Fuel system fans linked with cooling?

koen.govers · Posts: 81

Cooling and ventilation are seperate in my project, but somehow they are linked in DB.
I'll explain;
- DB simulation gives me an annual 10kWh/m� fuel for system fans with cooling.
- if I turn off cooling its reduced to 3kWh/m�, all others (heating, room electr., etc) remain the same
- I've tested with higher setpoints for cooling and it resulted in lower energy needed for the system fan.
Is there a way to prevent DB for adjusting the ventilation.
(I've also simulated a week's hourly air flow, and there was no difference between the case with and without cooling)

Laurent_Support · Posts: 292 Location: Vincennes

Hello,

Well, I don't know your settings, so excuse me to ask you these questions.
- are you using VAV ?
- is fan are continuous or cycling ?
- do you use a central AHU or unitary system ?

Thank you,
Laurent

koen.govers · Posts: 81

Thx for the quick reply.
Here is some additional info:
- fan is continuous
- 1-unitary zone, so no VAV

koen.govers · Posts: 81

Ive also tested it with multi-unitary zone, and it gives the same results.
Somehow ventilation energy is consumed for cooling, but can't find how to make them seperate.
Lighting consumes 25kWh/m�, heating 11kWh/m�. So the 10 kWh/m� seems to high.
I figure the 3 kWh/m� will be more realistic...

Andy Tindale · Posts: 2432 Location: Stroud

Fan energy is related to required airflow rates which in turn is related to cooling loads etc. Are you using economisers? If so you may get extra free cooling flow rates in summer which will add to fan loads and reduce chiller loads. I'll check this next week.

If this doesn't help please send your model.

Andy

koen.govers · Posts: 81

Again thx for the reply,
Economizers are not set in the project...

When trying to get more output we realized the problem is bigger than expected:
- the ventilation fans work 7/7d and 24/24 (hour simulation data) when they are supposed to work in the workweek 8:18h and to be switched off otherwise.

- heat recovery does not seem to work at all, except for a small amount in summer days where there has been heating and cooling in the same day.

The combination of the previous 2 points makes the losses through mechanical ventilation too high. Static and dynamic simulation results predict a +- 40kW installation needed for total zone healing and cooling, which comes to a 70kW/m�. Too high for the low-energy concept weve created with low U-values etc.

Ive sent the model now, along with all possible data. Hopefully this clears the road.

Andy Tindale · Posts: 2432 Location: Stroud

The connection between cooling and ventilation with unitary systems is that the max supply flow rate is based on max cooling loads. If the cooling set point is lower then the max load is lower and so the max supply flow rate is lower too. If cooling is switched off then the supply flow rate is sized to supply just fresh air.

Use the system availability schedule to control the fan (and heating/cooling availability) operation.

I'm still looking into heat recovery.

Andy

Andy Tindale · Posts: 2432 Location: Stroud

Initial investigations have shown that for Unitary systems with the economisor switched off, heat recovery is only possible during cooling operation (return air temperature colder than outside air temperature). It seems that you must switch the economisor on to obtain heat recovery during heating periods. Be aware however that switching the economisors on will affect outside airflow rates.

I've requested more information from EnergyPlus support on this and will let you know when I hear back from them.

Andy Tindale · Posts: 2432 Location: Stroud

Following consultatiom with E+ support we have made a fix which will allow you to model heat recovery correctly in unitary single zone systems in v.1.4.0.43 and later.

koen.govers · Posts: 81

The heating setpoint temperature for heat recovery is a great new feature.

Thanks for the help,

Hans

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