Coffee with Caleffi: Proper Component Selection for Boilers and Application Fundamentals

Training and education manager leads technical webinar for contractors, designers and wholesalers
Coffee with Caleffi: Proper Component Selection for Boilers and Application Fundamentals
Bob “Hot Rod” Rohr, training and education manager for Caleffi Hydronic Solutions

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Bob “Hot Rod” Rohr, training and education manager for Caleffi Hydronic Solutions, leads a free webinar, “Part 1: Proper Component Selection for Boilers and Application Fundamentals,” from noon to 1 p.m. CT Thursday, Nov. 19. “Part 2: Proper Component Selection in Renewable Energy Hydronic Systems” will be held Thursday, Dec. 17. There is no cost to participate in the webinars, although registration is required.

Topics for the Nov. 19 webinar include:

  • With so many component and feature choices, how do I decide which are best for my application?
  • Pressure, temperature, flow rate – what are the components’ role in their control?
  • What problems can arise with undersizing or oversizing components?
  • What other performance problems can occur? Why?
  • Combination devices – what is their attraction? Trade-offs?
  • What are practical component maintenance tips?
  • What are effective ways to protect high-efficiency equipment, including boilers and pumps?

For any boiler, overall efficiency is driven by the burner. In last month’s Coffee with Caleffi presentation, Jody Samuell, manager of engineer education for Caleffi Hydronic Solutions, examined the combustion process and how it affects hydronic system operation.

The following is a question-and-answer session from the webinar, “Designing for Condensing Boiler Performance.”

Q: What is the dew point for propane?

A: The dew point is dependent on excess air and the actual makeup of the fuel. The normal dew point used for propane is 122 degrees F at 10 percent CO2. If the excess air is changed, then the dew point will change accordingly.

Q: What impact does near boiler piping design have on efficiency?

A: The efficiency is primarily a function of burner setup, return water temperature and cycling. If the design impacts cycling (improper hydraulic separation, for example) or if the design impacts the return water temperature (over pumping the boiler loop, for example), then there will be a reduction in boiler efficiency.

Q: What about hydro air with air handlers?

A: Hydro air systems can work with condensing boilers, but as explained in the webinar, it is all about return water temperatures. It is very similar to the approach needed to be taken with baseboard. The first step is to determine what water temperature is needed on the design day. If the air handler is oversized, you can use a lower supply water temperature and/or you can increase the Delta T across the air handler to maintain a lower return water temperature back to the boiler. Then run the boiler with outdoor reset to lower the water temperature to what is actually needed on any given day. The key to comfort is to limit the minimum water temperature to prevent a cold blow on milder days. This will maximize the performance of the system and still provide a comfortable environment for the building occupants. From an efficiency standpoint, approach the efficiency of a baseboard system with a properly setup hydro-air system.

Q: What about Delta T pumps?

A: Typically the boiler loop runs with a fixed-speed pump and the system side runs on the variable-speed pump. Because of this, a Delta T pump will maintain the differential temperature by changing the flow rate on the system side of the hydraulic separator. Since the boiler loop is running at a fixed speed, if the system flow is less than the boiler flow, supply water will mix in with the return water elevating the return water temperature, resulting in no performance gain as compared to a non-Delta T pump.

Q: Please explain outdoor reset.

A: With outdoor reset, the system does not run at one fixed water temperature. It is called outdoor reset because the water temperature is changed (or reset) based on a change in the outdoor temperature. This is used to match the heating system’s output to the heat loss of the structure. As the outside temperature lowers, the rate of heat loss increases; therefore the water temperature has to be increased to match the load. Outdoor reset manages this.

Q: Obviously some condensing will be due to venting in unconditioned space. Just measuring the amount of condensate is not accurate.

A: For the certification testing of the boilers, only the condensate produced in the boiler is accounted for. In any instance the amount of condensate is only part of the story. A gallon of condensate represents a different latent gain for a 100,000 Btu/hour boiler than it does for a 500,000 Btu/hour boiler. It also doesn’t tell the sensible heat loss story. If you are looking for an accurate picture of the efficiency of a condensing boiler, then be prepared to do a bit more work. You will have to start with clocking the gas meter to get an accurate reading of the boiler input. The output is going to be calculated by accounting for the losses and gains.


Refer to the slide below (courtesy of Viessmann U.S.):  

Q: Have you seen many condensing boiler installations for DHW only, using a coil in a steam boiler? How do you deal with the mixing valve required temperature differential limiting how low the water temp can be set at, which would limit how much it can condense?

A: With condensing boilers, the return water temperature is the important factor in determining if the boiler will condense. The question is how large a Delta T can the indirect water heater’s heat exchanger generate? The larger the Delta T, the lower the return water temperature and the higher the efficiency. In a commercial application, it also may help placing a preheating heat exchanger in series with the water heater to open up the system Delta T. I might obtain 30 degrees F across the water heater if I can get an additional 20 degrees F across the preheat, which will result in better performance.

Q: For a radiant slab job to achieve proper balance, is that possible with Caleffi manifolds only?

A: Unless all loop lengths are equal on the manifold and all loops require the same flow rates, a manifold with flow gauges/balancing valves will be needed.

Q: Don't forget the 15 percent "heating effect" in the footnotes there.

A: The 15 percent mentioned here is the difference between the DOE heating capacity of the boiler and the Net IBR rating. It is there to provide enough heat for “piping and pick-up losses.” For many designers, this is a thing of the past. If the piping is insulated and the system is running constant or near constant circulation because of outdoor reset, this 15 percent extra is not needed.

Q: Combi units (DHW/space heating) – Can these be efficient? Radiant heat might be 90 degrees F, DHW 130 degrees F. Radiant heat is via flat plate exchanger but the boiler must maintain the 130 degrees F. I say "no" to the efficiency question and do not recommend the Combi units, but rather a stand-alone boiler with indirect water heater. Am I right?

A: Depends on the Combi boiler. It can be efficient if the DHW is run by a flow sensor in the DHW that brings the boiler up to the proper temperature to provide DHW only when there is a call. In this situation, the boiler can run off of outdoor reset for space heating, providing good performance based on the lower water temperatures that outdoor reset dictates. When DHW is required, the flow switch signals the boiler to ramp up to the temperature needed to provide DHW and maintains that temperature as long as the demand is there. Once the demand stops, the flow switch opens and the boiler goes back to a space heating mode running off of outdoor reset.

Last month’s webinar, “Designing for Condensing Boiler Performance,” can be viewed below:


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