I recently read a trade journal article about what’s new in the world of “wet heat.”
The article focused mostly on boilers and how the current market trend is toward higher efficiency. This is true. It has been true for a least 25 years. Another takeaway from the article is it’s easier to sell boilers that operate at high thermal efficiency, especially as the price of fossil fuels is rising. Again, that’s true and it has been this way for at least the last 25 years.
Although I agreed with most of what was mentioned in the magazine, I was disappointed that hydronic distribution systems were barely mentioned beyond the statement that radiant heating still is creating sticker shock and continues to be a hard sell against cheaper systems.
As an industry, we have to move beyond judging the quality of a hydronic system based solely on the heat source. We must focus more effort on complete systems that complement the high-performance heat sources now available, systems that leverage the many benefits of water-based heat delivery vs. the “acceptability” of forced air.
Ferrari meets Cub Cadet
Suppose I decided to get into NASCAR racing. My plan is to go to Italy and buy the best racing engine Ferrari has to offer. I hear they run about $500,000 these days. I’m going to ship this engine back to the United States and install it in my Cub Cadet lawn tractor. I’ll paste some decals on the Cub Cadet and trailer it to the next race. Is that going to be an awesome racing machine or what?
Most of you are probably thinking this is absurd. What good is a finely tuned racing engine in a garden tractor? There’s no way to channel all the power it can generate through the belts, pulleys and gears to make the Cub Cadet fly around the race track at 200 mph.
Suppose you consider a boiler to be the “engine” of a hydronic system and the distribution system to be the “drive train.” Isn’t it just as absurd to connect a highly tuned mod/con boiler to a distribution system dating from an era when 200° F or higher water temperatures were the norm? How about going one step further by connecting the engine to a wall full of neatly aligned circulators that collectively draw more wattage than a blower in a generously sized furnace?
To top it off, how about installing all this over a backdrop of chrome diamond plate? Industry award, here we come ...
In North America, this stuff happens every day. I get unsolicited emails and phone calls every week from people seeking guidance on how to correct one of these hydronic mismatches. Just when you think the industry has turned the corner on such practices, another case arrives for triage.
At times, it seems we expect our North American hydronics industry to be judged solely by the “box” that heats the water. If it’s a shiny new mod/con mounted on some shiny chrome diamond plate, we’re supposed to assume the balance of systems look and perform as good as the boiler. Ditto for geothermal heat pumps and those flashy new pellet-fired boilers.
5 design principles
It’s time for our industry to get as serious about the quality of distribution systems as we are about heat sources. Here’s a list of five principles I believe should be embraced by those claiming to design modern hydronic systems.
1. 120º F and below.I suspect everyone reading this article has heard the following principle several times: Mod/con boilers are built for low-temperature distribution systems. They can only achieve their highly publicized mid-90% efficiency in such systems.
The idea of buying a mod/con boiler because it’s small and light and, according to its manufacturer, can be safely vented with PVC pipe is simply the wrong mindset. It’s even worse to crank up the boiler’s limit setting to 180° because the existing fin-tube in the house was designed for such a water temperature.
Getting back to racing — you probably wouldn’t hitch a thoroughbred race horse to a hay wagon. Why do the equivalent with a high-performance hydronic heat source?
My recommendation is to design all new hydronic distribution systems so they are capable of delivering design load output using a supply water temperature no higher than 120°. This lets a mod/con boiler hum along at peak efficiency. It also makes the system compatible with many renewable energy heat sources such as solar thermal collectors and heat pumps.
Even if these heat sources are not initially installed with the distribution system, that system stands ready for them in the future. It only makes sense to “future-proof” distribution systems that are expected to last for several decades.
2. Get serious about circulator wattage.It’s just plain wasteful to use multiple kilowatts of electrical power to move heat from where it’s produced in a basement mechanical room to heat emitters that, even in a large house, are probably no more than 100 ft. away from the boiler.
Just think about it for a moment: Using several thousand watts of power just to move heat through a house. If this doesn’t bother you, it’s probably best if you get out of the design part of this industry.
3. Don’t over-complicate your systems.I just did the forensics on a system with meticulous piping craftsmanship. It had primary loops, secondary loops, tertiary loops and even “quadinary” loops. Some with circulators, some without. Aside from some thermosiphoning, heat was not making it from source to load. Instead it was loop-de-looping through several hundred feet of uninsulated cooper. Well, at least it helped heat the basement.
It was a shame most of that shiny new copper and beautifully done soldering got scheduled for disassembly. It was the only way to make it work. The solution involved far less piping and far fewer circulators. Check out www.radiantandhydronics.com’s March 2014 The Glitch & The Fix article to see the before and after designs.
It’s also easy to overindulge when it comes to controls. Just because you can put a thermostat in every room doesn’t mean you should. Just because you can create a system capable of creating five simultaneous water temperatures doesn’t mean it’s the best approach. Simple is better. Always has been, always will be.
4. Forget about long series of circuits. In new installations, long circuits of series-connected heat emitters linked by soldered copper tubing should be avoided. This is the customary way to install residential fin-tube baseboard. It’s also a setup for complaints about why the room at the end of the circuit is too cold and the room near the beginning of the circuit is too warm.
In theory, if you’ve accurately determined the design heating load of each room and you’re willing to do calculations or run some software, it’s possible to get the right amount of baseboard into each room. The problem is few contractors are willing to do the upfront design work. They would rather just install wall-to-wall fin tube and call it a day.
Move away from the traditional series circuits used with baseboard and instead use homerun distribution systems that supply each baseboard with 1/2-in. PEX or PEX-AL-PEX tubing. Each baseboard is now in parallel with the others. Each receives the same water temperature and each can be flow-balanced as needed to fine-tune output.
Much less soldering is involved and installation time is reduced. The fittings to transition from the PEX or PEX-AL-PEX to the copper fin-tube element are readily available. Try this approach once and it’s unlikely you will ever go back to series-connected baseboard.
5. Don’t fixate on primary/secondary piping.Sure, when properly done, P/S systems work. We’ve designed plenty of them but none within the last eight years or so. Why? Because there are other ways to achieve the same benefits offered by P/S piping that involve less hardware, less labor and less dollars.
Primary/secondary piping is one way to achieve hydraulic separation of circulators, but it’s not the only way. Installing one of several commercially available hydraulic separators between the boiler and load headers is one option. So is installing a buffer tank as shown in Figure 1. Just think of the buffer tank as a bigger hydraulic separator with thermal mass. It keeps peace among the circulators and provides thermal stability for low-mass mod/con boilers.
Perhaps our industry needs a new contest. Contractors can submit photos of their installations but must cover up the heat source. It’s there, but can only be identified as a conventional boiler, mod/con boiler, heat pump, etc. The balance of the system would be the basis by which the entry is judged. Evaluation criteria could include the following:
- Is the distribution system well-matched to the heat source?
- What is the wattage required to operate the distribution system at design load?
- How simple is the piping (fitting count, pipe crossing over pipe, extra bends and offsets, etc.)?
- If multiple circulators are used, what are the provisions for hydraulic separation?
- If valve-based zoning is involved, what are the provisions for controlling differential pressure?
- If multiple zones are used, how is short-cycling of the heat source avoided?
- How many simultaneous water temperatures are required?
- How well can the installed components be accessed for maintenance?
- How well does the system avoid thermal migration issues?
- How polished is the diamond plate behind the heat source?
I hope you know I’m kidding on that last one.