Data Logging a Heating Hot Water System; Where’s the Load?

In the previous string of posts, I have been discussing how I went about deciding what to monitor in a data logger deployment associated with a heating hot water system that I am currently studying.  The system diagram is reproduced below and  additional information on the system (including larger images) can be found in a previous post titled Data Logging a Heating Hot Water System; An Overview of the Targeted System.


By being clever in how I deployed my limited logging capability, in addition to understanding how the different pumps in the system interact with each other, as discussed in the post titled Parallel Pumps; An Indicator of a Retrocommissioning Opportunity, I also hoped to somehow gain a feel for how the load was proportioned through-out various portions of the system.  

Since I did not have a lot of loggers, nor did have sophisticated sensors, I needed some sort of fundamental indicator of where the load was and if it moved around from day to day.  I was not looking for an exact quantification of the load so much as I was looking for an approximate indication of where it was and how much it was in each portion of the system
relative to the total  load.

After thinking about it for a while, I realized that if I knew the return temperatures from different portions of the system I might be able to find out what I wanted to know, especially if I combined this information with any insights gained from the pump current data logging and flow testing I was planning.

My reasoning was as follows:

  • The supply temperature to all portions of the system will be about the same, neglecting losses though the insulation for the time being.  While there were a few fittings here and there that were missing insulation, in general, the the insulation on the system was intact and the system was not extensive (probably less than a mile of pipe vs. the miles of pipe I encounter in some large, high-rise facilities).  Thus, neglecting losses through the insulation seemed reasonable, at least for a first assessment.  The bottom line is that I concluded that by monitoring the supply temperature leaving the central plant, I would have reasonable indication of the supply temperature everywhere in the system.


  • The return temperature from the various loads will be a function of the flow and load.  Given that the flow on the HVAC portion of the system would tend to be constant due to the three-way valves, then the return temperature from that portion of the system should vary fairly directly with the load.  The temperature data could be supplemented with the pump tests I did when I deployed the loggers, which would give me a feel for the flow in the two pumping circuits in the system.

The bottom line is I decide to use an open input on a logger at the boiler room to monitor system return temperature, which would reflect the combined load on the system (the magenta dot on the system diagram). I had two open inputs on the logger that I was going to deploy at the HVAC pump location to monitor pump current so I planned to use those inputs to monitor return temperatures from the AHU loads (blue dot on the system diagram) and the reheat loads (red dot on the system diagram).

This data, when combined with the flow implied by the pump test I performed with the existing pump gauge when I deployed the logger (which in theory, should be constant) will give me a feel for the HVAC loads.  (For more on pump tests, see the Energy Design Resources Design Brief titled Pumping System Troubleshooting.  Starting on page 2, you will find a description of a pump test, including why you might want to perform one, what some of the limitations are, and a general procedure with illustrations of how to interpret the data.)

Combining the HVAC load data with the system return temperature and the information I planned to collect by monitoring the boiler amps ( the subject of the next post in this series), will imply the magnitude of the process load (the light blue dot on the system diagram).

So, will my logging effort paint an exact picture of the hour by hour accumulated energy consumption of the various loads in the system?  Absolutely not;  but it will give me a feel for the relative magnitude of the various loads and help me understand the dynamics of the system and focus additional efforts.  And it will tell me more than I know now, and
probably more than anyone else really knows about the system.  Not a bad deal for a couple hundred dollars of hardware and a bit of planning.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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