MVHR Spec??

[Visti]

We've been quoting for MVHR's left right and center these past few weeks and have come to several conclusions:

1. We're confident we can DIY the installation
2. We think we can design the layout of the ducting (for a radial system with semi-rigid ducting)
3. We have no idea why spec's (and therefore costs) being quoted to us vary so immensely (suppliers & units below)

 

 

Calculations for Whole Dwelling Ventilation Rates (Table 5.1b)

• Calc 1 (# rooms) 4 bedrooms: 25 l/s = 90 m³/hr

• Calc 2 (internal floor area) 0.3 l/s/m² * 184m² = 55.2 l/s = 198.72 m³/hr

• Calc 3: (max # inhabitants)  14 * 4 l/s = 201.6 m³/hr (given our plans, we can sleep up to 14 people at peak times like Christmas and Easter)

 

That leaves us with a unit requiring a flow rate of at least ~200m³/hr. Clearly we don't wish for the system to run at 100% due to noise and maintenance, so a unit with a capacity greater than this would be ideal. But is 50, 60 or 70% the best measure? And what pressure differential should be be considering? 50, 100, 150 or 200? There seems to be no standard way to compare these units!

 

I'm an aerospace engineer by trade, so I'm certain that we can go ahead with the installation ourselves as well as the design. Then again, the same profession makes it difficult to select THE unit we should go for without thoroughly understanding that is it indeed the correct specification for us.

 

Another compounding factor appears to be that Genvex is highly recommended in previous posts despite the units purchased by @PeterStarck and @JSHarris being almost double the cost of the units above (~£4k). Is this due to the inbuilt heat pump or combi boilers? or am I missing something?

 

Edit: I should add that the total internal volume of the house is about 586m³, larger than the typical volume per m² which makes the sizing even more tricky!

Edited November 11, 2017 by Visti

[le-cerveau]

You can definitely DIY it.  I just have, see my latest blog entry:

I did all the calculations my self, I have a floor area of 438m² which was my defining limit giving a unit requirement of 472.92 m³/h, so beyond one standard unit, I ended up with 2 x Brink Renovent Excellent 400 plus units.  From the blog you can see it can be installed however I did have the help of my builder (with all the tools) but you definitely need 2 people when running pipes.  I spent a long time working out duct runs which paid dividends during the fitting even though we varied, as I knew the plan back to front alterations on the fly were easy. 

 

All my equipment came from CVC, they also supply Ubbink ducting, though it is not on the website, just e-mail them and they will send you a price list for all the components.

 

For power I went with aiming between the 50-70% mark, with my theory being the slower the fan runs the less noise and if you look at the performance tables they are more efficient (heat exchange) at lower settings, so personally I would go towards the 400m³/h.  As for pressure difference that depends on your duct design, the longer more complex the duct/manifold/terminal arrangement the higher the pressure!

 

Reading into @JSHarris blog and subsequent conversation he states that he probably wouldn't fit a Genvex in hindsight (wait to be corrected) as it is not really needed in his situation. and @PeterStarck units also does DHW so not just an MVHR unit.

 

For your calcs personnel calcs is based on 2 personnel for the first room and one thereafter, not what you can cram in, it is all building regs based, you need to work out if your extract requirements exceed your floor area supply requirement and base your system on the greater of these.

 

[Gone West]

Yes I have a Genvex Combi 185LS which is a compact unit housing a 185l UVC, EASHP, which provides warm air and heats the DHW, and the MHRV unit.

[Visti]

Thanks @le-cerveau, that was a very good read. I'll be keeping that in my back pocket for a refresher before I start on my own installation in a few months *I hope*

 

The CVC quote has certainly been the most substantial by far amongst all of those I've engaged, and they appear to have quite a balanced price given the spec of the unit chosen and ducting combined.

 

I'm tempted to DIY source the components but I'm not sure I'll get it substantially cheaper for it to be worth the lack of support! We'll see following some more research.

[IanR]

Have you tried Airflow direct... for the Adroit unit.

 

Airflow will do the system design in house (free), which is very comprehensive, and then provide you with the BoM to hawk around their distributors to get the best price.

 

I found that BPC don't get such good terms with Airflow so their pricing wasn't keen. Travis Perkins ended up getting me the best price. They had no idea what they were selling me, just quoted on the Part Numbers from the BoM provided by Airflow.

 

Airflow provide you a target price which you should be able to achieve better than through a distributor. That price provided was also "negotiable". When I hadn't placed an order within a few weeks of getting the system design I was chased up by their sales team who promptly re-quoted when I suggested I might go with a different brand of unit. When I sent that re-quote through to TP they came back with an offer that was a few hundred pound less that the target price.

[Bitpipe]

I have a 400m2 house and went for the Sentinel kinetic plus from BPC - when I calibrated it to meet the regs for whole house ventilation, it required both supply and extract fans to be at 40% (I easily exceeded all the other requirements).

 

In reality I run it lower than this (supply 30%, Extract 37%) which gives me a balanced system and ideal conditions.

 

Lots of threads on installation and commissioning - it's almost a self build rite of passage.