Robert Bean on Fluid-Based System Concerns in HVAC Engineering

Abby Thompson

On the Integrated HVAC Engineering Discussion Board…

Robert Bean discusses losing boiler efficiency, vs. losing radiator efficiencycycling a boiler to maintain average temperature in a system, using 3 way mixing valve (instead of a circulator) to control the temperature of the on the load side, and more!

Student 1:

1) The pressure differential control valve shown around 22 minutes [in Lesson 8 of this course] at the end of the system acts as a bypass, and I believe it will increase the return temperature to the boiler. If the return temperature increases the boiler efficiency decreases. What are your thoughts on the trade off of losing boiler efficiency, vs losing radiator efficiency? Can the boiler be cycled to maintain some average temperature in the system, while trying only to operate during times when the efficiency would still be high?

2) The bridging layout a few minutes after looks interesting, but I just know what my old bosses would say, there’s no way we can add that many pumps (the circulators) to a system because they are all a point of failure. Instead of the circulator could a 3 way mixing valve control the temperature of the on the load side? Or perhaps do the pressures of the main circulator somehow not provide the correct pressure differences to circulate through varied load loops?

Robert Bean:

Re: The pressure differential control valve shown around 22 minutes at the end of the system acts as a bypass, and I believe will increase the return temperature to the boiler.

Yes and decrease the return temperature in cooling.

Re: If the return temperature increases the boiler efficiency decreases.

Yes, likewise if the return temperature decreases the chiller/heat pump efficiency decreases.

Re: What are your thoughts on the trade off of losing boiler efficiency, vs losing radiator efficiency.

Due to the variables, it’s complex…as it relates to the boiler efficiency; considering so much of the return temperature depends on what the design mean temperature is for the zones and the weighting of loads open at any given time. If 10 zones are served adequately with 90F supply with a design return of 75F, the consequences of 9 closed zones and one flowing zone but of a significant load, the consequences might be minimal for that period of time that the condition exists. However the same zoned system with a design temperature slightly above the condensing temperature of the boiler might make all the difference in the world. As you can tell I don’t have a definitive thought as, “it all depends.”

Re: “Losing radiator efficiency.”

Without the bypass valve or use of a variable speed circulator the differential pressure across an open zone in the presence of closed zones can cause overflow and unstable control valve operation due to destabilization of the room temperature – a result of radiator output exceeding room losses (see discussion on log mean temperature differences, LMTD). The pressure bypass ensure this conditions is mitigated.

Keep in mind the bypass valve can also be located around the circulator; i.e. it’s not necessary to locate it at the end of the circuit. However, for zones that are sensitive to changes in loads – having design pressure (flow) and temperature available at the inlet of a closed valve can sometimes be an advantage… especially in very large projects with remote zones.

The use of a variable speed circulator achieves the same thing but with motor control versus valve control. The latter having a power penalty since resistance is used to control pressure versus reducing power in the circulator.

Finally, as noted in the introduction, it is not possible in this curriculum to address every conceivable schematic and I would encourage those following and interested in the, “can you do this” or “shouldn’t you do this” discussion to obtain copies of Total Hydronic Balancing by Prof. Robert Petitijean published by Tour Andersson (sometimes free, in North America see your local Victaulic Rep) and Modern Hydronics by Prof. John Siegenthaler (order hard copy from Appropriate Designs) and the idronics Series published by Callefi (free and available as a PDF download)…between the three references you will find hundreds of options for application appropriate schematics.

Re: Can the boiler be cycled to maintain some average temperature in the system, while trying only to operate during times when the efficiency would still be high?

Though it is not shown (but it is recommended), mean temperature reset control is a better representation of the heat exchanger(s) and system design. Prof. Bjarne Olesen has talked about this in several papers on radiant designs but I’m not aware of any “off-the-shelf” control system which does this…

Re: The bridging layout a few minutes after looks interesting, but I just know what my old bosses would say, there’s no way we can add that many pumps (the circulators) to a system because they are all a point of failure.

Where real estate is location, location and location; hydronic schematics are application, application and application. My ideal system is a mid-efficient boiler with standing pilot using analog reset control, a single fixed speed pump, and thermostatic radiator valves with a spring loaded bypass control valve in a high performance building. It is considered sacrilege by all those who are virgins in the world of design, commissioning and maintenance. It’s only when you have a bad back, gray hairs, poor eyesight and failing memory do you appreciate the simple bullet proof systems. Young people who thrive on technology want the zippy stuff until it needs fixing while the customer is holding a baseball bat over your head… then, they get why old technicians are grumpy and long for the old days.

Re: Instead of the circulator, could a 3 way mixing valve control the temperature of the on the load side?

You can do whatever you’re comfortable with so long as it works. 2 way injection valves, 3 way mixing valves, 4 way mixing valves or variable speed injection circulators all accomplish the same thing, just keep in mind when it comes to systems the water molecules don’t care nor do customers. Pick what works for you and stick with it so you don’t leave behind a smorgasbord of different systems when you retire.

Re: Or perhaps do the pressures of the main circulator somehow not provide the correct pressure differences to circulate through varied load loops?

Again, this is application based, as noted in the control module when you control pressure you control flow and when you control flow you have authority over the system. A complex network of pressures can be simplified by hydraulically isolating the suspect groups. This can be accomplished with primary/secondary piping and bridging piping techniques.

Regards,
Robert Bean

Integrated HVAC Engineering students discuss pressure theory, flow theory, velocity and head losses in fluid-based systems during Week 5 of instruction. Do you want to learn more? Enroll today!

About Instructor Robert Bean – Pres., ICC INc.

Robert Bean, R.E.T., P.L.(Eng.) is a registered engineering technologist in building construction and a professional licensee in mechanical engineering. He is president of Indoor Climate Consulting Inc. and director of www.healthyheating.com. He is a volunteer instructor for the ASHRAE Learning Institute and serves ASHRAE TC’s 6.1, 6.5, 7.4 and SSPC 55 Thermal Environmental Conditions for Human Occupancy; and is a special expert on IAPMO’s new Uniform Solar Energy and Hydronics Code committee. He has developed and teaches numerous courses related to the business and engineering of indoor climates and radiant based HVAC systems.

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Abby Thompson

Abby is HeatSpring's Product Marketing Manager located in Boston, Massachusetts. She is passionate about people and education, particularly in diversifying the burgeoning fields of science, technology, engineering, and mathematics. Abby works with instructors to build new courses and engages with our community of students and experts through HeatSpring Magazine and social media.