There is a long running debate over whether it is more efficient and economic to 1) push in external air continuously (positive input) so that humid or polluted air finds it’s way out through cracks, 2) remove vapour or polluted air continuously from source, such as a bathroom, or 3) remove it as and when vapour is generated; bathroom, drying clothes, kitchen and respiration.
To add to this mix you should consider extracting pollutants;
- VOCs (Volatile Organic compounds) such as from new furniture, carpets, glue, cleaning products, hairspray, air fresheners etc,
- Carbon dioxide from respiration,
- Small and large particulate matter (PM – such as from building works and break dust),
- Radon gas (over granite e.g. Cornwall),
- Nitrogen dioxide if you live near a busy road – in such cases it would be best to draw in and filter air.
The most common alternatives are;
- Continuous flow in from a central unit such as PIV, (Positive Input Ventilation) or dMEV (decentralised Mechanical Extract Ventilation). These units push in cold dry air to be displaced by warm air of average humidity.
Typical costs are £1,200 including installation.
They should not be used instead of bathroom ventilation and are at risk of “short circuiting”, that is vapour push in, blowing out of the nearest gap in a window, without removing vapour. - Continuous flow out, such as from a bathroom extractor fan.
These pull out warm humid air, to be replaced by cold air through small cracks around the property.
Typical costs are £110 plus £200 – £250 for installation by an electrician – or general builder if the original fan was light switch operated with an overrun. - Standard extractor fans fall within the category of intermittent or purge ventilation. They need to overrun by 30 minutes after a bathroom is used, with an extraction fan running for at least 30 minutes after a vapour is generated. These fans tend to cost £15 – £50, but require an electrician to install initially, making sure to be wired up to both the light switch and electrical mains. Typical installation costs are around £200 – £250 plus cost of the fan.
All methods above should be accompanied by an externally ducted kitchen extractor fan (used when cooking) and clothes should be dried outside, in a vented tumble dryer or in a vented room.
Cost comparison – continuous-flow to intermittent
This does account for any additional background airflow, such as opening windows, which may be required more often with intermittent ventilation to achieve the same air quality.
Continuous flow running costs considering heat loss
| Continous flow extractor | 7.5 l/s |
| Seconds in a year during heating season (6 months) 60*60*24*180 | |
| Air flow during heating season, in M3 | 116,640 M3/year |
| Specific heat capacity of air at 20 °C and 1 atm pressure is 1.005KJ/Kg/K | |
| 1 M3 air has a mass of 1.205 kg | |
| kJ for each degree | 141,254 kJ/°C |
| Ave. temperature difference between inside to outside during heating season. | 8 °C |
| kJoules lost per year | 1,130,032 kJ |
| Convert to kWh: 1 KWatt hour (KWh) is equal to 3,600 kilojoules (kJ) | |
| kWh lost over year | 313.9 KWh |
| Average energy cost for KWh @ 15p per kWh | |
| Cost of replacing heat from extractor | £47.08 |
| Plus electrical cost of running fan | £6 |
| Running cost per year | £53.08 |
To this you should add depreciation of fan over about 5 years and annual maintenance (easy to do) – see tips.
*The increased kWh over standard (purge) ventilation is a release of about 120Kg atmospheric CO2 p.a.
Standard bathroom extractor (30 minute overrun)
| Intermittent flow rate | 15 l/s |
| Assume 1 hr per day, seconds per year when heating 60*60*180 | |
| Annul flow in M3 | 9,720 M3 |
| Specific heat capacity of air at 20 °C and 1 atm pressure is 1.005KJ/Kg/K | |
| 1 M3 air has a mass of 1.205 kg | |
| kJ for each degree | 11,771 kJ/°C |
| Temperature difference between inside and outside | 8 °C |
| kJoules lost per year | 94,169 kJ |
| Convert to kWh: 1 KWatt hour (KWh) is equal to 3,600 kilojoules (kJ) | |
| kWh lost over year | 26.2 KWh |
| Average energy cost for KWh @ 15p per kWh | |
| Cost of replacing heat from extractor | £3.92 |
| Plus electrical cost of running fan | £0.5 |
| Running cost per year | £4.42 |
To this you should add depreciation of fan over about 5 years and annual maintenance (easy to do) – see tips.
Expert opinion
Professionals are invited to comment. Some say there is no right or wrong solution. While in theory this might be true, users deserve debate and consensus over better and worse methods of vapour control, especially given Covid and the need to reduce carbon consumption.
I asked a ventilation expert who I admire to check my calculations. He heads-up one of UK’s leading ventilation manufacturers. I have bought his products for personal use, and have influenced many landlords and owners to do the same, without personal profit.
He says that my calculations “are flawed and the overall ventilation heat loss in a home is similar with systems that adopt intermittent fans as part of the overall ventilation strategy in a home, to those which adopt continuously running fans. In fact, [his] view is that if a guaranteed minimum whole dwelling ventilation rate is required to be provided 24/7/365 to control not just water vapour but all the other pollutants in a home, then a ventilation system using continuously running extractor fans is likely to cost the occupants less to run.”
Conclusion
I understand why a manufacturer would point out the need for whole dwelling ventilation of all pollutants, and I don’t wish to sound like I am questioning the efficiency of their products. However, I still think my question is reasonable and calculation a valid attempt.
For example, say I have two bathrooms, one with a continuous flow extractor fan, one with a standard “intermittent” fan – as is the case. What is the thermal impact of replacing the standard fan in the second bathroom with a continuous flow extractor fan, all else being equal?
The comparable thermal loss of running one additional continuous flow fan compared to a standard fan can’t be a zero thermal loss, or even a thermal saving. I believe an additional £50 p.a. is in the right ballpark, but would gladly update this with improved calculations.