Air supply mechanism

[btysoe]

I'm trying to develop my understanding of what happens when changing the Air Supply mechanism from 'Centralised balanced A/C or mech vent system' to a 'Zonal supply system with remote fan' and when it is appropriate to use either type?

I setup a simple one room building using activity type B1:Office and assigned a fan coil system with heat recovery using default efficiency settings.

Changing the auxiliary supply mech from centralised to zonal generates no difference in the actual building but does generate an increase in heating energy in the notional building which originates from an increase in Aux Vent gain. what does this represent?

is there a definition of the air supply mechanisms somewhere please?

[btysoe]

There is a definition in the non-domestic building compliance guide as follows:

Central system: means a supply and extract system which serves the whole or major zones of the building

Zonal system: means a system which serves a group of rooms forming part of a building, i.e. a zone, where ducting is required.

But when does a group of rooms become a major zone? and this definition doesn't explain the increase in heating energy in the notional building?

[JohnLloyd]

I don't know for sure, and haven't got time to go check, but I would assume the Notional building is seeing an increase in heating load because it is now using a 'zonal extract' system which does not have heat recovery.

[btysoe]

That would make sense and checking the figures it matches the energy difference.

Would it make sense for IES to disable the heat recovery in the actual building when using zonal supply as the Air supply mechanism?

paragraph 46 of the NCM guide States: The Notional building has heat recovery with sensible efficiency fo 70%, where appropriate (i.e. zones with mechanical ventilation providing supply and extract)

[RossThompson87]

I have just been taking a look at this and it seems a bit odd.

Taking for example a hotel with supply air to bedrooms and extract through en suites all served from a central unit.

You could create two systems one for zonal supply and one for zonal extract and put heat recovery on both. This would give you heat recovery in the actual and not the notional making compliance much easier!

All the IES inputs would be accurate and paragraph 46 the modelling guide met as the zones technically don't have both 'supply AND extract'.


On a separate issue, what is the best way to model toilets with local extract fans and undercut doors?

Obviously you can put the exhaust flow rate and SFP on the zone level, but for your system should you choose 'Nat Vent' or 'Mech Vent' with 'Local ventilation-only units.. e.g toilet extract'?

Should it depend on the probable make up air?

Trying a simple room gives the same BER but TER changes slightly. The difference seems to be down to 'Free Cooling Vent Gain' variable (only active for nat vent).

[pran]

I'm finding this a little bit frustrating as well. I just want to check if I'm right. I'm using dynamic sim 2014 FP1.

From what I can see, the model only ever applies heat recovery to the notional building when you use the 'centralised, balanced A/C or mech vent system'.

The NDBSCG 2013 lists 12 different types of mech vent system, including the following two, which I would deem the closest match to a local MVHR unit:

Zonal supply and extract ventilation units, such as ceiling void or roof units serving a single room or zone with heating and heat recovery

and

Local balanced supply and extract ventilation system, such as wall/roof units serving single area with heating and heat recovery

In IES, we do not have the same range of systems. We are limited to 5 types of system:

Centralised balanced A/C or mech vent
Zonal supply
Zonal extract
Local ventilation-only units
Other local ventilation units

If we have a local MVHR unit with no heating or cooling element, serving a single room, my instinct would be to either choose local ventilation-only units or other local ventilation units. It's certainly not a centralised system in the sense described in the NDBSCG.

But neither of these options apply heat recovery to the notional building, or apply the SFPs as you might expect.

So it seems to me that the only option for any heat recovery system is to choose Centralised balanced A/C or mech vent. Is this right? It seems odd to me, and I think IES should change the wording of the options in line with the NDBSCG definitions.

[RossThompson87]

Hi,

Yes it is really frustrating. Assuming 2010 Part L, the NCM modelling guide states:

Paragraph 46: The notional building has heat recovery of 70%, where appropriate (i.e. zones with mechanical ventilation providing supply and extract).

The IES interface contradicts this. With your MVHR unit (no heating or cooling) it is technically a 'local ventilation only unit'. So IES would do no heat recovery in the notional building!

It would be so much simpler if the wizard just asked the direct question that references the regs i.e. 'Does the room have supply and extract' yes or no?. What are all the options for? How do they actually help us with compliance?

Maybe someone from IES can explain them.

Personally I would put your system in as balanced to comply with paragraph 46 of the modelling guide. But it is obviously not 'centralised'.

Ross