Figure 1
Recently I had an opportunity to walk through some very impressive radiantly heated "trophy homes" near a large ski resort. Miles of tubing, hundreds of valves, chimneys that easily accommodate Santa's girth, and control boards that would make most NASA engineers jealous.

For the most part the systems were well-installed. They provided the owners and their multiple attendants with the comfort only radiant heating can deliver. Most were installed with a directive to make the heating system as impressive as the house it serves. Cost, give or take a few ten thousand dollars, was usually of secondary importance.

Now don't get me wrong: I love designing hydronic comfort systems for houses like this. Especially the challenge of developing a system that ideally suits the owner's expectations and life style.

Most clientele in this strata understand that top-shelf systems carry top-shelf prices. Penny pinching is seldom encountered on these projects. These folks don't ask if they can provide the materials while you provide the labor. Instead, they seek comfort and reliability. If achieving this requires lots of complex hardware then -- to paraphrase a typical gated-community client -- just do it!

Humbled Hydronics

Many of us -- myself included -- at times fall prey to the notion that lots of hardware, particularly lots of controls, exemplifies the ultimate heating system. One that's theoretically capable of squeezing every last Btu from its fuel, and delivering that Btu to the exact square inch of the building, at the precise second it's needed.

We approach the design of such systems as if attempting to guide a hypersonic missile to the exact spherical coordinates and instant in time necessary to hit another hypersonic missile. The most sophisticated technology available is arguably necessary for the latter, but we can probably leave a bit more slack on our slide rules when it comes to designing a good heating system.

Last year, I was asked to lay out a system that could tie a fully condensing 200,000 Btu/hr. gas-fired boiler to whole-house radiant heating. The system would start at the bottom with a fully heated basement slab. The first and second floors would use 1.5-inch poured gypsum underlayment thin-slabs. Even the garage would be maintained warm and dry with embedded tubing. The system would also include an indirect water heater to keep several bathrooms supplied.

The foremost objective was to keep the system simple while still providing multiple water temperatures, and taking full advantage of the condensing boiler.

If designing the system around a conventional boiler -- which requires protection against sustained flue gas condensation -- I likely would have used multiple mixing devices, each set to the supply temperature requirements of the various radiant floors. These subassemblies would have been tied to a primary secondary system as shown in Figure 1.

This same approach could be used with the condensing boiler. However, forcing such a boiler to run at higher water temperatures only to mix down before supplying the radiant floors essentially wastes the efficiency gains achievable through low-temperature "condensing mode" operation. Scratch that idea, and instead focus on the potential of full-temperature reset using the boiler and relatively simple piping rather than multiple mixing controls.

The system was broken down into five zones. Two serve different areas of the basement. One serves the first floor, another the second floor. The fifth zone serves the garage. The piping concept used is shown in Figure 2.

Boiler water is delivered to the "horseshoe" through a pair of closely spaced tees. These tees uncouple the boiler pump (P1) from the other circulators in the system. Flow through the boiler remains constant regardless of what distribution circulators are operating.

The horseshoe supplys five small circulators. Each circulator comes on when its zone calls for heat, and is equipped with an internal check valve to prevent reverse flow when off.

When operating, circulators (P3) and (P4) send a stream of water through closely spaced tees in their crossover bridges. These tees are the pick up and return points for water that's circulated through the first- and second-floor manifolds. The closely spaced tees provide a U-turn for constant circulation in each zone (a design objective). They also prevent any flow interference between circulators regardless of what zones happen to be operating.

Circulators (P5), (P6) and (P7) are injection pumps, but not variable-speed injection pumps. When operating, they inject water from the horseshoe into the subsystems for the garage and basement zones. The flow-setter valves determine the rate of injection, which remains constant once the valve is set.

Zone valves and a single circulator were considered in lieu of individual crossover circulators. Flow-rate variations in the horseshoe depending on which zone valves were open was a concern. This could have been corrected by installing a differential pressure bypass valve, but that adds complexity. We also found that individual circulators were less expensive to install than the zone-valve option. The 2-inch copper horseshoe piping creates very little flow resistance between the return and supply side of the crossovers. This minimizes flow-rate variations in the crossovers as various circulators turn on and off.

Water temperature in the horseshoe is controlled by a simple boiler reset control. It's the only microprocessor-based control in the entire system. Upon a demand for heat, the reset control calculates the target temperature for the supply side of the horseshoe based on outside temperature. It then fires the boiler as necessary to maintain the supply side of the horseshoe close to this temperature.

When domestic water heating is required, the boiler is allowed to climb to 180 degrees F to drive heat through the tank's heat exchanger as fast as the boiler can produce it. The circulator between the boiler and horseshoe is turned off, as are the zone circulators. The distribution circulators are also off for the relatively brief periods of DHW heating.

Ideally, the distribution circulators would provide constant circulation through the floor slabs during the DHW cycle, but this required another relay and more wiring. In keeping with the simplicity theme, we felt the compromise was minimal. When the DHW load is satisfied, the zone circulators and distribution circulators restart as necessary. The small volume of hot water in the boiler blends with cooler water from the return side of the horseshoe before heading out the floor circuits.

Figure 2

Temperature Transformers

The thin-slab areas require water at approximately 130 degrees F under design load conditions. The basement floor requires water at about 100 degrees F, and the garage floor about 110 degrees F at the same conditions.

To provide the lower temperatures to the basement and garage, we used proportional reset. 130-degree F "injection water" from pumps (P5), (P6) and (P7) is mixed with return water from the garage and basement circuits to yield the reduced supply water temperatures. The flow-setter valve determines the rate of injection. The greater the injection flow, the warmer the distribution subsystem becomes.

If you do the math, you'll find that by maintaining fixed mixing proportions, while resetting the injection water temperature, you also get full reset in the lower temperature loop (at a lower reset ratio). We discussed this concept of proportional reset in the May 1999 Hydronics Workshop column "Two for the Price of One." (If you want a refresher, check it out in the archive section at

The Ice Storm Cometh: I sleep better knowing that garage floor circuits are protected from freezing under shut down or deep setback conditions. I realize that if the circulator were kept running, the water in the garage floor circuits would probably not freeze for several days without heat input. However, another massive ice storm, like the one that hit the upper northeast a few years ago, taking out electricity for over a month in some locations, quickly renders this irrelevant. Like it or not, the only sure way to prevent freeze-up is to install antifreeze.

In many systems, we've used a brazed-plate heat exchanger between the glycol-filled garage circuits and the balance of the system (that's usually filled with water). The heat exchanger, along with the associated glycol-side trim (expansion tank, air separator, pressure relief valve, fill/purge valves and circulator), obviously add cost and further complexity to the system. Keeping with our goal of simplicity, we elected to fill the entire system with a propylene glycol solution and eliminate the heat exchanger. The fringe benefit is that the entire system is now protected against freeze-up during a prolonged power outage.

Actuator Awareness

One of the beauties of modern hydronic heating is the ability to provide extensive zone control when necessary. The hard part is deciding when such control sophistication is necessary.

Some would say that installing a separate thermostat in each room of a building is the ultimate form of zoning. True, this can be done. Each room could provide a totally independent call for heat. In the house being discussed, this would have added a couple thousand dollars worth of thermostats, valve actuators, differential pressure bypass valves and wiring, not to mention the labor cost of installing this hardware.

Have you ever installed an extensively zoned hydronic system only to find that the owner "experiments" with the thermostat settings for a couple of weeks, then leaves them wherever they're set? The owner uses the thermostats to tweak the room temperatures to the desired levels. Once achieved, those room temperatures are seldom changed. There has to be a simpler, less expensive way to do this.

The heat output of radiant panels as well as other types of hydronic heat emitters can be altered through flow-rate adjustments. Reducing the flow through a radiant panel circuit reduces its average temperature and hence its heat output. In many cases this provides the desired room temperature variations without the added complexity of thermostats, valve actuators and miles of wire.

True, this approach doesn't allow each room to call for heat independently of the others, but with the exception of bathrooms, how often do most people really want to do this? If a brief temperature boost is desired in the bathroom prior to a shower or bath, it can be achieved with a small panel radiator or a fast-responding electric radiant panel. Yes, even Wet Heads should think about electric heat in some circumstances, and this is one of them.

In summary, here's what the system we've discussed delivers:

  • Maximum efficiency of the fully condensing boiler is achieved by operating at the lowest possible temperature.

  • Full reset of the distribution system temperature yields minimum variations in comfort.

  • Constant circulation in all distribution circuits allows heat parked in thermal mass as well as internal heat gains to be redistributed within the zones when necessary.

  • Priority domestic water heating without heat migration into the space-heating portion of the system.

  • Simplicity of control: A single boiler reset control and a multizone relay center.

  • Three different supply water temperatures that are fully reset as the boiler temperature is reset.

    Arguably, these objectives could be provided through other approaches. More hardware, especially control boxes, could have found a home in this system. Money may even have been "left on the table" so to speak. Still, what this system delivers in comfort, reliability and low fuel bills is probably worth more to its owners than what it lacks in colored LEDs in the boiler room.