05/06/2018: Is the use of drain water heat recovery relevant ?

décembre 6, 2017 issamsemlali No comments exist

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The use of the drain water heat recovery has become usual when owners seek some energy recovery in new constructions or during renovations.

However, it is obvious that due to the design of this particular piece of pipes induces a relativeley consistent pressure loss (too many loops with a D shape tube). So at the end there is an expected energy economy against an increase of energy demand on the pump side to compensate the pressure loss. The response that gave me all the professional I asked was that this pressure is not assumed by the owner but by the tape water supplier, which means that – in a sustainable development point of view – we are simply transferring the problem outside the house and maybe the initiative do not leads to a real economy after all.

To be factual, I took the example of a specific model listed in NRCAN website which indicates the flow, the pressure loss and the heat transfer power, measured and certified by a laboratory:

  • Heat recovery power = 7 KW; Cold water flow = 1.58 10-4 m3/s; Pressure loss = 9100 Pa

On the other side, the power pump increment =DeltaP (Pa) x Flow (m3/s), so:

  • Power pump increment = 1.44 W

In conclusion the economy is indisputable. But there is two facts that have to be highlighted:

  • The cold water flow taken into assumptions corresponds obviously to the last economical models of showers and taps (flow around 9.5 L/min). If the construction is expected to be or remain with a standard shower or tap, the instantaneous flow can exceed 60 L/min which can impact dramatically the pressure loss and a certified measurement has to be made again with realistic flow values.
  • The measures made in accordance with CSA B55.1 are in permanent regime. However, our water demand is characterized by various short and pic demands with a duration from 1 to 10 min, which will put the system in a transient heat transfer probably due to thermal inertia. And the energy recovery cannot be concluded to be interesting unless there is a modelization of the process, and before that a highlight of several hypothesis…

Actually, the initiative is encouraged with 2 LEED credits (according to Ecohabitation) and up to 0.9 point (according to a manufacturer)…

Questions:

  • Are these LEED credits given even if the house maintain an old tap/shower system ?
  • Was the conclusion of the indisputable energy recovery based on real measures in-situ? Based on modelization for transient heat transfer process? Or simply based on the following argument which seems to be acceptable?

The tap water supplier ensures a minimal fixed pressure at the user entry. This minimal fixed pressure is in all cases lost at the opening of the shower/tap when it get in touch with the atmosphere. In that way, the installation of such “tool” will be beneficial for houses which still have old non-economical shower/tap because the pipe will play the role of a flow reductor. And this will be beneficial also for the tap water supplier if he applies pressure regulation in its network.

Finally, if we assume that the subject was simply a false alarm, at least, because of estimations are based on permanent regime, we are not underestimating the savings. So we are far from being “conservative” (which is one principle of several measurement & verification protocoles like the IPMVP) and cost saving estimation would not be reliable.

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