At this point in my series of columns on snow-melt systems, we must begin to assemble the components to drive the heat from the heat source to the snow-melt areas. I’m assuming that the base load has already been calculated and the total Btuh load factor determined.
Once this has been done, we next have to determine if the needs of the system can be met with one or several boilers. This decision is a combination of the system designer’s choice, and possibly weighted by the consumer’s needs. Ideally, if the load can be split between two boilers, it allows for load diversity, greater boiler efficiencies and system redundancy in case of component failure.
The choice of a boiler can vary greatly with all the boiler designs that are available on the market, and not one particular boiler is perfect for all needs. Selection of the boiler should be based on the consumer’s expectations about overall system performance, and budgetary constraints.
Certainly, as mechanical contractors we wish that every installation used nothing but the best high-efficiency, fully condensing boilers, but we are not paying the bill to install the system. It is important to discuss these factors with the system owner early in the conceptual design stages so that the consumer can give you a clear picture of his wants, needs and expectations. Installing a high-efficiency system may cost more initially, but the additional expenses can be justified by the fact that the operating costs will be substantially less. Also, installing a high-efficiency, fully condensing system will eliminate the need for some items that are required when installing a less efficient system that needs protection from long-term condensation production and thermal shock.
Make sure that the consumer is aware of all these conditions prior to estimating the system or you may be faced with trying to cut the budget at the last minute by trading efficiency for system upfront costs.
A typical high-efficiency, fully condensing system may require the use of a heat exchanger that may not be necessary when using medium efficiency equipment. Most condensing boiler manufacturers limit their boilers to operating with a 30% glycol content to avoid the boilers flashing the fluid to steam due to glycol’s inefficiency in heat transfer ability. Although a heat exchanger is an added expense, the cost of a condensing boiler and heat exchanger is frequently recovered because of the boiler efficiency at a high steady-state efficiency.
High-efficiency boilers operating with 30% or less glycol content can be piped as secondaries to a primary system. This allows the coldest fluid to enter the boilers equally. Remember, condensing boilers love to see as low an entering fluid temperature as possible.
Don’t forget to take the acidic nature of the boiler’s condensate into consideration. If you dump it into a cast-iron drainage system without neutralizing the fluid, you are guaranteed to eat the bottom of the cast-iron drain out in short order. This can cause problems with retaining a good trap seal to avoid accumulation of dangerous sewer gases in the mechanical room. Even if the building is served by plastic drainpipes I recommend first neutralizing the condensate before dumping it into the sewage disposal system.
If your boiler of choice is a non-condensing boiler, you must make sure that the heat source is protected against long-term condensation production and thermal shock. This can be performed with any one of the “Btu bridges” that are available to the contractor. They can vary from something as simple as a return temperature thermostatic operator placed in the boilers return lines, or a simple motorized four-way valve, which will also protect the slab from thermal shock, or even a variable-speed injection control system that will also protect the boiler as well as the system.
This is usually the designer’s choice, again, based on experience and or system costs. These designs require the use of an intelligent control logic as I’ve discussed in earlier columns.
I prefer the use of either a properly sized four-way motorized valve or variable speed injection when possible. It allows you to protect the equipment from long-term condensation production and can also be used to protect the slab from thermal shock when married to the proper controls.
If mid-efficiency boilers are used, they too should be piped as secondaries to a primary loop. This allows both boilers to see the lowest allowable return water temperatures available to keep the system efficiency as high as possible.
Tune in next month as we continue to look at what makes these systems tick.
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at firstname.lastname@example.org or by phone at 303/778-7772.