17 Mar 2009

Demand Control Ventilation

There was an interesting product on display at EcoBuild that I hadn’t seen before. It’s a whole house ventilation system from a Dutch outfit called Itho and they call it Demand Control Ventilation. You can read the blurb on their website, but the crucial factors here are that:
• it’s extract only
• there’s no heat recovery
• it only works when CO2 detectors say GO
• it uses much smaller ducting than MVHR systems

Why should it be of interest? It presents a challenge to the orthodoxy of built tight, ventilate right which is enshrined in SAP Appendix Q. This dictates that first you should seal up your house, as if it was an aircraft fuselage, and then introduce an air handling system which should work 24/7, even in the summer months. OK, by adding heat recovery, you minimise the heat loss, but nevertheless it’s an awful faffle to get all the kit in place and it needs to be balanced to ensure that it works as designed. There are bound to be a lot of MVHR systems out there that don’t perform anything like they were intended to.

In contrast, extract only on demand ventilation is a much simpler concept. The idea of using CO2 detectors to turn the system on and off is theoretically sound, as it tends to be CO2 concentration in rooms that makes the atmosphere stuffy. CO2 build up in an enclosed bedroom overnight can be a big issue (background reading at www.veetech.co.uk) and it’s one that MVHR deals with very crudely — by pumping warmed air INTO the bedroom rather than extracting it.

In contrast, having a system that turns on only when CO2 levels call for it seems both logical and economical. Also, if people sleep with their windows open (which they do precisely to avoid CO2 build up) then the ventilation system won’t come into play. With an MVHR system, you risk double ventilating.

At the moment, it seems CO2 detection ventilation systems don’t really cut the mustard with Appendix Q and Itho are faced with months if not years of expensive testing to get the system accepted by UK regulators. But I wouldn’t be surprised if, in the long run, ventilation systems like this don’t come to predominate in the UK climate.

POSTSCRIPT: Just spoken with Simon Kaye, one of Itho’s UK sales team, and he confirmed that this system is not Appendix Q compliant, and therefore will not meet the higher levels of the Code for Sustainable Homes. Thus far, only Holland has approved the use of Demand Control Ventilation in low energy housing and they have found that it’s more energy efficient than MVHR, which Itho also market. However, to get accepted into the UK market, this product will have to undergo extensive testing, costing upwards of £40,000. At this point in time, Itho have no plans to do this because it’s not clear that there will be a large enough market to justify testing, so for the time being its likely to remain a curio, rather than a practicality. However, selfbuilders not having to reach Code Level 3 or 4 may be interested in the product.

I also learnt that there is only one CO2 detector but that each outlet valve has a sensor which feeds via Cat 5 cable to the detector. If CO2 is detected as being high in one room (say a bedroom overnight), the fan switches on until CO2 levels are reduced to normal. The other valves dampen while this is happening. There is also a humidity detector built into the unit which is wired to sensors in the wet rooms. The success of the system obviously depends on the detectors continuing to operate correctly. I would think that it would only make sense if there was a service contract in place to check all this out. But then, ideally, an MVHR system should be regularly serviced as well, and I bet very few actually are.


  1. Mark

    From their website they claim: 'This considerably reduces the loss in heat associated with standard whole house mechanical ventilation systems.'

    But this may be reference to conventional MEV rather than MVHR? Perhaps this was clarified when you spoke to them.


  2. You can't pump air into a bedroom without extracting the stale air. All HRV systems I've seen in Canada both extract and supply - in my case the extracts are in the bathrooms and corridors and the supplies are in all the rooms (as we have a full forced-air heating system that's linked into the HRV). Our HRV system is controlled with a humidstat - plus there are 20 minute demand buttons in the bathrooms. We don't open windows in either summer or winter. The balancing of an HRV is not such a big deal either unless you have combustion devices that you don't want spillage from.

  3. OK so there appears to be smoe energy saving trhough lowered ventilation rates, albeit lower than regulatory minimum for health and wetness removal. what about the amount of energy the control system uses ni operation. From reading another report the system controls woudl appear to use a bout 200 kW of energy per year. Taking this away from the energy saving of lower ventilation mya show no energy saving at all.

  4. Maybe I'm certainly not quite in the topic, but are all of the such ventilations http://hardware.nl/ventilatoren/itho demand control? I just want to know whether they are worth the money at all? Or you are talking about other kinds of Itho ventilations?