|What is positive input ventilation?
Low energy positive input ventilation is the usual name for systems in which fresh air is supplied to a dwelling from the roof space by means of a small fan. It was developed in the 1970s to deal with condensation by providing continuous (and quiet) ventilation throughout a dwelling, without cold draughts and without affecting the operation of open-flued combustion appliances. Input ventilation systems are now sold by several companies in the U.K. According to these companies input ventilation is claimed to be an effective means for overcoming condensation and mould problems in existing homes, mostly Local Authority or Housing Association owned properties. New housing has been identified as another sector with scope for the use of input ventilation.
Why was the project needed?
While there is considerable anecdotal evidence of input ventilation system performance the true energy and environmental impact of these systems is not well understood. By taking air from the roof space, which is generally several degrees warmer than the outside air, they should in principle use less heating energy than conventional extract systems. Innovative units are now available with high efficiency fan motors which have the potential for worthwhile savings in energy cost and CO2 compared with other means of ventilation.
A project was set up, under the DETRs Partners in Innovation scheme, to begin addressing the lack of measured performance of positive input ventilation. The study ties in with ongoing work at BRE investigating the ventilation and airtightness of dwellings. Increasing airtightness, with its implied need for purpose provided ventilation, is likely to make input systems more attractive in the future.
What were the project objectives?
There were two main objectives in this project:
- Determine the conditions for which low energy positive input ventilation will save energy whilst providing adequate ventilation through an understanding of its performance;
- Promote this knowledge to potential users.
How was the investigation done?
There were two parallel parts to the project:
a) Detailed investigation of performance in a BRE test house
The way in which the system performs was investigated under controlled conditions of heating and water vapour production (but not external weather) by testing in an unoccupied test house on the BRE site. This involved monitoring of temperatures, humidities, energy use and weather over several two/three week periods, and also tracer gas studies to indicate indoor air movements and ventilation rates. Tests were made with internal doors both open and closed. BRE carried out the testing, monitoring and data analysis with Home Ventilation Ltd (a division of The NuAire Group) providing and installing an input ventilation unit. The test house was very airtight, with an air leakage rate of 4.5 ach @ 50 Pa having been measured by a fan pressurisation test (Reference 1).
b) Field studies in 16 dwellings
Positive Input systems were installed in 16 occupied dwellings in Merthyr Tydfil and Aldershot. Temperature and humidity levels were monitored in the kitchen, living room, bedroom and roof space (attic) over several months. Some weather data was measured on site and daily weather data from nearby weather stations was bought-in for the data analysis. For alternate periods of three to four weeks the input ventilation units were switched on and off whilst monitoring continued. The occupiers were asked about ventilation and condensation conditions in their houses before and after the monitoring. Home Ventilation Ltd arranged access, installed input ventilation units and carried out the monitoring work on site. BRE carried out the analysis of the data obtained and the interpretation of the results.
Does it save energy?
Installing a low energy positive input ventilation system will not directly save any energy, but it may give an energy saving compared with a conventional extract system providing the same level of ventilation air exchange. This is because input ventilation takes its air from the roof space where temperatures were found to be about 3 &Mac176;C higher on average than outside. Energy performance proved difficult to establish in the project but the relative saving is estimated to be a maximum of about 150 Watts in an average modern family house (equivalent to about 550kWh over a heating season, or 10% of annual space heating cost). Actual relative savings will be less than this (possibly even halved) because of recirculation of room air via the roof space.
How well does it remove moisture?
Input ventilation was found to be effective in reducing relative humidity levels by around 10%RH in the test house, even when internal doors were closed. When absolute humidity excess over that outside was examined it was also found to be effective [reduction of about 0.2 kPa] when internal doors were open. With doors closed a similar overall reduction was found but it was more effective upstairs than downstairs.
In the field monitored houses input ventilation was not consistently effective in reducing relative humidity. When absolute humidity excess over that outside was examined it was effective in the most humid houses but did little in the dryer houses. Even in the cases where it was effective there were often inconsistencies between rooms in the same house.
Is the roof space a good source of ventilation air?
In both the test and occupied houses, the roof space was consistently more humid than outside (excess vapour pressure of about 0.1 kPa), implying that moisture was being transmitted to the roof space from the rooms below. The results showed this moisture transfer regardless of whether input fan was operating or not. Tracer gas measurements in the test house confirmed that much of this was airborne moisture. This recycling of air from the rooms below reduced the actual ventilation rate to about half of the 0.7 air changes per hour which the fan air flow rate would suggest. This clearly shows the importance of making the upstairs ceiling airtight when input ventilation is installed.
What did the occupants think?
The occupants were more enthusiastic about the effectiveness of input ventilation than the results would suggest. Those who previously had the highest humidity in their houses were the most impressed. Some occupants also claimed relief from severe respiratory illness but these claims could not be substantiated in this project.
Environmental Engineering Centre
John Bradley and Keith Maddock
Home Ventilation Ltd
Western Industrial Estate
e-mail: firstname.lastname@example.org, email@example.com
(Home Ventilation Ltd is a division of The NuAire Group)
The results of this project are to be published in more detail by means of a technical paper in a journal (possibly Building Services Research Engineering and Technology) and a BRE Information Paper.
This article forms part of a DETR Partners in Innovation project entitled Low energy positive input ventilation in dwellings. The work is published with the agreement of the Department. The BREs project partner was Home Ventilation Ltd, a division of The NuAire Group.
1. Stephen R. K. Airtightness in UK dwellings: BREs results and their significance. BRE Report BR359, ISBN 1 86081 261 9. London, CRC Ltd, 1998.
The author of this material is employed by the BRE. The work reported herein was carried out under a Contract jointly funded by Home Ventilation Ltd and the Secretary of State for the Environment, Transport and the Regions placed on 1st July 1998. Any views expressed are not necessarily those of the Secretary of State for the Environment, Transport and the Regions.