What is Comfort Ventilation?
Comfort
Ventilation is the solution to the ventilation needs of energy
efficient buildings. Comfort Ventilation, mechanical ventilation heat
recovery (MVHR), or heat recovery ventilation (HRV) are all names for
the same thing. In many ways the term Comfort Ventilation is the most
apposite, as it describes the result rather than the process. A Comfort
Ventilation system properly fitted into a house provides a constant
supply of fresh filtered air maintaining the air quality whilst being
practically imperceptible.
Comfort Ventilation works quite simply
by extracting the air from the polluted sources e.g. kitchen, bathroom,
toilets and utility rooms and supplying air to the ‘living’ rooms e.g.
bedrooms, living rooms, studies etc. The extracted air is taken through
a central heat exchanger and the heat recovered into the supply air.
This works both ways, if the air temperature inside the building is
colder than the outside air temperature then the coolth is maintained
in the building.
At what level of build is a Comfort Ventilation system worth while?
Although Comfort Ventilation can be installed in any building, there is a rule of thumb that its use is not justified unless the air permeability of the thermal envelope is at or below 3 air changes per hour when tested at 50 Pascal (equivalent approximately to 3 m3/m2.h @ 50 Pa for average dwellings). If this level of air tightness is not achieved then the natural breathing of the building is such that alternative strategies to ventilation are more appropriate.
To explain the previous statement in slightly more detail. If there is no temperature difference between inside and outside then there is no energy to be recovered. When you have a significant temperature difference between inside and outside then the thermal stacking effect (or chimney, hot air rising effect) becomes the driver that will cause your building to ventilate itself through the leakiness of the envelope. If it is windy then you will have wind as the driver ventilating your house through the envelope leakiness. By using intermittent extract at point of need, e.g. shower, kitchen, you will simply reverse some of the leakiness.
If your building has poor insulation/thermal bridging and or other damp ingress due to for example lack of proper damp proofing, then it can be tempting to look for technological solutions. In such a situation you will have problems with condensation and probably mildew where the condensation is forming. Yes Comfort Ventilation can help reduce these symptoms; however, our advice would be to sort out the problem not cure the symptom.
Frost protection
As the technology currently stands there are no mechanical ventilation recovery systems that can function with an out going temperature of below zero Celsius. The reason for which is as follows:
In winter as the outgoing warm humid air passes through the heat exchanger it drops below dew point forming condensation which flows from an outlet. If the intake air is below -10C then out going side of the heater exchanger will drop below zero and the condensation will start to form ice and block the narrow air ways. This usually takes around 1-2 hours of below -10C before the out going side is closed by ice. If the system is left running the heat exchanger no longer has the benefit of the warm air passing outwards and will simply freeze to the temperature of the incoming air flow.
To compound this issue water has a high latent heat. This means that to change ice back to water it require the same amount of energy as it takes to raise the same quantity of water from 0 to 800C, so as the intake air temperature rises above zero it still takes sometime for the ice to thaw.
There are several possible solutions to this problem. All the solutions use an energy source to raise the temperature in the heat exchanger to above the critical level.
The conventional mainstream units sense that the outside air temperature has fallen to a critical level and reduce the intake flow whilst maintaining the exhaust flow so as to have more heat in the exchanger. If the temperature continues to fall the intake will be turned off completely and the extract fan will continue to run. Referring back to the air tightness issues, plainly if the building is truly airtight this is a nonsense, you can not extract more air out of a building than is going in.
In the real world buildings are never completely airtight and the air has to come from somewhere. The air will be pulled in from where ever possible creating draughts and energy loss. The cold air being drawn in through unintended routes will cause cold spots, condensation and possible damage over the longer term to the building fabric. This will occur at the time when the maximum benefit of having invested in a low energy build should be being realised. In a very low energy building, the heating system may be too small to cope with this extra demand, resulting in sub-comfort conditions.
Particular requirements for frost protection to Comfort Ventilation in low energy / PassivHaus buildings
In very low energy buildings such as those at or approaching the German Passivhaus standard, the design requirements for Comfort Ventilation are a little different from other buildings. It is very important that these different requirements are fully understood by the system designer, as without this understanding the system will potentially not function correctly. Passivhaus certification specifically excludes the form of frost protection out lined above.
There are three main alternative solutions for advanced buildings.
PTC electrical resistance pre-heater
The simplest solution is to put a PTC electrical resistance pre-heater on the intake controlled by a thermostat. PTC stands for Positive Temperature Co-efficient and means that only the sufficient heat is put in to the air to raise the temperature to just above zero. The advantages of this solution are that it is relatively cheap and simply to install. The disadvantage is that it uses electrical energy which is relatively expensive.
Ground to air heat exchanger
Alternatively the intake air can be drawn in through a ground to air heat exchanger, also known as a ground pipe. This usually takes the form of a 20 – 40 meter length of 200mm diameter polypropylene pipe, laid at 1.5 – 2 meters depth and with a minimum 2% fall, so that condensation when it forms runs to an outlet or gulley. Laid at that depth the ground has a year round temperature of between 8 to 120C. The energy source here is solar.
The advantages of this system are that it helps stablise the building temperature by giving a moderate warming in winter and cooling in summer. It can be fitted in many circumstances with no moving parts (except when a sump pump is needed for the condensation). It can when appropriate be laid in the same trench as the foundations of the building. Running costs are only a minimal extra load on the ventilation unit fans. The disadvantages are the pipe must be laid very carefully. If a new trench has to be dug specially installation gets expensive. Some sites/ground conditions make it impracticable. Some people have expressed concerns regarding microbial growth in the pipe (e.g. mould) though I have seen no evidence for this.
Ground to brine to air heat exchanger
The ground to liquid to air heat exchanger system uses a small bore polypropylene pipe laid in the ground at a depth of 1.5 – 2 meters filled with anti-freeze solution (usually glycol). This again is using solar heat. The liquid is circulated through a duct heater when required to pre-heat (or cool) the intake air. The advantages of this system is that it is simpler to install. It is easy to turn on and off and does not have the issues of possible microbe growth. The disadvantages are that it requires energy to run albeit less than PTC electrical resistance heater but more than the ground pipe. It has moving parts that ultimately wear out. Though simpler that the ground pipe it still requires ground works and space for them.
Other important considerations
If you are intending to fit MVHR comfort ventilation into a new build or refurbishment then it should be considered from early in the planning stage. For any MVHR comfort ventilation system to give its best performance the devil is in the detail. System design will make the difference between a highly efficient system working un-noticed in the background and one that is constantly in the awareness of the inhabitants and wasting both heat and electrical energy. These systems necessarily require ducts to be run through the building and the routing is important both from the point of view of the building and the efficiency of the ventilation system. Indeed, layout of rooms can be influenced by ventilation needs.
The installation of Comfort Ventilation in a building will also affect the heating system design. One of the effects of Comfort Ventilation is to equalise the temperature through out the building. Therefore it is important prioritise the heat supply to the rooms that should be warmer. If the building is of sufficiently high standard then it might be only necessary put heat directly into certain rooms. In contrast with conventional building standards, in very low energy and Passivhaus buildings it can be pragmatic choice to put all or some of the heat requirement in through the ventilation system. Again, it is vital that the system is designed with a clear understanding of the design requirements when ventilation air heating input is to be considered. At Green Building Store our staff have a good understanding of the Passivhaus building standard, and can advise on the suitability of ventilation air heating. We are able to advise and supply Comfort Ventilation Systems with integrated controls that also manage the heating and other ancillaries.