DHW Delivery Efficiency

[Warwick]

Should both DHW delivery efficiency and circulation losses be included?

IES Guidance on the knowledge base states:

'If secondary circulation and/or storage losses are used, then the DHW delivery efficiency should be set to 1. Otherwise the losses are accounted twice.'

https://www.iesve.com/support/knowledge ... e/faq/4326

The SBEM Technical Manual P93 states:

'Note: If the hot water is provided by the HVAC system’s heat generator, rather than by an independent generator, the efficiency of the heat generator is reduced by 5% for the water heating calculation in order to take account of the reduction in the efficiency at part load (since the generator has been sized to accommodate both the space and water heating loads).'

Based on the SBEM technical manual wording the DHW delivery efficiency in IES is representing the 5% reduction in heat generator efficiency. My understanding is therefore that both DHW delivery efficiency and circulation losses should be included.

An alternative thought was that IES was applying the %5 reduction if generator type is set as 'same as space heating', but this doesn't appear to be the case.

Would appreciate other peoples thoughts and interpretations of this.

thanks

[Terence]

Hi there, your choices are to enter the losses directly and then set the DHW Delivery efficiency to 1, or don't enter the storage losses and instead enter a DHW Delivery Efficiency value less than 1 to account for storage losses.

[Warwick]

Hi Terence,

Thanks for your response - I also contacted CIBSE Certification and they have confirmed what you have said - but how does IES account for the 5% reduction in heat generator efficiency when DHW is generated by the HVAC system's heat generator as required on page 93 of the SBEM technical manual? When you change the DHW generator type in IES it makes no difference (so the 5% reduction isn’t applied automatically here).

thanks

[Terence]

Hi David,

Our DSM doesn't take that into consideration as it isn't covered in Approved document L2a or the NCM Modelling Guide used to define how approved software should function. I've not heard of any expectation on users of DSM software to reduce the heating efficiency of their boiler by 5% because it serves both Space Heating and DHW. I would have thought that it would be expected that the efficiency entered by the user would have already considered this.

Regards,
Terence