Five Questions about Hydronic Heat Sources

On the Mastering Hydronic System Design Course discussion board with expert instructor John Siegenthaler…

Takeaways:

• Although closely spaced tees are great for hydraulic separation, a modern hydraulic separator provides air and dirt separation in addition to hydraulic separation.
• When considering whether to use electric boilers for radiant hydronic heat or electric resistance radiant heating, it’s important to design the distribution system for the life of the building.
• Perhaps in some buildings where uninsulated piping passes through attics or ventilated crawl spaces it might be true that 15% of the boiler’s output is lost between the boiler and the heat emitters, but in modern buildings the heating piping is almost always within the building’s thermal envelope.
• Because the solar thermal market has changed so drastically, it’s no longer a “given” that the combination of PV + heat pump water heater will always have a better life cycle cost than traditional solar thermal. However, this combination may become a competitor to traditional solar thermal, especially where net metering is available.

Student 1: Is short cycling an issue also with conventional boilers? It seems that compared to a boiler that modulates down to 20%, a conventional boiler of the same output must have 5 times the buffer capacity to be less susceptible to short cycling. Is that true?

John Siegenthaler: Yes, all other conditions being equal, a fixed output boiler would require 5 times the buffer volume relative to a boiler of the same peak heat output, but with a 5:1 turndown ratio.

Student 1: Since hydraulic separators seem to do everything that closely spaced tees do and more, why are closely spaced tees used instead? Does the separator cost significantly more than installing the individual components (air separator, etc.)? Here’s what Caleffi says.

John Siegenthaler: If the ONLY objective is hydraulic separation, a pair of closely spaced tees is definitely lower cost than a hydraulic separator. However, the appeal of a modern hydraulic separator is that it provides high efficiency air and dirt separation, in addition to hydraulic separation. When the cost of all the materials and labor required to provide closely spaced tees, high efficiency air separation, and high efficiency dirt separation is considered, a “3 function” hydraulic separator is of comparable installed cost. It also takes up less space in tight mechanical rooms.

The best way to view hydraulic separation is that it can be accomplished multiple ways, with multiple hardware configurations. Learn all the methods and apply each of them where they make the most sense. For example, if you know a buffer tank will be present, and understand how to pipe it to achieve hydraulic separation, there is no need for another hydraulic separator (I’ve seen systems where this was not understood, and booth the tank and separator, as well as a circulator in between have been installed).

Student 1: When would it make sense to use an electric boiler for radiant hydronic heat versus electric resistance radiant heating? It seems like the latter would be as comfortable, efficient, and easier to install. Or is the electric boiler mainly used with other types of emitters?

John Siegenthaler: From an energy cost perspective each approach should be equal. However, the use of hydronics allows the possibility of changing the heat source in the future, whereas electric resistance panels do not.

For example, perhaps the electric boiler would eventually be replaced by an air-to-water heat pump.

My preference is to design the distribution system for the life of the building. Keep it a “low temperature” design (max 120ºF supply water temperature at design load) to “future proof” it. This allows the possibility of several future heat sources as technology allows, or future energy costs mandate.

Student 1: It makes sense that DOE heating capacity should be > IBR Gross which = IBR Net/85%, but that does not seem to be the case with manufacturers’ specs. For example, in Figure 3-27, taking any IBR net water figure (say, 68.7 in the first row) and diving by 0.85 will yield a IBR Gross number that is a bit higher (not lower) than the corresponding DOE rating. Is there some other factor in play here?

John Siegenthaler: I agree that DOE heat output should be higher than IBR gross rating because the DOE output assumes boiler jack heat loss is useable output whereas IBR gross doesn’t.

I’m not sure why this is the case with these ratings. Perhaps the manufacturer was using some other method to establish the IBR net rating, or perhaps there was some other factor involved in establishing the DOE ratings given.

I typically only use the DOE heating capacity rating in boiler sizing. I think that the rationale between the IBR gross and net ratings is not well-founded technically. It assumes that 15% of the boiler’s output is essentially lost between the boiler and the heat emitters. Perhaps in some buildings where uninsulated piping passes through attics or ventilated crawl spaces this might be true, but in modern buildings the heating piping is almost always within the building’s thermal envelope. These rating methods date back decades, when fuel was cheap, and concern about over-sizing was minimal.

Student 1: Martin Holladay argues that for domestic hot water, solar PV with a heat pump water heater is more cost effective than a solar thermal system.

I’m curious as to whether a similar argument can be made for hydronic space heating. Under what conditions is solar-thermal is likely to be the more cost-effective solar choice?

John Siegenthaler: The solar thermal market has changed drastically over the last 5 years as the price of solar PV modules has dropped, while the price of the materials in solar thermal collectors (and the balance of system) have increased.

Although I cannot make an all encompassing statement that the combination of PV + heat pump water heater will always have a better life cycle cost than traditional solar thermal, I’m sure this combination will become a major competitor to traditional solar thermal, especially where net metering is available. The only way to know would be to get installed cost and simulated performance data for both approaches in a specific system at a specific location. The performance of solar thermal system can be simulated using f-chart of other readily available software packages.

I’m not sure what’s available to simulate the PV+heat pump combination. One factor that I would inquire about is the likely life of a heat pump water heater. There are different approaches being used. Some have a copper coil for the condenser immersed directly in the tank. I doubt that this coil is cleanable or replaceable in most residential systems. I would prefer to use a system where the heat pump and storage tank can be separated, and the water side of the condenser cleaned if necessary.

Another factor is the potential changes in refrigerants over the next 10-20 years. There’s no guarantee that the refrigerant originally used in a present day heat pump water heater will be available if needed for service in the future.

My own leaning is toward a low ambient air-to-water heat pump supplying both space heating, cooling, and domestic hot water. If interested, check out the new low ambient heat pump just introduced by SpacePak in Westfield, MA. It maintains a COP of 2.4 at 0ºF ambient. This appears to be an excellent heat source / cooling source, especially in combination with PV and net metering.

About the Instructor:

John Siegenthaler is a licensed professional engineer, a mechanical engineer, Professor Emeritus of Engineering Technology at Mohawk Valley Community College, a Hall of Fame member of the Radiant Professionals Alliance and graduate of RPI. John is the author of many trade publication pieces about hydronic heating and solar thermal system designs as well as the Modern Hydronic Heating textbook.

ADDITIONAL RESOURCES:

• Expert Course: Mastering Hydronic System Design with John Siegenthaler
• Expert Course: Condensing Boilers in Hydronic Systems with Roy Collver
• Expert Course: Integrated HVAC Engineering with Robert Bean
• Free Course: Integrating ASHRAE HVAC Standards with Robert Bean
• Free Course: Radiant Cooling & Heating with Robert Bean
• Free Tool: Hydronics Design Studio 2.0 Demo by John Siegenthaler