The use of the pre-heat margin in heating system design: a review based upon published literature

Abstract

The size of a pre-heat margin for heating system design has been an engineering design decision for the past 35 years. This paper examines this issue through published literature. It is seen that although design techniques exist to ascertain the margin size and its consequent pre-heat time engineers do not appear to have used them or the full economic analysis called for in the 1970 IHVE guide. Engineers are reported as using empirical margins in most cases. The 1999 CIBSE guide is seen as an improvement, but it is seen that for over 20 years it has been suggested that empirical methods for sizing heating systems are no longer sufficient and that simulations may be the correct method to ascertain heating system size.

Challenges for the Transition to Low-Temperature Heat in the UK: A Review

“…The use of margins is presented in the CIBSE guides with methods and tools to limit oversizing, however, it is common practice for engineers to rely more on empirical margins than CIBSE guide methods [47], hence "the reduction in uncertainty can only come from improved knowledge" [17]. The lack of monitored and analysed feedbacks is identified as a key problem [8,47,48], and the lack of complaint from the occupant is sometimes the only feedback used to assess a successful design [8]. For the design engineer, over-sizing can present less risk than under-sizing, and there are no incentives to correctly size a system [49].…”

“…However, under-sizing is also mentioned in the literature by [61] where "under-sizing is a well-known problem in the UK". Finally, [8] highlights that oversizing is a "largely hidden topic" but detailed values are rarely available [47].…”