Zone heat balance output

Bobby Gilbert · Posts: 20

In the help system topic "Simulation detailed results" it says...

Note: taken together the Internal gains and the Fabric and ventilation data above give an approximate Zone Heat Balance, i.e. the data represents all of the heat flows into and out of the zone.

Could you explain why this is only approximate? I have done a simulation and over the course of a year the 100m2 house I am modelling in UK weather is 400kwh short of heat to acheive a heat balance over the year. It ends the year at almost the same temperature as it starts the year so this does not appear to be due to it cooling down.

Also, possibly related, there don't appear to be any casual gains for DHW accounted for when DHW is turned on. Are any casual gains included for hot water? if so how do I find how much?

Thanks

Bobby

Andy Tindale · Posts: 1148 Location: Stroud

Three inaccuracies in the heat balance spring to mind:

1. The solar gain is not 'solar absorbed by the zone' but rather 'solar transmission through glazing' so this is not useful in the heat balance.

2. More esoteric, a heat balance must be carried out at a particular point in the heat flow network, for example the air point or the 'rad-air/environmental' point. But the fabric heat transfer (Walls, Roof Heat Gain etc) is 'heat flow at the surface', whereas the ventilation gains are to the air point. Strictly speaking it is not correct to add gains through walls to gains from ventilation and this could cause a discrepancy.

3. Start up temperatures throughout the building will be different from end conditions. Over the year this effect should be quite small but will cause a non-zero balance.

You didn't say how big 400kWh was relative to the total heat flow but it does sound like a significant error.

There may also be other reasons I haven't thought of.

Andy

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