IES-VE User Forum

Hi All.

I am trying to determine the best way to model a Central AHU with terminal reheat.

The AHU will have heat recovery and a DX condensing (electric) unit providing heating and cooling. This will provide an air supply to each space at a constant volume but the temperature will be varied depending on the mixed return and internal room conditions (averaged) . In each space an LPHW duct heating coil (via gas boilers) will be added to trim the air temperature to achieve the desired room condition.

Most of the energy for heating and cooling will be provided by the condensing unit but obviously some will be via the LPHW duct coil.

What wold be the best way to represent this in the Part L calculation? I was thinking a Bivalent system with air source as the main energy?

Any help would be appreciated, how would I calculate how much energy should attributed to each fuel?

Cheers

I'd select the NCM system type "Terminal reheat (constant volume)" with heating and cooling systems set to electric air source heat pumps (word of warning, the delivery efficiency for that system is quite poor).

If the duct heating is to be provided via gas LPHW then I suppose adding it via the bivalent tab is probably the best way. You have to input the estimated % of annual heat demand you think will be provided by the boiler compared to the total for the boiler and condensing unit combined but there's no automated way to do this. I'd recommend asking the mechanical designer to provide you a quick calculation based on the controls and specifics of the connected rooms.

Alternatively, if you can show quickly that the percentage is likely to be very small you could probably omit the gas side altogether as a justifiable simplification.

Thanks for the quick response CP