Correct usage of AHU cooling coil

[SteveBurrows]

Hi all,

I have a question that I suspect I may already have the answer to, but I was posed the question by a colleague and had trouble giving a complete answer.

My scenario is this, say within your building you have an area that is heated via radiators and has supply/extract air delivered by the AHU. However this AHU has a cooling coil to temper the air in summer (i.e. reduce it down but not directly help with the internal temperatures). What should your NCM system selection be?

As the space isn't being actively cooled then the selection of a system with cooling (i.e. to a setpoint) is inappropriate. However if you pick a system without cooling then the cooling load for this fresh air is lost and inaccurate.

I believe that when I tackled this in the past I attempted something clever with setting one system as the primary, and a completely different auxiliary vent system.

Any thoughts would be welcome.

[PCully]

Hi,

This is logged under UK Compliance regulations so I assume we are restricted by the NCM here, this will have a bearing on what options you have for modelling your system.

the NCM gives a definition of what it considers to be air conditioned space and if the building falls into this criteria then you have to choose a system with cooling and it will condition to the NCM Activity setpoints.

In your case I don't think it would be considered this way, I'd have normally opted for a tempered air exchange where you can specify the conditioning of incoming air and have that energy consumption calculated but you can't do this under NCM. Similarly with NCM you can't have a different system for the auxiliary energy and the space conditioning so that won't work.

Not looked in my NCM Modelling Guide but I don't think you would have the tempered incoming air (NCM is set to outside air temp) or associated energy consumption in the model

Phil

[PCully]

I should also note - these answers should be coming form your accreditation scheme. they are the authority on regulations and will validate any modelling approach you choose to take.

Phil

[Complex Potential]

The NCM specifies that fresh air should be modelled as supplied to rooms at outside condition which is basically saying that AHU heating and cooling is to be omitted/ignored.

As such, the system type I would recommend in your scenario would simply be radiators with mechanical ventilation.

Obviously this means there may be no cooling energy consumption in your results when in reality there would be, but the NCM is about compliance, not realism.

CP