Over the last 20 years, there has been a tremendous increase in the types of controls available for hydronic heating systems. They range from simple bimetal thermostats to sophisticated microprocessor-based system management controllers. Control actions range from basic on/off switch action to fully modulating devices operated by PID control algorithms.

PM Engineer surveyed 1,000 randomly selected readers regarding their preferences in controls and control methods for hydronic heating. The survey consisted of 12 questions and had a response rate of 14%.

Survey questions 1 and 2 dealt with the demographics of respondents. Ninety-seven percent of those who responded to the survey were directly responsible for specifying, recommending, or purchasing hydronic controls. Forty-one percent of the respondents dealt with hydronic systems in large commercial or industrial buildings. An additional 24% dealt with light commercial buildings, and 25% dealt with residential buildings. These percentages indicate the results will be skewed a bit toward controls used in medium and large systems.

Question 3: Which one type of the following thermostats do you most frequently specify for your hydronic systems?

Nearly 2 out of 3 respondents indicated they specify electronic programmable thermostats for their hydronic systems. The remainder were almost evenly split between non-programmable electronic thermostats (19%), and electro-mechanical thermostats (18%). It appears electronic thermostats are now the dominant room temperature control device in larger hydronic systems.

Question 4: How often do you use the following zoning methods on your hydronic systems?

The numbers in the always column indicate most specifiers aren’t locked into use of a single method of zone control. The high numbers under the “Sometimes” column suggest that traditional zoning by either circulators or thermostatic valves is still popular, with 20% of respondents indicating they never use non-electric valves.

Question 5: Which two methods of reducing boiler water temperature to that required by lower temperature distribution systems do you use?

It seems three-way mixing valves are the dominant means of mixing hot boiler water with return water for lower temperature distribution systems. Motorized three-way valves were preferred over thermostatic valves 56% to 36%. Interestingly, the use of variable speed injection pumps for mixing control exceeds four-way motorized valves 22% to 4%.

Question 6: What percentage of your systems use the following strategies to control boiler water temperature?

a) Fixed high limit temperature—46%.

b) Variable boiler water temperature based on outdoor reset control—54%.

The energy saving of variable water temperature seems to be evident to increasing numbers of system designers. More than half use outdoor reset control to determine boiler water temperature. The savings associated with higher seasonal efficiencies are obviously greater in the commercial and industrial systems designed by a majority of respondents.

Question 7: Other than the primary operating control and pressure relief valve, what percentage of your systems use the following safety controls:

a) Low water cutoff—66%. b) Manually-reset high temperature limit—25%. c) No other safety controls—9%.

Two-thirds of systems have controls that prevent burner operation under low water conditions. A bit surprising was that only one-quarter of the respondents use manually-reset high limit controls which could stop a runaway burner condition long before the pressure relief valve opens or the system water level drops. I now consider it standard practice to install both a low-water cutoff and manually reset high limit on every system. Also surprising is that 9% of the respondents indicated they use neither of these controls, which are usually mandated by code on all but some residential systems.

Question 8: In hydronic systems using several zone valves, how do you typically control differential pressure across the circulator? (Multiple responses are allowed.)

Forty-five percent of the respondents indicated they use three-way zone valves, which produce less flow variation through the circulator, and hence, don’t force it to operate high on its pump curve. Forty-one percent of the respondents indicated they use differential pressure bypass valves. Less frequently used were variable speed distribution circulators (used by 35% of respondents), and manually set bypass valves (used by 31% of respondents). Twenty-seven percent indicated they use no means of differential pressure control.

Question 9: When regulating the water temperature supplied to a radiant floor heating system, in what percentage of your total jobs do you use each of the following methods?

It’s pretty close between the reset camp and non-reset camp. Fifty-five percent of the respondents indicated they use variable water temperature controls (outdoor reset) versus 45% of the respondents who use fixed supply water temperature (non-reset) on floor heating jobs. Given the larger system represented in this survey, I expected the use of reset water temperature control to be more dominant.

Question 10: What do you feel is the minimum acceptable on-cycle once the burner starts?

Many modern hydronic systems are divided into several zones. In some cases, the load a given zone generates may only be a fraction of the boiler’s heat output. Such situations set the stage for short-cycling, especially when the thermal masses of the boiler and zone load are small.

Efficiency decreases and emissions increase with decreasing cycle length. The life expectancy of the fuel controls and ignition components also decreases as a result of short-cycling.

The survey indicates that 25% of the respondents feel that anything above a 15-minute burner on-cycle constitutes a short-cycle. Almost the same number (22%) felt that a 1- to 4-minute burn is the minimum acceptable on-cycle for a gas-fired boiler. The rest were almost evenly divided between minimum on-cycles of 5 to 9 minutes and 10 to 14 minutes.

For oil-fired boilers, 39% of the respondents felt that 5 minutes was an acceptable on-time, and 9% felt 6 to 10 minutes was acceptable. Twenty-six percent of the respondents felt that a 1- to 4-minute burner on-cycle was acceptable. Another 26% felt an 11- to 19-minute cycle was the minimum acceptable on-cycle. These numbers indicate there isn’t a consensus about what constitutes a short-cycle for either a gas-fired or oil-fired boiler.

Perhaps these results point to a need for better design tools that model system dynamics rather than just steady-state design load performance. Such tools would allow designers to simulate the full and part-load performance of a system in its various zoning modes. Boiler cycling could be predicted, along with its consequences, before presenting itself as a difficult-to-correct field problem.

Question 11: How often do you use the following control methods to enable operation of hydronic snow/ice-melting methods?

Roughly a third of the respondents indicated they use a combination snow/ice temperature sensor either always or sometimes when initiating hydronic snow-melting systems. A relatively high percentage indicated that they sometimes use either a manual switch or a switch with a timer for shut-off. About a quarter of the respondents indicated they never use a combination snow/ice temperature sensor. Then again, 57% indicated they don’t use either a switch or a switch with a timer. It looks like no particular device has a lock on the market for initiating operation of hydronic snowmelting.

Question 12: Which of the following control/piping methods do you typically use for systems that require room-by-room zoning?

It looks like the traditional two-pipe parallel system with individual control valves at each heat emitter is used by 79% of the respondents. It surprised me that 35% indicated they have used a central manifold system with “home run” circuits to each heat emitter. Twenty-six percent indicated they have used diverter tees (one-pipe system) with an electric zone valve at the heat emitters.

Here are a few conclusions drawn from this survey:

  • Electronic thermostats are now the norm for room temperature control.

  • Injection mixing using variable speed pumps is gaining ground on traditional four-way mixing valves, but still lags behind the use of three-way mixing valves.

  • System designers have a wide variety of opinions on what constitutes a short-cycling boiler.

  • No one method of snowmelting control has a lock on the market.

Perhaps the fact that no one control method dominates in most of these categories speaks well of the versatility available to hydronic system designers, as well as their ability to use controls appropriate to the job at hand.

The future of hydronic heating will no doubt be profoundly influenced by new developments in control technology. The objective will be to better manage components such as boilers, circulators and valves, as well as overall system operation. Collectively, these controls will further enhance the comfort and efficiency provided by hydronic systems.