Controls Are a Pain
Controls are a pain, right? How many times has a technician uttered the phrase, “The controls weren’t set up right?” Here’s a question, would it be smart to spend an enormous amount of time and effort, and thousands of dollars on materials and equipment, only to throw the switch and walk away on the last day of a 1 year project? The final startup of a heating and cooling plant is typically done as the last part of the job, which may be over-budget, behind schedule, and the boss is screaming for the installers to get out of there, “Because we’re out of hours.” This may just be why the controls aren’t set up right.
For this article, I’ll be referencing a home, “Joe’s house,” where we installed the mechanical systems and automated controls a few years ago. Joe’s house is a 15,000 square foot estate complete with over 30 zones of radiant heat, 12 fan coil systems utilizing chilled water for cooling, forced air zoning, low-temp hot water loop for radiant/supplemental heat, 4 water to water heat pumps, 2 heating/snowmelt/domestic hot water serving boilers, pool and spa heat exchangers, and a pair of 120 gallon horizontal side-arm water heaters. A typical Joe-blow home owner’s pad right?
With the above in mind, the heart of the system is the 4 water-to-water heat pumps that simultaneously produce the 110 degree F heating water for the radiant floor and fan coils, and the 40 degree F chilled water which is used year round as it also serves the wine cooling cave (yes they have one of those too!) and AV/computer loads. The staging and sequence of the heat pumps is critical to getting the expected performance, efficiency, and longevity out of the heat pumps.
In Joe’s system we used TCS Basys DDC controls, one of their boiler controls, and one of their chiller controls to stage the heat pumps. This system is priority cool as the cooling has year-round critical loads, and the heating side is backed up by the boiler. The four heat pumps are Climate Master water-to- water, 3 phase, 5 ton, piped to a geo-exchange loop, with fifteen 400ft boreholes into the Rocky Mountains.
Joe’s heat pumps also have automatic 2-way valves on the ground loop side, and dual 3-way valves on the house side to connect the heat pumps to both the chilled loop and heating loop, and yes the systems are cross-connected. Are you starting to see why controls set up is critical? The heat call or cool call must move the ground side 2-way valve, both 3-way valves the correct directions, and hit the reversing valve when needed. If one valve goes the wrong way it’s a disaster, and Joe would have fired/sued us if we didn’t thoroughly check these valves. Don’t mess with Joe!
Throughout the mechanical system are some very small heating and cooling loads. For this reason Joe’s engineer had the foresight to design 80 gallon buffer tanks on the chilled and low temp loops. The boiler and chiller controls stage the heat pumps based on temperature sensors in each of the tanks. Our staging controls have the ability to rotate and equal run their connected heating or cooling loads; however, we’re not using this feature. In our system, the first of the four heating stages is the very last of the four cooling stages. The same for the cooling stages, only the first cool stage is the very last heat stage. During mild times of the year, it’s not unusual to have the first of each stage only cycle off a few times a day, as they are handling the base of Joe’s house loads. The majority of cycling is happening between the middle units. Trend log data from the DDC system proves to Joe that he got what he paid for.
Through observing this and other systems in operation there are some general rules of thumb and pitfalls I follow on staging:
- Pet peeve pitfall #1 with boiler staging controls is when the control is set higher than the boiler operator control. In this scenario the staging control is doing nothing, as it can never satisfy. Set the controls to the design temperature and the operator 5-10 degrees above that. Do not exceed the boiler operating temperature range.
- Generally slower staging is better for efficiency and longevity, with a minimum goal of 5 minute run times. Don’t be impatient with stages and set differentials and dead bands too low.
- Temperature sensor locations are key! Does the sensor that comes with the staging control go on the boiler loop or the system loop? Usually the system, but if you’re not sure, ask!
- Whenever possible, and this means a 2nd or 3rd trip out, observe your new system during design conditions, and in part load conditions. Systems may run great on design days, but short cycle on partial load days. Extra trips are way cheaper than lost customers or property damage!
I’m happy to say that Joe’s house is still running great, and Joe is still our customer. It took 20-40 hours of observation time to optimize the heat pump staging, and would not have been feasible without DDC controls and remote access to the data. Great control systems are cheaper, better, and easier to set up than ever. Embrace the end of the job!
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