TM52 - WTF?

How overheating is getting developers and consultants all hot & bothered.

The latest TLA (two letter acronym) that is doing the rounds is TM (Technical Memorandum) number 52.

This is getting many building services engineers and mechanical consultants all light headed, as it deals head on with the growing problem of over-heating, particularly in high rise and multi-unit developments.

A combination of better insulation levels (example external wall ʯ values of 0.35 w/m2 in 2010 now down to 0.18 w/m2 or better) and improved air tightness (typically 10m3/hr per M2 and heading to below 5) means that heat losses have drastically reduced.

The increased use of glazing has been made possible by the improvements in insulation levels elsewhere. Increased glazing has been recognised as having a positive effect on mental health and well-being as our exposure to natural light, and engagement with the outside world improves the perceived quality of life.

This is great news for the home owner or occupier as light and space are increased and running costs are commensurably reduced. But this presents challenges of its own in other areas.

For some years the concept of ‘thermal mass’ has been de rigueur in polite M&E circles as it been postulated that flattening out the demand curve for heating somehow leads to reduced running costs. The car analogy of ‘just ticking over’ was often used to visualise the slow, steady input of heat rather than a frantic rush to target temperature as if this was saving energy (although I cannot think of a single time when I started my car at 2am on a frosty February morning just so it would be de-iced and warm for my 8:00 am commute).

It’s true that a heavyweight structure can provide something of a buffer to rapid shifts in outside temperature, but when temperatures are in a steady state, the inherent inertia of the building can become a factor. This is especially true when ‘heavyweight’ heating systems are also employed.

Heavy weight radiant heating systems, often embedded in 50~100mm of screed to act as a ‘thermal store’ took hold as the preferred option and fast acting heating systems such as radiators took a back seat. Coupled with the increasing pressure to maximise plot size and footprint, this seemed an ideal way to remove the space-stealing radiators, and sell the dream of reduced running costs. Win-win, right?

Err, no.

Trouble reared its head in 2003 when there were an estimated 15,000 deaths across Europe attributed to overheating, mainly in the over 65’s. The ability to keep cool in summer suddenly became of interest and not just warm in the winter.

With so many UK houses still standing from before WWII, the great drive has been improving heat retention in the form of double glazing, cavity and external insulation, loft insulation etc. It wasn’t until 2015 with publication of (DECC, 2015DECC. (2015). "Guidance on preventing overheating in the home: Identifying and preventing overheating when improving the energy efficiency of homes" that the issue gained traction.

The issue came to prominence in around 2009 when there were various developments in city centres such as London, that began to report instances of over heating. The increased mortality as a result of the UK heatwaves of 2006 & 2009 were put at around 700 and 300 respectively. Not massive compared to the European figures, but still a cause for concern.

The UK is a relatively small, if densely populated, island with mild winter weather and modest summer conditions. Situated off mainland Europe between 50 and 59°N, the ambient temperature in the warmest part, London, has only ever exceeded 26.°C for less than 1% of the time in the last 30 years (CIBSE, 2015CIBSE. (2015). Guide A. Environmental design (8th ed). As a result air-conditioning is very rare and not popular with developers or planners due to the added cost, complexity and impact on running costs.

The nagging concern is that by 2040, there will be a significant rise in the number of heat attributed mortality due to global warming. More pressing is the fact that for increasing periods of time, occupants of apartments are finding living conditions unbearably hot, and this will ultimately stagnate sales and rentals if word gets around that a building is “like an oven to live in”.

Competing Factors

The construction industry is heavily cost and time focused and as a result, modern methods of construction (MMC) or offsite construction is being deployed even in buildings classed as ‘traditional’ construction to save site time and costs.

Many apartments have lower ceilings (<2.4m) and are often with a single aspect, preventing cross-ventilation. The buildings may have large windows to create a modern, airy and light aesthetic appearance which can act as solar collectors. Large windows may be difficult to open; and unless on upper floors, have security concerns and traffic noise often preventing them from being opened.

 Night-time ventilation, which could do much to alleviate overheating risk, is especially problematic if in a noisy city. MVHR (Mechanical ventilation & heat recovery) systems are often installed to overcome these problems, but again they add cost, complexity and if in cooling mode actually increase power consumption. Where ceiling heights are limited, and there already are competing services, finding the routing space for MVHR ductwork is not straight forward

So what’s to be done?

The emergence of smart sensors and thermostats in the last decade, not only has precipitated a flood of new data, but also has presented opportunities in controlling energy consumption and potentially overheating.

Learning sensors that map the heat-up characteristics of a room or building and adjust accordingly can restrict the inappropriate use of energy and similarly those with occupancy sensors or GPS location, can ensure that the heating is not ON when nobodies home.

Experts have tried to ‘map’ the subjective issue of ‘overheating’ Source: Lomas and Giridharan Thermal comfort standards, measured internal temperatures and thermal resilience to climate change of free-running buildings:2012

All these technologies are great, but ultimately only make a difference if the heating system can respond accordingly. If a heating system with high thermal mass can only raise (and thus lower) temperature by 1°C per hour, it doesn’t make much sense to have a thermostat that can locate your phone and turn the heating ON when you are leaving work. Does it?

A ‘light weight’ heating system heating the room directly and with low water content (in the case of a ‘wet’ system) makes a lot of sense even if the structure is of heavy weight construction. Heating the walls and floors (effectively becoming storage heaters) and then expecting them to cool down ‘at the flick of a switch’ is cloud cuckoo physics.

The UK has a maritime climate, and we can get cold but sunny days where solar gain through the double glazing can render heating superfluous and in some instances unpleasant. If the heating has a high thermal mass and has to be on at 5am to achieve a target temp of 20°C by 8am, then by the time the sun is beaming through the windows it’s too late to turn the heating off.

Far better to have the heating come on at 07:45 (and off at 8:05 if needed).

TM52 – What do developers have to do?

TM52 outlines the assessment criteria that need to be considered when evaluating the potential for overheating:

1.     Hours of Exceedance No more than 3% of the occupied hours during the non-heating season should be more than 1°C over the adaptive comfort threshold defined in BS EN 15251. The CIBSE guide says the non-heating season is typically 1 May–30 September (153 days)

2.     Daily Weighted Exceedance The exceedance of the chosen threshold on any one day should be less than 6 degree-hours (°C.hr).

3.     Upper Limit Temperature There must be no single occupied hour more than 4°C over the threshold. Criteria 2 and 3 are not constrained to the summer period

All this can be very complicated, and affected by factors other than the construction itself such as location, elevation and orientation (South facing for example). It’s one of the reason that several high profile city centre developments have gone back to the drawing board and undergone significant M&E design changes to mitigate against the risk of over-heating.

Reverting to ‘above ground’ panel heaters especially direct electric, has been a necessity in some cases, if significant extra expense in the form of auto blinds and shading, air con or forced ventilation are to be avoided. But with every m2 worth around £3~4,000, maximising useable wall and floor space becomes a contradictory pressure to reinstating wall hung heaters.

Answers on a Postcard, please.

So what to do?

Maximising useable floor and wall space by embedding the heating into the structure of the building can lead to poor controllability and thus energy useage, and potentially overheating as it struggles to adapt rapidly to our fickle maritime climate.

Reverting to wall hung panel heaters seems be raising the white flag, and with every inch of space precious can lead to clashes of furniture layouts and reduced useable area, impinging on kerb appeal and potential sales.

It’s for these reasons that there has been a huge surge in demand for skirting heating both wet and direct electric. Above ground, instant controllable radiant heat, without reducing the wall space, which given the current architectural penchant for large glazed areas, is in somewhat short supply.

Added benefits are elimination of trip and fall hazards, prevention of damp and mould as there is no temptation to dry washing indoors on radiators and reduction in the risk of fires (electric version) as they can be offered with a failsafe to prevent burning out - even if covered over.

In Summary

Whilst TM 52 presents challenges to building design, there are alternatives and technologies out there including new types of glazing and responsive heating systems and controls that can mitigate the effects of solar gain, and rapid changes in external temperatures, without the recourse to forced air changes such as air-con or MVHR.

M&E consultants and developers alike need to embrace the new paradox of improving insulation and air permeability to reduce the overall SAP rating and carbon footprint of their new builds, without exposing the occupants to unbearable living conditions when the sun is out.

The solution is out there.

Martin Wadsworth is Managing Director of Discrete Heat, and a survivor of BBC's Dragons Den