Heat recovery extract fans- payback?

[hairyben]

I see heat recovering extract fans can be had for around £150-£200 or so and seem like a smart idea, anyone have a general idea or able to calculate how much in £££ you blow through the wall using an extract fan?

[caziques]

The answer is...........it depends.

In winter an extractor fan will be wasting energy, if a heat recovery unit is incorporated some of this can be reclaimed (perhaps 50%).

The main factor will be how long the fan is on for.

In most cases I doubt a heat recovery unit extractor fan will actually have a sensible payback time.

More details needed.

[TooLateForAName]

Depends how good the house is.

If the building is decently air tight you need forced ventilation anyway and the heat recovery makes lots of sense.


We have a heat recovery unit in the bathroom. Runs at low speed |(2W) continually and goes high on humidity. It has certainly been an improvement on the previous ordinary extractor.

Look around though. Ours had an RRP of £200+ but I bought it through an ebay guy (masterspartan I think) at a huge discount. Vent Axia unit.

[hairyben]

I suppose another way of asking is, any ball part figures or calcs on how much gas/cost to heat a cubic meter of air by 20c?

Using a good condensing boiler.

[944fan]

hairyben said:

I suppose another way of asking is, any ball part figures or calcs on how much gas/cost to heat a cubic meter of air by 20c?

Using a good condensing boiler.

That calculation would be irrelevant. You need calculate how much energy your house looses through the walls, floors and roof and the rate of ventilation in the property - which depends on the U-Value of your buidling elements.

Once you know how well the property retains heat you can then calculate the heat demand for the dwelling - i.e. the energy required to heat the property to 20 degress (for example) based on an avergae outside temperature.

In order to calculate how much energy you can recover from a MVHR system you need to know the Specific Fan Power of the system, the efficency of the heat exchanger, whether the ductwork is rigid or flexible, and whether the duct work is insulated.

[JM]

hairyben said:

I see heat recovering extract fans can be had for around £150-£200 or so and seem like a smart idea, anyone have a general idea or able to calculate how much in £££ you blow through the wall using an extract fan?

When you say extract fan are you meaning like a bathroom or kitchen extract fan?

[944fan]

JM said:

When you say extract fan are you meaning like a bathroom or kitchen extract fan?

I assumed he means a Mechanical Ventilation System with Heat Recovery - a whole house system with a fan and heat exchanger in the loft and ducting to the bathrooms.

[hairyben]

944fan said:

That calculation would be irrelevant. You need calculate how much energy your house looses through the walls, floors and roof and the rate of ventilation in the property - which depends on the U-Value of your buidling elements.

Once you know how well the property retains heat you can then calculate the heat demand for the dwelling - i.e. the energy required to heat the property to 20 degress (for example) based on an avergae outside temperature.

In order to calculate how much energy you can recover from a MVHR system you need to know the Specific Fan Power of the system, the efficency of the heat exchanger, whether the ductwork is rigid or flexible, and whether the duct work is insulated.

I'd have thought the energy required to heat a cubic meter of air by a given temp would be a constant regardless of losses? All increased losses mean is you're heating more air, but aside from boiler workload/efficiency issues it's the same gas per m3 whether it's 1 or 1,000 meters.

JM said:

When you say extract fan are you meaning like a bathroom or kitchen extract fan?

Yes, I'm considering the benefits of a heat recovery system instead of boggo extractor fans

[herewego]

hairyben said:

I see heat recovering extract fans can be had for around £150-£200 or so and seem like a smart idea, anyone have a general idea or able to calculate how much in £££ you blow through the wall using an extract fan?

I reckon it costs around £12/year to run one 80m3/h extractor fan for one hour/day in lost air heated by 10 degreesC. This is using electrical heating at 12p/kWh. gas heating is normally 1/3rd of that price.

Edited by herewego on Saturday 22 October 15:34

[944fan]

hairyben said:

I'd have thought the energy required to heat a cubic meter of air by a given temp would be a constant regardless of losses? All increased losses mean is you're heating more air, but aside from boiler workload/efficiency issues it's the same gas per m3 whether it's 1 or 1,000 meters.

No exactly, forgetting the extact system for a minute as you heat the air you are constantly loosing heat through the building fabric so it takes more energy to heat the air to a specified temperature. The bigger the temperature difference between the inside and outside temperature the greater the energy loss. The fan creates an air change so yes you will be heating new air.

If you really want an assessment of how much money you can save with a MVHR system then send me the following details and I will do a SAP calculation for you and model do a before and after calc with and without the MVHR system:

Age of construction, house type and detachment, wall type, wall insulation, roof insulation, heating system (boiler type and which controls), how your hot water is heated, number of stories, approx size in m2, approx length of wall exposed to air, double glazing coverage (and approx year of insatllation), number of wet rooms (i.e. rooms you want to connect the MVRHR system too), any renewable technology you have, number of habitable rooms (not kitchens, bathrooms, utlity rooms.