Hot Water and Warm Water Calculations

Cold Water + Hot Water = Warm Water

We have seen some misinterpretation or misunderstanding amongst users with warm water vs hot water and how they affect energy so thought we’d post on this topic.  Warm water use is very different from hot water use.  Warm water use is assumed static in the eTool default residential hot water templates.  That is, we assume you have the same number of showers for the same length of time regardless of the climate you live in.  This assumption is probably incorrect but is a relatively small driver compared to other factors.  We also assume that the output temperature of a shower is the same in different climates.

What this means that the amount of hot water need to change if the input water temperature to the building changes.  Let’s compare a building in a tropical climate with an input water temperature of 30oC to one in a cold climate with an input water temperature of 5oC.  If you want a shower at 39oC this will be the ration of water you’ll need:

Climate Ambient Water Contribution and Temp Hot Water Tank Contribution and Temp Water Use and Temp at Shower Head
Tropical 7L/s at 39oC 3L/s at 60oC 10L/s at 39oC
Cold 4L/s at 39oC 6L/s at 60oC 10L/s at 39oC

We’ve excluded decimal places for simplicity.

Note that the cold climate requires twice as much hot water from the hot water tank to deliver a comfortable temperature at the shower.  That is, the user will need to set the mixer to deliver more hot water than cold water.  That also means twice as much hot water for the same amount of warm water.

Water Heater Energy

But the energy the water heater uses in the cold climate will be more than double that of the tropical climate.  This is due to the hot water heater working harder to get the incoming water to 60 degrees in the cold climate:

  • In the cold climate the water heater needs to take the 5oC incoming water to 60oC which is a 55oC difference.
  • In the tropical climate the water heater needs to take the 30oC incoming to 60oC which is a far lower 30oC difference.

Warm Water Demand

Estimating warm water demand is difficult.  eToolLCD uses measured water demand studies to estimate warm water demand.  This is different from a theoretical number derived from estimating shower run times and flow rates (plus other hot water uses).  We have made the decision to use empirical data for the following reasons:

  • Estimating shower run times is hard. I really think that has to be done empirically rather than theoretically.
  • In general there is a significant “performance gap” in building energy estimates. Design teams consistently under-estimate energy use. This is recognised widely. So I tend to trust measured data more than theoretical data.
  • Greenwashing is real in the water labelling space.  Even in Australia where water labelling is heavily regulated it’s far from perfect.  I recently purchased two “high performance” WELS 4 star shower heads that advertised a flow rate of 5L/s. Neither of them used that much water when I measured them.  In fact, one one used 9L/s and the other use 7.5L/s.  Was it high pressure?  Well, I’m inclined to think not as my existing shower heads (WELS 3 star 9L/s) measured at 9L/s flow rates.  So I think there are suppliers who are gaming the system. This is of course anecdotal but I’m personally wary. The response from WELS regulators was pretty poor too, as far as I know there was nothing more than a slap on the wrist for the suppliers and the offending equipment is still on the WELS database.

Reconciling with Other Estimators

If users are trying to reconcile the eToolLCD template hot water energy figures with other hot water use or energy demand estimates please consider the following:

  • Warm water is not the same as cold water:  Make sure you’re comparing apples with apples.  For example the Green Star potable water calculator has an output of “Hot Water Use” in kL/annum for buildings.  This figure is hot water not warm water, so it’s the akin to putting a meter on the output of the hot water tank (not at the tap or shower head).
  • Climate differences: Does the alternative estimator account for climate differences when estimating both the mix of warm vs hot water and the energy required to achieve hot water in the tank?  That is, is the mains water temperature factored into the calcs?
  • Tank Heat Losses: For systems with tank does the alternative estimator consider tank heat losses?  These can be significant.

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