Tube Heater Basics
TUBE HEATER BASICS
Essentially, a tube heater is a torch into a steel tube, which heats it to a temperature high enough to radiate. Because the operating temperature is not high enough to make the tubes glow, it’s called low-intensity infrared. Some systems blow, and some suck. No, really. Some brands opt for blowing the hot gases through the tube, which is commonly called a forced-draft or positive-pressure system. Others use an inducer fan to suck the gases through vacuum-vented, induced-draft, or negative-pressure systems. Although we prefer and recommend vacuum-vented, we stock both types from Space-Ray.
Whichever type you install, it’s important to keep in mind that the tube heater's output is lopsided. Where the flame enters the tube, temperatures will reach their peak at up to a thousand degrees, so the radiant output is far greater near the burner than at the vent end. Bear this in mind when you’re planning a layout. Put the burner where the heat load is greatest, such as near an overhead door. If the mounting height is sufficient, diffusion will spread the heat evenly. At lower heights, you’ll get uneven heating from a straight tube: too much near the burner, not enough near the vent. That’s why, more often than not, we end up recommending U-tube models. Their counterflow (up-and-back) design ensures even coverage. Different mounting heights require different input ranges: we’ll help you with that and all aspects of design.
How much will it save on operating costs? That depends on the building’s height and infiltration (air changes). The higher the building, the more you save due to the elimination of heat stratification. With unit heaters, heat collects at the roof due to hot air rising (that’s why hot-air balloons work!). Radiant heat doesn’t fight the buoyancy of hot air, but delivers its heat the same way as light, releasing its energy as it’s absorbed into the floor and objects in the space. That’s also why opening overhead doors don’t lose all the heat: it’s sunk into the floor (heat sink).
Bottom line: commercial and industrial spaces save at least 30% and often 50% compared to forced-air heating while providing warm surfaces, reduced noise and dust, and better comfort.
Getting All the Air Out
Most of the boiler callbacks our customers receive are caused by residual air left in the system. The plumber thinks he has purged it right the first time, but he ends up wasting time and money doing it again, as his once happy customer stands over his shoulder with crossed arms. Sound familiar?
Air left in a hydronic system can cause many problems, such as noises in piping, low flows, corrosion, uneven heat patterns, and more. This is why all hydronic systems need to be properly purged before the system is placed into operation. In this article, we’ll cover purging a hydronic system with homerun distribution piping; in an upcoming issue, we’ll tackle the more involved process of purging a primary-secondary system. A fast and effective way to accomplish this is with fast-fill purging. Fast-fill purging is fast-moving water, starting near the boiler's outlet, moving through the system piping, and then out of the system through an open boiler drain valve installed upstream of a full-port ball valve located near the inlet of the boiler. This is a good location for the ball valve because it lets you flush any other foreign matter out of the system before it can enter the boiler’s heat exchanger. It also ensures that we have gone through the entire system, from beginning to end.
The first step in fast-fill purging is connecting one end of a garden hose to the open boiler drain valve, with the other end of the garden hose placed into a five-gallon bucket. The five-gallon bucket is to help us see air bubbles as they are expelled from the system. The automatic make-up valve is then placed into the fast-fill mode. Some automatic make-up valves don’t have this fast-fill feature; in this case, a neat trick is to install and open a full-port ball valve in parallel with the automatic make-up valve as a bypass.
As the system begins to fill, water flows out to the distribution system, pushing air through the piping and eventually out into the five-gallon bucket, as shown in the figure. The purge procedure should continue until the water being expelled is free of any visible air bubbles for at least one minute.
Systems with more than one heating zone should be purged one zone at a time. Each zone being purged should also be purged one circuit at a time. This will allow the maximum flow through each circuit, forcing as much air out of the system as possible. This can only be done if the return manifold has isolation valves installed on each circuit. Start by closing all of the circuit isolation valves on the return manifold, except for the one you are purging. If you are zoning with zone valves, you will need to manually open one zone valve at a time, starting with the purging zone. After a circuit has been purged and has no visible air bubbles for at least one minute, open the next isolation valve on the distribution manifold and close the previous one. Once you have purged each circuit on a zone, it’s time to move on to the next zone. After all, zones have been purged, the boiler drain valve, and (if installed) the bypass valve can now be closed. The automatic make-up valve should then be placed back into normal operation mode. Don’t forget to open the full-port ball valve near the inlet of the boiler. The last thing to do is set the system pressure at 12-15 PSI. If the system pressure is higher than you want, bleed some water through the boiler drain valve. Any small or dissolved air bubbles left in the system will be pushed through the piping back to the air separating device located near the boiler, where they can be expelled from the system. If you follow this procedure, you should have a quiet and smooth-running system.