updates, I might have to copy that Grin

Desp
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still a crazy old duffer!
AndrewE
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« Reply #12 on: September 12, 2020, 07:51:11 PM »

Looks good, I couldn't realy be sure if I could feel a draft through our MVHR system on low setting, but if I put a piece of paper over he inlet in the bathrooms it would stay put, whereas if the system was off it would just fall.

I was warned not to use flexible pipe on any sharp bends or for any more than a meter length as it supposedly creates turbulence and restricts the flow (don't know how significant that is). The semi-flexible has an extra layer on the inside that is much smoother - but then that is narrower (70mm e.d.) and probably sued in longer lengths since it goes to each room from a central location (some runs as 15m).

If your attic is warmer than outside beware of getting condensation on the inlet pipe especially  during still frosty periods. Don't know whether that can be a real problem or not, but again I was warned about that however ours is within the thermal envelope right upto going through the wall, so would be much more prone. As a result our pipes to theoutside are wrapped with polystyrene (about 1") and then rockwool above and below (150mm pipe runs between the joists). 
A friend was warned not to use flexible pipe specifically because all the sags accumulate condensation unless they are effectively inside the room.  His heating engineer friend had been called out to a system which had failed because of the condensate forming water-seals in the U-bends...
Gerry has built his system with all rigid pipes (hung off rafters or propped off ceiling joists with plywood cradles) and tried to get a fall towards the MVHR unit.
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