Reusing Warmair ducting for MVHR

[severnside]

We are renovating our 1970s house, there are couple of things we need to resolve. One of them is ventilation. 

Currently house has warm air for heating which is working well but we will change it to UFH and radiators (First Floor). Warm air system has vents in every room (floor level) and 3 extraction vents at wall/ceiling level in hallway. This is solid ducts and designed for high airflow rate.

Some Questions we have:

- Could we reuse the ducting for MVHR by replacing the water to air heat exchanger unit with MVHR?

- The MVHR unit will be going in cold garage, will this be OK?

- Current vents are at floor level and it's OK for warm air as warm air rises. Will they work well for MVHR

- I have read on forum that cooling via MVHR need higher flow rate, I believe this ducting should help achieve that but will probably need bigger MVHR unit

 

[Bitpipe]

1 hour ago, severnside said:

We are renovating our 1970s house, there are couple of things we need to resolve. One of them is ventilation. 

Currently house has warm air for heating which is working well but we will change it to UFH and radiators (First Floor). Warm air system has vents in every room (floor level) and 3 extraction vents at wall/ceiling level in hallway. This is solid ducts and designed for high airflow rate.

Some Questions we have:

- Could we reuse the ducting for MVHR by replacing the water to air heat exchanger unit with MVHR?

- The MVHR unit will be going in cold garage, will this be OK?

- Current vents are at floor level and it's OK for warm air as warm air rises. Will they work well for MVHR

- I have read on forum that cooling via MVHR need higher flow rate, I believe this ducting should help achieve that but will probably need bigger MVHR unit

 

 

A few thoughts...

 

1) MVHR uses ceiling vents as you want to maximise cross flow of air and reduce 'dead zones'. You extract from warm 'wet' rooms (kitchen, bathroom, WC) and supply to habitable rooms (beds, living, dining etc).  Doors need a 7600mm2 gap at the bottom (10mm on a standard 760mm door) and the idea is that the airflow goes from the supply to the extract with the maximum room coverage, under the doors etc. Risk of your system is that it just moves at floor level in the supply rooms and does not ventilate the upper parts of the room. Your extraction locations will also not work to clear 'wet' rooms of stale air, so they will still need through wall ventilation which will render your MVHR system somewhat useless.

 

2) MVHR is for ventilation and not heat transfer as the airflow is quite low. The ventilation is energy efficient as the heat from the extracted air is used to warm the incoming air so losses are minimised. However it is not an effective heating or cooling system unless you house meets very high insulation and airtightness standards - i.e. a passive new build.

 

3) A renovated 70's build is unlikely to meet the airtightness requirements (< 2 m3/hr/m2) for the heat recovery element of a MVHR to be effective. Even a bog standard new build only needs to meet 10 m3/hr/m2. Passive houses would be well below 1 m3/hr/m2. As a ventilation system it could work ok but only if your vents are in the correct locations.

 

4) If the unit is in a cold space then all the ducting between it and the house needs insulated.

 

I think it's a nice idea but you're probably better off to remove the heating ducts and start again with MVHR manifolds, ducting and vents that are designed for that purpose. Are you looking to improve airtightness? You should be able to improve your current performance but will hit a practicable limit due to the construction design of the house - the most common loss zones are at joist and roof level where the timbers pass through the wall.

 

 

 

[severnside]

Thanks for the feedback, I did doubt that this might not be most efficient solution. The reason I considered this options is

1. It's solid and bigger ducting. Currently it heats our 250m2 house to 20C without any issue. Current air handling unit circulates 1200m3/h.

2. It should be quieter because of size 

3. It's already there so no additional cost and disruption to install

 

Some negatives as you pointed out

- Extraction is not at optimal points, It's in hallway downstairs and upstairs. So moist stale air from Kitchen/Bathroom need to go via Hallway

- Input is at floor level and extraction at wall level

 

 Also house won't be very airtight, though we will pay attention to any new work being done to good standard. We will also try to do less disruptive work like sealing plaster around perimeter from loft side and sealing any light fittings.

 

So I guess I would be better off putting new ducts at ceiling level.

 

Edited April 22, 2021 by severnside