Improvised summer bypass

[haddock]

Really struggling with these hot nights and unfortunately not having a summer bypass fitted on my Brink Renovent HR Medium. Can't leave windows open because it's a bungalow and, well there aren't really any, only patio doors.  Unfortunately not a good idea to leave those open at night around here.

 

Tonight I've tried improvising a summer bypass by simply removing the heat exchanger (and the filters) from the unit. To be honest it's rather late and I can't quite get my head around the effect of this. Will it just be pumping air in and directly out without going around the house? It doesn't seem to be. The temperature at the floor output vents has dropped by 2 degrees already so I'm wondering if this harebrained scheme might actually be working!

 

[Ferdinand]

Welcome to buildhub.

[Jeremy Harris]

Welcome.

 

For summer bypass to work and be effective, it has to connect the external fresh air inlet to the internal fresh air supply and separately connect the internal extract to the external

exhaust.

 

It's a mixed feature.  Having full bypass on when the outside air temperature is hot will make the house hotter - you are better off having the heat exchanger in place, as the chances are that may cool the incoming air.  Having full bypass on any time the outside air temperature is cooler than the house internal temperature will help to cool the house a bit (not muc, as the flow rate is pretty low)

 

If you have an MVHR with an adjustable bypass (like some Genvex models) then you can see from the display when they start to ease in bypass as the outside air cools down, and shut bypass off as the outside air rises.

 

For models that dont have an integral motorised bypass valve, you can usually buy a dummy heat exchanger, often called a "summer box".  This is the same size as the heat exchanger but just has two clear internal passages that connect the fresh air intake to the fresh air supply and the extract to the exhaust.

 

It would be fairly easy to make a summer box, using thin plywood or aluminium sheet, made the same external dimensions as the heat exchanger, with a vertical internal divider and four openings on each of the four angled sides that live up with the duct pipes/fans.  If you do a web search you may well find images of a summer box, like this one for Genvex units that don't come with the automatic bypass option:

 

 

 

[Temp]

4 hours ago, JSHarris said:

For summer bypass to work and be effective, it has to connect the external fresh air inlet to the internal fresh air supply and separately connect the internal extract to the external

exhaust.

 

I think you may only need to do the inlet bypass. I think you could leave the extract air going through the heat exchanger - it just wouldn't be able to heat the incoming air.

[Temp]

I think having a motorised valve in the bypass would be useful. You could arrange for it to open and close depending on the relative temperatures or time of day. eg bypass at night when it's cooler outside than in. Heat exchanger in the daytime when it's hotter outside then in

[Jeremy Harris]

1 hour ago, Temp said:

 

I think you may only need to do the inlet bypass. I think you could leave the extract air going through the heat exchanger - it just wouldn't be able to heat the incoming air.

 

 

You're right, looking at the back of ours that's exactly what it does.  It has a channel running externally in the rear case with a position-controlled motorised flap valve, that allows any proportion from 0% to 100% of the incoming fresh air to bypass the heat exchanger.  The exhaust still goes straight through the heat exchanger and out.

[jack]

Ours is the same, but the flap isn't proportional. 

[haddock]

Many thanks for the background on this. Main takeaway for me is that the flow rate is too low to cool the house really effectively, but it's still something I'd like to get sorted out. I've had a quote of £1,000 to retrofit the official Renovent automatic bypass but wonder whether the money might be better spent getting a new unit from another manufacturer that has a built-in bypass and also humidity/CO2 sensing. I'm currently running the Brink at full blast (300m3/hr) all the time because there's a problem with huge amounts of condensation forming in the ceiling voids. That may be a posting for another day.

[Jeremy Harris]

You can build a bypass if you have room.  Buy two of these (shop around for the best price and get the right size and type for your ducting):

 

 

Two of these (same comments above about size and type):

 

 

And whatever duct and bends you need to connect the bypass up.

 

Connect one arm of each of the two "Y"s to the MVHR extract and exhaust, with one of the two iris valves fitted on one or the other side (this shuts off the heat exchanger).  Connect the "bottom" of each "Y directly to the room extract duct and the duct leading to the external exhaust.  Connect a bit of duct as a bypass between the two remaining "Y" connections, with the second iris valve.

 

To switch to full bypass, close the valve that connects to the MVHR and open the valve in the bypass arm.  The iris valves allow you to blend the amount of bypass if you wish, much as the Genvex system does. 

 

You can buy motorised valves/dampers and so make the bypass operate electrically from a switch or control system at a fair bit of additional cost:

 

 

 

[ProDave]

On 27 July 2018 at 13:23, Temp said:

 

I think you may only need to do the inlet bypass. I think you could leave the extract air going through the heat exchanger - it just wouldn't be able to heat the incoming air.

That is exactly what my Kingspan unit does.

[jack]

2 hours ago, ProDave said:

That is exactly what my Kingspan unit does.

 

My Brink unit is the same. It makes sense - why provide two bypasses and valves when one is sufficient?