Holdfast Training Services Trial Blog

You have a heritage building with an inoperable Low Temperature Hot Water heating System due to reservicing works, how do you control the internal conditions such that you can be confident the building fabric is preserved, especially throughout the winter period?

Logically you would start by defining what ‘acceptable’ internal conditions are and then formulate a design. The key parameter for preserving the building fabric in a heritage building is ‘relative humidity’ which is predominantly a function of the absolute humidity and temperature. Relative humidity levels greater than 65% cause issues such as mould growth, insect attack and the expansion of hygroscopic materials (a material that readily absorbs moisture) due to the absorption of moisture. Alternatively relative humidity levels lower than 40% cause contractions in hygroscopic materials and ‘flaking or cracking’. It is a lot more complex than this especially when it comes to the conditions required for specific materials in museum archives (not required in this situation due to the widespread removal of valuable objects). Also worth noting is that RH and temperature fluctuations (not more than 10% RH and 4°C change in 24 hours) are undesirable.

For context, nearly all buildings cater to human thermal comfort requirements and typical setpoints of 21°C are specified all year round to ensure comfortable working conditions, the added benefit of this setpoint is the creation of a ‘humidity buffer’ i.e warmer air is able to hold more moisture before saturation and will therefore experience lower relative humidity levels than colder air for the same absolute moisture content. This means that throughout winter your 21°C setpoint will help prevent excessive RH. However, it is not always necessary to provide heat for human thermal comfort, the energy efficient method of ‘Conservation Heating’ (Using heating to control RH) can be used where human thermal comfort is not required and works on the principal that a 6°C temperature differential with external conditions will maintain acceptable RH levels. Essentially conservation heating is used in buildings that don’t require the energy intensive 21°C setpoint for human thermal comfort in order to save energy but still maintain acceptable RH levels.

In more recent times, due to the increased emphasis on sustainability the use of dehumidifiers (relatively low energy demand) to control RH has been explored and succesfully employed in numerous buildings across Europe. Typically a dehumidifier requires low infiltration rates to be at its most effective.

The strategy for the building in question (The contractor is adamant about only using 110V electrical appliances to manage the fire risk) is to use a combination of heating and dehumidification units to control the internal parameters. The constraints in the title of the blog refer to the fact that the units can only be operated during the hours 0800-1700 during working days and that the need for asbestos enclosures require the temporary removal of the units from certain areas.

This solution is going to be in-situ for circa 6 months which equates to roughly 1000 hours of operational time. The way the units are used will have significant consequences for the energy consumption and cost of the design. For example, if you had 60 heating units drawing full power during the six month operational time slots you’re looking at a circa £25,000 electricity bill (based on 14.3 pence per kWh).

Ultimately both the units are controlling the RH of the room, the dehumidifier does this directly by reducing the absolute mositure content to achieve a RH setpoint whereas the heating unit achieves this indirectly by heating the room up to a pre-determined setpoint to reduce the RH. The dehumidifier has much better control as it will reduce the RH to the exact level you want whereas the heating setpoint (dictated by the user) will be constant throughout the day and is likely to be ill specified to realise the exact RH you want e.g you could find a situation where a 15°C setpoint reduces the room RH to lower than 40%.

It has been found that the entire building has high infiltration levels, the moment the units are switched off the absolute moisture content of the room equalises with external conditions (If dehumidifiers have been active) and RH increases and the heated room air quickly dissipates (if heating units have been active). Ultimately there is a complete lack of control outside of the operational window. The fan convection units are effective at heating the room air up and not the thermal mass of the building.

Initially a 10°C lower temperature limit for the building was set. Upon reviewing this limit it has been found that up to 400 national trust properties regularly flout this limit with temperatures as low as 6°C in order to maintain acceptable RH levels (Using conservation heating). Due to residual heat gains the building is only expected to drop as low as 8-9°C, coupled with the lack of human thermal comfort requirements and the reduced energy intensity of dehumidifiers the argument can be made that the convection heating units are unecessary.

It would be interesting to have trialled a range of different heating units such as oil filled radiators, radiative heating units and storage heaters to see how effective they could have been at heating the thermal mass of the building and maintaining out of hours control. It is my belief that fan convection units are poorly suited to the task of controlling RH levels in a heritage building and that the dehumidifiers should be the principal means of RH control as they achieve the same effect and with reduced energy consumption.

Effectively controlling the parameters 24/7 in light of the constraints specified in this blog is like entering a boxing match with both hands tied behind your back. Based on the data that is accumulating I envisage a compromise can be made between the constraints and the preservative requirements of the fabric.