The one number that governs everything: lift
A heat pump moves heat rather than creating it, which is why it can deliver more thermal energy than the electrical energy it consumes. How much more depends almost entirely on the lift: the temperature gap between where it takes heat from and where it has to deliver it. Small gap, high efficiency. Large gap, poor efficiency. Everything else is detail.
For domestic hot water in a Malaysian hotel or resort, the delivery temperature is usually around 55 °C, and often held higher for Legionella control. That end is fixed. So the design question becomes: what is the warmest source of waste heat available in this building?
Two places to take it from, and they are not close
Roughly 30 to 33 °C, on its way to the cooling towers to be thrown away. Lifting that to 55 °C is a gap of about 25 K.
Roughly 7 to 12 °C, on its way back to the chillers to be cooled further. Lifting that to 55 °C is a gap of about 45 K, nearly double.
Why the chilled water option quietly backfires
Taking heat out of the chilled water return does not just make the heat pump less efficient. It changes what the machine is. Removing heat from chilled water is cooling, so the heat pump has become a second chiller on the system, whether or not anyone intended that.
At a 45 K lift it does that job at well over 1 kW per ton of refrigeration. A competently run water-cooled chiller plant does the same job at roughly 0.6 kW/RT. So the building ends up displacing efficient cooling with cooling that costs more than twice as much, in order to make hot water. The hot water looks free. It is not: it has been paid for on the chilled water side, where nobody is looking.
Why the condenser water option counts twice
Heat in the condenser water return has already done its job. It is waste, in transit, on its way to a cooling tower that will spend fan and pump energy rejecting it to the sky. Intercepting it does two useful things at once.
First, it makes hot water at a high coefficient of performance, because the lift is small. Second, it removes heat the cooling towers would otherwise have had to reject, which lowers the condenser water temperature the chillers are working against. Chillers run measurably better against colder condenser water. So the same intervention produces hot water and improves the chiller plant, from one piece of equipment.
When this does not apply
This is not a universal recommendation, and the cases where it fails are worth stating plainly.
- The building has no meaningful hot water demand. Most offices and most malls do not. Without a sustained load to absorb the recovered heat, there is nothing to justify the plant.
- The cooling load and the hot water demand do not coincide. Recovery only works while the chillers are running. A building whose hot water peak is at six in the morning and whose cooling peak is at two in the afternoon needs storage, and storage changes the economics.
- The condenser water circuit cannot take the modification. Some plant rooms genuinely cannot be worked on without an outage the building will not accept.
- The numbers are marginal. If the recoverable heat is small relative to the capital, the honest answer is to leave it alone and spend the money on the chillers.
What to ask whoever is proposing it
- What is the source temperature, and what lift does the design actually run at?
- Is the heat taken from the condenser side or the chilled water side, and if the latter, what is the effective kW/RT of the cooling it is doing?
- What happens to the recovered heat when hot water demand is low?
- What is the measured hot water demand, from meter readings rather than from a design assumption?
- How will the saving be verified once it is running, and against what baseline?
We have designed and priced condenser water heat recovery and air-source heat pump systems, and the load figures behind those designs come from our own energy audits rather than from a manufacturer's estimate. None of those plants has yet completed a full verification cycle, so we publish the engineering and no performance figures. When there are metered results, they will appear on our results page with their measurement basis, in the same form as every other number we publish.
Start with what the building is actually doing
Recovery only makes sense once you know the real cooling load and the real hot water demand, measured rather than assumed. That is what an energy audit produces, and the government will usually fund it.
Check whether your audit can be funded