Commonly asked questions about Dwell-Vent

Why is it that 'supply-air' windows are so energy efficient?

The flow-regulating vents are fitted at the top of the window where the ventilation air enters the room. They control the airflow so it is smooth as it passes between the glass panes of the window. In this way an uninterrupted barrier is formed across which it is difficult to heat to pass by conduction. Consequently the only mode by which heat can cross the gap is by radiation. Research has shown that the use of low-E glass for the inside pane forms a very effective radiation barrier. 'Supply air' windows can as a result achieve phenomenally efficient insulation values, in excess of any window that is commercially available at the present time.

Two sheets of glass and low 'E' coating will not give a particularly good 'U' value. Presumably the 'U' value is also going to depend on the wind conditions (speed of the air between the panes) , so how are you going to quantify the benefits?

The U-Value does indeed depend upon the flow through the windows. 8l/s per metre squared of window area will give a U-value of 0.7W/m2K. The vents on the window are self-regulating and are engineered to provide 8-10l/s per window under pressures differences typically created by a Passive Stack, which is the driving force for the ventilation. As they are self regulating they damp down varaitions due to changing wind conditions outside. We use advanced simulation tools, coupled with the experimental results we have gathered over time, to make predictions of system performance.

Where very wide windows are involved, will the same system apply?

The system will still apply but a U-value of 0.7 is only achieved if the flow rate per metre squared is 8 l/s. A wide window with a larger area will therefore have a larger U-value with this flow rate. The flow rate could be increased by having more vents but does the room require this much ventilation? It may be better to have a portion of the window ventilated and a protion as standard but it would depend ultimately on the overall building configuration of the system.

Surely since air is drawn from outdoors through the windows the inside of them must get dirty?

They most certainly do. We had considered fitting the windows with filters but what we have found is that the window itself acts a filter, as the inside gets dirty the window tells you how much dirt it has captured, dirt that otherwise you would have been breathing in! The windows are designed with catches holding together the sashes that can readily be separated for cleaning. The windows are also reversible so every surface can be cleaned from inside the room.

The rate at which dirt accumulates within a 'supply air' window is dependent on the location of the building and the storey at which the window is located. If the building is situated near to sources of particulate pollution then the 'supply air' windows will get dirtier quicker. Testing of the windows in Norwich has shown that first floor windows, not near a busy road, need cleaning on the inside about once a year.

Can the vents be adjusted or closed for example to reduce the amount of heat into the room in the warmer months or should air outside become contaminated?

There is no bespoke method of closing down the ventilators as they tend to be shut down and not re-opened if occupants are able to close them. The flow regulation provided by Titon's vents is intended to prevent draughts and thus remove the most common reason for closing ventilators. One of the outcomes from the current Carbon Trust sponsored project in Norwich is to record users' behaviour and responses in relation to the system.

How does the system work in summertime?

The windows are opened as normal in the summer so air flow to the room bypasses the window cavity which stops pre-heated air entering the building.

How easy is it to clean the vents of debris?

Both the upper and lower vents have a fine metal grill on them to stop debris penetrating. Debris is not likely to get on the vents as they are nowhere near the ground.

How has the problem of air flow been overcome in relation to high wind, does the air flow increase and decrease in relation to the wind patterns?

The vents are self-regulating so variations due to changing wind conditions are damped down, although some variation does still occur.

For the air to flow through the dwelling, do the internal doors also need vents?

Internal doors should be undercut up to 2cm. In extreme cases, where there are multiple intervening doors between stack and window, door vents may be required. In these cases advanced simulation can be conducted to verify the design.

How easy is it for disabled or elderly people to open and close the windows?

As easily as for any standard double glazed window.

Do the windows require special treatment?

The low-E coating on the inside of the window is quite durable but should not be cleaned with anything abrasive as this can reduce its effectiveness. Cleaning with a damp cloth is however perfectly OK.

Do the windows meet current building standards?

The windows meet all relevant British Standards and building codes and offer a high level of security.

Does the Dwell-Vent system require significant maintenance?

The passive nature of the system i.e. it has no mechanical or electrical components, not only means that the system is extremely quiet but also very low in maintenance. Apart from window cleaning, and occasional cleaning of the passive ventilation ducts, there are no other significant maintenance requirements associated with Dwell-Vent.

Doesn’t the internal surface of the window get uncomfortably cold?

No, the low-E coating maintains the glass surface temperature at around 15 deg. C even when the temperature is 0 deg. C outdoors and 20 deg. C within the room.

Should radiators be located beneath supply air windows?

It used to be the case that the radiators were located beneath windows to stop the window being an uncomfortably cold surface; highly insulating windows don’t have the same requirement. In fact, in the absence of a radiator, if the incoming air falls down the surface of the glass, the resulting air layer is of lower temperature than the general room air temperature. The supply air window consequently has a lower Ue-value than would otherwise be the case, which argues in favour of radiators being on side walls rather than under the window. The surface temperature of the external walls, even if highly insulated, will however be low relative to the rest of the room surfaces, which may still argue in favour of radiators being under windows.

The graphs of monitored results from the EU sponsored project showing the variation in the Ue-value of the supply air windows over time have periodic ‘spikes’, what does that mean for their performance.

These are due to solar gain, the same effect can be seen on the U-values of the conventional windows that were monitored in the control houses. In fact the Ue-value of the supply air window is lowest when there is the greatest temperature difference between indoors and outdoors, which is advantageous. The ‘supply air’ windows if shaded from the sun have very consistent Ue-values.

How durable are the flow-regulating vents?

The anticipated life of the vents is the same as for the windows. They have been adapted from a design for flow regulation that Titon Hardware have produced for many years. The backflow damper is fixed without adhesive or hinges for long term durability.

What is the correct orientation of the windows?

The windows act as a heat reclaim device independent of orientation. For windows facing towards the sun, solar gain is an added benefit. On very sunny days, for example with solar incidence of 800 W/m2, the solar gain and resulting pre-heat offsets approximately 70% of the energy load due to the incoming ventilation air.

At everyday wind speeds of 3-4m/sec the operation of the passive stacks is driven by thermal buoyancy so that as warm and humid air from kitchens and bathrooms rises it is expelled from the building. At higher wind speeds wind forces start to dominate but the Danish ‘wind catcher’ terminals are designed to catch the wind coming from any direction. Provided the terminals are near or above the ridge of the roof they will function well irrespective of the orientation of the windows. Similarly passive stacks on monopitch roofs need to be near the high point of the roof. To restrain the projection of the stacks above roof level against wind loads the stacks should be adjacent to, and restrained by, the party wall between dwellings.

An important consideration is if the building has no obstruction to the cross circulation of air because, on upper floors, the pressure of air entering the building on the windward side of the building may prevent an adequate quantity of air entering ‘supply air’ windows on the leeward side. For this reason the layout of the building has to be carefully considered when proposing installation of the Dwell-Vent system.

What is the maximum height of a building suitable for Dwell-Vent?

Even at very high levels of air tightness it is difficult to maintain the neutral pressure line above the 2nd floor above ground floor i.e. so the interior is at negative pressure relative to the windows. For that reason two interconnecting storeys is the maximum that can be ventilated as a single zone. Dwellings having a third floor, for example as a room in the roof, will require the top floor to be isolated (by doors opening out onto the stairwell being draught-stripped) the 3rd floor having its own passive stack and supply air windows.

How does the requirement for the top floor in 3 storey houses to be separated by fire doors from the stairwell.

This also in favour of the separation of the top floor as indicated above.

Does every kitchen and bathroom require a separate PSV extract?

Yes, to avoid exchange of exhaust air from kitchens to bathrooms and vice versa, each wet room should have a dedicated passive stack. If wet rooms are vertically above one another it should however be possible to bundle the ducts together to form a combined stack with a single ‘wind catcher’ terminal. Passivent rectangular ducting 60mm x 200mm is useful for this purpose. If each duct has a 10mm thickness of acoustic insulation between it and the next, three ducts can be combined to form a 200 x 200mm ‘bundle’.

Are the windows patented, can anyone make them?

The principle of the ‘supply air’ window is long established, they are manufactured in Finland without IP protection. The windows now manufactured by Howarth Windows and Doors Ltd are however a heavily engineered component and a step-change removed from the prototype windows we used for the demonstration projects in Ireland, Poland and Denmark. The Howarth windows are dimensionally coordinated with the rest of their range so they can be combined with conventional windows or doors within a single house or flat. This is advantageous because part F of the UK regulations requires that the passive stacks extract from ‘wet’ rooms, which should not have either supply air windows or trickle vents providing ingress of air from outdoors. Instead air is drawn through the adjoining rooms, and in through the supply air windows, to form a whole house ventilation system. Consequently the kitchen and bathroom windows will be conventional Howarth windows.

The windows have been tested for all standard BS tests for the highest exposure to water and air penetration and ‘secured by design’ (so they are burglar-proof) as required by UK housing associations.

Do the stacks need to be vertical and planned in to the layout of the unit?

For best operation the stacks ought to continue vertically through the height of the building and be terminated by a Danish ‘wind catcher’ terminal, since offsets and sloping sections inevitably compromise the flow rate. This requires that the building is planned for vertical continuity of the ducts.