Geert,
We've been looking into this in more detail and noticed the following:
1. Reducing the HW plant loop volume from "Autocalculate" (E+ calculated 4.1m3) to the minimum allowable value of 0.1m3 gives results much closer to what you may have been expecting (<1% difference between loads and boiler fuel).
2. Timestep results give an even greater difference between loads and boiler fuel. This is expected because in the first few timesteps in the morning the boiler is working flat out just to heat the HW loop water and there is no averaging effect of timesteps later in the hour where the water is already heated up.
3. Regardless of the differences in peaks you have seen due to the thermal mass of the water in the loop, as expected the overall energy consumption over the year is almost identical (total coil heat delivered vs boiler fuel consumption over the year).
4. The way you size the boiler is your decision as an engineer, however based on the data you are seeing I think convention would say that you would ignore the mass of the water in the circuit when sizing and use the sum of the zone loads. Some might include a design factor to help heat up quicker under an intermittent operation regime. Alternatively start heating earlier and save on boiler capacity!
Perhaps others might share their experience of using of simulation for plant sizing?
Andy Tindale
DesignBuilder
From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]
Sent: Monday, April 20, 2015 7:20 AM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Total heating load boiler 60 % higher than total heating load rooms
I've done a designbuilder simulation of an office building with fan coil units heating and cooling every room.
In Designbuilder you quickly get the total system heating load (the sum of all rooms) and the Boiler total capacity for heating.
I work with efficiency 100% for the boiler, so normally the heating load of all the rooms, must be the total capacity for the boiler.
In this model, the boiler needs 60 % more capacity ( at 7 AM on the coldest morning in the year) then the sum of the heating load of all the rooms. Later the day, the two are loads are almost equally, so it's just at startup in the morning.
According to the DB support this could be the difference between the inertia of water (fan coils) and gas (boiler).
I've never had such a difference. < span sty le="word-spacing:normal;line-height:1.25;">Is this normal?
What will I communicate to the client as Heating capacity for the boiler? The sum of all the rooms ? Or the boiler capacity as calculated ? (which is a lot higher than the sum of all the rooms)
Geert
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Posted by: Andy Tindale <Andy.Tindale@xxxxxxxxxxxxxxxxxxx>
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