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It Doesnt Seem to Work Correctly

“It Doesn’t Seem to Work Correctly”

I once had the opportunity to give a training class for the people in the HVAC apprentice program for Local 355 in Suffolk County. I was impressed by the facility and the interest and sincerity of the folks there. It’s good to see people who want to be the best at their trade. One fellow asked me a few questions on the side about a particular job he was having some thoughts about. He had been called in after the installing contractor left the job and he felt that the two hot water boilers were not sequencing or staging properly in response to the demand. We talked a little about what was happening and I agreed with his thoughts. Not having seen the job I couldn’t pinpoint the exact problem but I had a good idea where to look. We are all aware of the energy and global environmental crisis we face and I think designers, installers and service groups are trying to do the right things to maximize equipment and system efficiencies and be kind to mother nature in the process. Indeed performance for equipment has risen dramatically in my 30+ years of watching. So why was this new installation, with its energy efficient boilers, burners, and smart controls not operating very intelligently? Design problem? Manufacturing Defect? No, it was the control settings. Far too often I visit job sites only to find the boilers operating inefficiently due to controls not integrated or set properly. We want to wring out the BTU’s on a boiler so we need to have the combustion efficiency dialed in and the cycle times maximized. So here is this job with both boilers starting and stopping at the same time and, to add to the horror, are shutting off in the high fire position resulting in a high amount of cycles per hour. No energy savings here. Examining the equipment and controls for the job, it becomes apparent that the control set points are incorrect and need to be reset. It’s a challenge today to select the right control devices that will be compatible with the equipment in the system and give you the results you want. Just looking at hot water type systems we have primary secondary pumping loops, reverse return pumping loops, pressure differential pumping loops, three way valve loops, fin tube zones, air handler zones, in floor radiant zones and so on. And for controls we can select stand alone, simple relay base sequencing, microprocessor based sequencing, optimizing, outdoor temperature reset and of course the interface with the BMS, the “master brain”. Sometimes the designer selects a control system that’s not compatible with the boilers. Sometimes the local boiler controls are not compatible with the BMS or the operating sequence the BMS wants to follow. Sometimes the controls combination may be perfect but the commissioning guy didn’t set the points correctly. This isn’t as simple as it may initially seem and it takes considerable expertise to design and install a system properly. I don’t have enough space in this newsletter to do this topic justice. Here are a few tips however for hot water systems that may help get you on track.

Consider the type of building, method of heating (fin tube, radiant, etc.) and check the temperature requirements of the distribution system. If you’re designing a low temperature distribution system, high efficiency condensing boilers work well. Not so good however on a high temperature systems. Don’t use a conventional “sensible heat” boiler on a low temperature system, it will condense and corrode. Check what type of fuel is available and how you intend to deal with the boiler exhaust. Select the boilers (copper, stainless steel, cast iron, etc.) based upon what’s best when looking at all the parameters.

With a specific type of boiler in mind, it’s now time to select the operating or firing mode. There are several common modes (on-off, low-high-low and full modulation) that may be available. Full modulation of the firing mode generally is the most efficient and some of the higher efficiency offerings may only be available as a fully modulating unit. That’s a good thing.

Now it’s time to start thinking about controls. You must consider the sequence of operation for the building. How do you want the system to work? How many boilers do you have? Did you design for 100% standby or a 70% - 30% load factor? How are you controlling the pumps? All this comes into consideration. Most all boilers come with a local operating and safety limit control but you want to incorporate some intelligence or a “master control”. Now, if you have a boiler that has a modulating burner there are two parameters to control. The start and stop of the boiler and the firing rate. Make certain that the master controller, which could be a simple relay based lead lag, microprocessor based lead lag, temperature reset controller or the BMS itself, has output provisions for both the start / stop and firing rate functions. If you using the BMS as the front end of the system don’t have the BMS and boiler master controller perform the same function. Talk to the boiler representative and the BMS system representative. Have them talk to each other. A little coordination beforehand saves a lot of time and grief later on.

Now you have a job that has multiple boilers, local controls, a master control and a BMS system. Working with the owner and various representatives you have created a sequence of operation for the system. That’s great but don’t stop there. Don’t leave it to the imagination of the commissioning guy to establish individual control set points. Create a schedule of control devices and provide suggested set points. Of course these points can be fine tuned in the field but it will help ensure the sequence of operation meets the design intent and we don’t have controls bypassing or overriding each other. Don’t fall into the “It doesn’t seem to work correctly” trap. Give us a call and discuss the application.

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