IES-VE User Forum

I have a building where a CHP system is used to generate 100% of the design hot water demand. This demand has been calculated using guidance from CIBSE and relative British Standards and the CHP system has been sized accordingly.

The assumptions of the NCM assume a much higher hot water demand than that as calculated with guidance from CIBSE and British Standards. When producing a compliance model in SBEM this difference between real design hot water demand and NCM assumed hot water demand would not matter because the all CHP input parameters are efficiencies and percentages.

However, my building is level 5 complexity and as such I must use DSM compliance software. But CHP input parameters in IES ApacheSim Compliance require an absolute CHP capacity. If I state the CHP size as designed for the real hot water demand it will not be large enough to provide 100% of the notional hot water demand and therefore in my IES ApacheSim Compliance model a portion of the hot water demand will be met by other means.

Please can you tell me how you would advise an assessor to model this situation in IES ApacheSim Compliance.

Many thanks

This is a problem I also have with the IES DSM and CHPs.

What you have to remember is that the NCM modelling guide is not software specific and so there is actually a fair amount of flexibility in how you inputs parameters into your software of choice, in this case IES. Ultimately it is only IES themselves who could audit your work and say you have done it "wrong".

To my mind there are two equally valid methods to account for a local CHP in an IES DSM compliance model; the first you have already alluded to, namely inputting the absolute CHP parameters and letting IES do the rest. This has the advantage of being simple and easy to "tick off" on an audit but, as you say, the percentage of heating demand served can differ hugely from the design specification due to the NCM profiles and assumptions.

The second method is my preferred option and basically involves modifying and iterating the CHP size/turndown until the CHP is serving the correct percentage of load (you can interrogate vista for this), essentially staying consistent with the SBEM method. The absolute CHP inputs are considered incidental and never reported on. This also avoids design teams sizing their CHP from a compliance model which should obviously never be done.

It is not ideal but it feels like the lesser of two evils. You could always email IES support for confirmation in writing that such a method is reasonable.

CP

All

Has IES ever given a view on the 'correctness' of the (second) method proposed by CP?

I ask this because it matters if the proposed method is used when issuing a BRUKL, and more significantly, when lodging an EPC.

This issue has arisen on a project where the Stage 3 designers have used CPs proposed method to size the CHP and demonstrate the required betterment against the London Plan, which now needs to carry through to the 'As Built' assessment.

Does anyone have any experience of successfully lodging EPCs this way? Has the method been accepted by any of the lodgment schemes e.g. CIBSE, Stroma, Elmhurst etc.?

Wasted