Fresh air supply increases cooling load

[Wasted Energy]

The scenario consists of a number of communal rooms of a residential block at lower ground level; cinema, meeting room, and gym.

They have a fresh air supply via local balanced supply/extract units with heat recovery. Cooling is required to meet the internal occupancy gains (there are no windows) and they are heated/cooled by local a/c split units.

The problem is that with the supply vent turned on the cooling load in the actual building is increased above the notional (but when the supply vent is turned off, the actual's cooling load is lower than the notional). This seems irrational because for most of the year this supply vent will provide free cooling. I’ve tried modelling some free cooling in the SBEM systems tab by adding 10 l/s/per ‘free cooling flow capacity’ but it has no effect.

Is there a way of representing the free cooling effect of the supply vent?

[Complex Potential]

For compliance modelling in IES, the room load associated with the fresh air requirement (ie, the aux vent in IES) is always active, regardless of how you specify the system type. The only difference is to do with aux energy and definitions of fan powers etc.

Are you sure it's the cooling load that is changing? I would imagine that it's the Auxiliary energy component that is actually changing and it's your fan powers that are pushing the BER over the TER when active.

CP

[Wasted Energy]

My understanding is that aux vent, as defined in the 'Air Exchanges' tab of the 'Thermal Conditions' templates, has no effect on the SBEM calculation at all.

Surely in SBEM the only allowances for supply and/or extract vent that affect the SBEM analysis are those made in the 'Systems (NCM)' tab of the 'Tabular Room Data' in 'SBEM Compliance', where SFPs are defined and where also the extract rate can also be defined (but not the supply rate as this is defined by the applicable NCM aux vent rate regardless of what, if anything, was defined in in the Air Exchanges tab of the Thermal Conditions template).

And turning on supply air here, to a room where cooling is also defined, increases the cooling load.

But most likely I am doing it wrong again.

[Complex Potential]

You are correct that anything you input into the air exchanges tab has no impact on NCM models. However, the pre-defined NCM templates that are applied do include a minimum fresh air allowance which stays the same regardless of how you describe your air system within the system tab. The logic here is that the min fresh air is required for the space occupants and so the heating and cooling should account for it regardless.

I'll caveat my advice by saying that I don't use SBEM often because I always go with the full dynamic sim methodology, however the rules governing the two methods are supposed to be the same. I'll admit that the bit about the cooling load increasing has me a bit stumped.

Without looking at the model I'm not sure I can advise further, sorry.

[Wasted Energy]

I've discovered the cause of this problem, and its solution.

The problem is the specification of heat recovery with the balanced vent. In reality this would be specified with a bypass damper so as to avoid the situation in the summer, (when in cooling mode), whereby the relatively cool external air could be pre-heated by the exhaust air, and would therefore increase the cooling load (if there were no bypass to avoid this). The SBEM just assumes that heat recovery is continuous through the seasons, and not bypassed in summer, when heat recovery would be counter-productive, and it thereby increases the cooling load in the Actual, but not the Notional Building.

My solution; don't define heat recovery where there is also cooling defined, (even though there will in fact be heat recovery, which will operate during the winter, and which ought to reduce the heating load, which ought to be a benefit to the Actual Building). Unless there is a better way of doing this?

Wasted

[Complex Potential]

What you describe is indeed a pain. The only thing I can suggest is that you experiment with adding free cooling to your model to offset the increase in cooling demand when heat recovery is active.

Otherwise, I generally just live with it for buildings where the heating load is significantly higher than the cooling.

[Wasted Energy]

I have found the solution to this problem - somewhat by chance.

The solution is actually described in the iSBEM user guide where it is applied at room level by turning variable heat recovery on (to simulate a bypass damper), but there is no corresponding option at this level in IES.

In IES what you need to do is enter the the NCM wizard that describes the A/C split system that is applied in the rooms where there is also ventilation heat recovery specified at the room level. In the wizard go to the Ventilation tab and at the bottom of this tab tick 'Variable Heat recovery ?' - and bingo - your cooling loads will go down in these rooms.

Wasted