FAQ MVHR Design/ Specification

Cooker hood: Can a cooker hood be connected up with the MVHR kitchen extract?

It is not recommended as grease still permeates the filters in the cooker hood. We recommend to install a re-circulating cooker hood with a separate HRV intake at least 1m away (filter protected)

Cooling in summer: How does the system provide cooling in summer?

Either the system has got a summer bypass, which supplies fresh air without heat recovery in the warm season, or the HRV system can either be switched to extract mose only or be seasonally be switched off. In this case the ventilation has to be achieved with window opening.

Extractor fans: Are extractor fans necessary with MVHR systems?

Whole house ventilation does usually eliminate the need of extractor fans.

Ground heat exchanger (ground pipe): Is such intake pipe advisable for the UK?

In Europen climate often ground heat exchangers were installed for frost protection of the HRVsystem and gentle summer cooling. Such a intake duct is usually 30-50m long, 200 to 250mm wide and burried normally 1.5m deep in the ground. It needs to be installed in an angle of at least 2.5% to allow condensate run off – together with a condensation drain. The pipe can be split into several smaller pipes to shorten the duct length or can be laid in angles. For protection against fungus growth, it is recommended have these pipes silver lined.

Such ground heat exchangers replace the need for an electrical defroster and provide a gentle summer cooling effect. However this is not as powerful as an air conditioning device. It is a fairly expensive investment compared to an electric defroster. As in the UK frost conditions are not as extensive and severe as in continental Europe, the energy consumption of the defroster is lower here in the UK. We would therefore often recommend to install an electric defroster.

Insulation: How well insulated should a building be to make MVHR efficient?

Heat Recovery Ventilation Systems (MVHR) are beneficial for almost all domestic properties, apart from very drafty ones, where there is a lot of uncontrolled ventilation. New homes are usually being built to standards of air tightness that make the use of a mechanical ventilation system (MV) recommendable in order to avoid humidity build up with all its consequences (mould, fungus mites) and ensure good indoor air quality. The Scottish Building Standards recommend a MV system if the infiltration rate is below 5 m3/h m2 at 50Pa (air tightness test result). Most new built homes achieve these values, even without dedicated air tightness layer unless the builder is building to very poor standards of quality. Especially wet rendered brick and block buildings can turn out to be very air tight. New research shows that even homes exceeding the 5 m3/h m2 still don’t achieve sufficient air quality with trickle vents and no MV system. On the other hand, we have had lots of positive feedback from our customers, what difference the MVHR system had made to their lives, in respect to the excellent indoor air quality. This refers to Passive House dwellings and low energy buildings alike.

To sum it up: It is the level of air tightness, not the level of insulation that makes the use of an MVHR system necessary. Air tightness is an integral part of an energy efficient building, being almost par in its effect on the heating demand as the level of insulation. We would always recommend house builders to ask their architect to concentrate on a well-insulated, thermal bridge free and air tight thermal envelope and make provision for a high performing MVHR system, even if Passive House standards cannot be achieved.

Noise: How noisy / quiet is an MVHR system?

If the system is designed and specified by specialists and correctly installed, it is designed to be inaudible at night and almost inaudible during the day.
Target values for bedrooms are 25 dB(A) (A-weighted noise pressure level of the unit) and 35 dB(A) for the installation room.

Retrofit: Can a MVHR system be retrofitted into an existing property?

The installation of ducting into an existing building is more complex than in a new built, but if e.g. floors are lifted and general renovation works are undertaken, a retrofit is possible.

Semi-rigid ducting: Is this ducting as efficient as rigid ducting and is it SAP tested?

The semi-rigid ducting system has been classified as equally efficient in performance as the rigid duct work. See notes on the SAP Appendix Q website:

“A Semi-Rigid duct system is defined as a unique product supplied by a manufacturer or supplier comprising the components required for assembly of that system. Systems listed within the SAP Appendix Q database have demonstrated that their performance, when installed in a variety of configurations, is at least equal in terms of aerodynamic performance to SAP performance data for Rigid duct. It is therefore possible to select Rigid duct performance data when listed Semi-Rigid duct systems are specified for a dwelling. Refer to Specification Requirements for further details.”

Trickle vents: Do we still need trickle vents, when an MVHR system is installed?

If the whole house ventilation system has been correctly designed, trickle vents in windows are not necessary any more. In Scotland compliance to BRE Digest 398 is often required.

Toilets: Can MVHR systems be connected to compost toilets?

Yes, this is possible and eliminates the need of other extractor fans in these rooms.

Wood burning stoves: What is to be considered when wood burning stoves are installed?

We would always recommend to install wood burning stoves with an air intake from the outside (into the combustion chamber). There is a range of such stoves available. However, if a conventional stove with room dependant air intake needs to be installed, it has to be made sure that no negative pressure in the respective rooms can occur to avoid the danger of drawing combustion fumes into the dwelling. Such stoves should only be installed in supply rooms and not in extract rooms (such as a kitchen). Our new units ‘focus’, ‘novus’ and ‘santos’ have volume flow constant controls, which increase the fan power when e.g. filters start to clogg up. Our other units can be fitted with such controls as well. These controls, in combination with the commissioning of the system, help to keep the system balanced; that means that the same amount of air is introduced as well as extracted.

FAQ MVHR operation

Filters: How often do the filters need to be changed?

Filters (in the HRV unit, at the defroster and in the kitchen) need to be inspected regularly (every 3 to 6 months) in order to prevent mal-ventilation and increased energy use. If the filters are visibly stained, they should be exchanged. In order to increase the life of a filter, it could be cleaned once. However they should be exchanged at least once a year.

Noise: The exhaust of our MVHR system has become very noisy.

The Paul novus and focus ranges and the ComfoAir Q systems have got an automatic adjustment of the pressure levels. With completely dust-soiled filters, the fan speeds can go up to boost level, which causes some audible noise at the exhaust grille. If the filters are changed, this noise will disappear again.

Traffic fumes: Can a MVHR system filter help filtering out diesel fumes from traffic in a busy city centre?

Yes, that is possible. We offer two kinds of active charcoal filters: Either for rural smells or for paint and diesel fumes. These help neutralizing respective fumes efficiently, but need to be replaced in regular intervals as the active medium is used up. Such filter can be installed in the ISO defroster box (instead of the G3 filter) or a filter box for the intake air.

Rural smells: Can a MVHR system filter help filtering out smells from farm animals and other farm operation?

Yes, that is possible. We offer active charcoal filters designed to neutralize rural smells. Such filters can be installed at a filter box for the intake air. These filters need to be replaced in regular intervals as the active medium is used up.

Summer bypass operation

The summer bypass is a passive way of cooling, which is by far not as powerful as active cooling (air conditioning), but it also uses only a fraction of the energy. The novus, climos and santos units have an integrated automated 100% bypass. , which allows outside air to enter the building without heat recovery. The bypass is opening and closing according to the temperature readings in the extract (indoor temperature). The threshold can be adjusted in the settings/bypass menu. The upper threshold is the main one. For your information: the lower threshold is only applicable in the rare instance when it is warmer outside than inside and you wanted to get warmer air into your house.

If you turn up the ventilation rate (level 2 or 3), the effect of the summer bypass increases. The best way to get rid of excess heat is still the opening of windows. Please do not feel restricted to open your windows, as it is the case with air conditioning. The MVHR system operates on a complete different level, without recycling of air and there is hardly any waste of energy if you open the windows in summer.

If you can open your windows, please feel free to do so and turn down the MVHR system to level 1.

If you cannot open your windows, e.g. if you are not at home during the day, leave the MVHR on level 2 and open your windows when you come home.