Snowmelting Boilers: The Condensed and Non-Condensed Versions
Fred and Stan were manning their company's booth, showing radiant floor heating and snowmelt systems. Stan was off getting yet another hot dog when a young couple walked into the booth. Compared to the traffic of the past hour, this was a stampede.
"May I answer any questions for you?"
Planning a Snowmelt SystemThere are many factors to consider when planning a snowmelt system. The tubing design and layout are obvious, along with determining the proper flow rates and headloss for proper circulator sizing, glycol percentages and control strategies. Most of these issues are fairly cut and dry-to do this, this and this, you'll need that, that and that.
But when the question, "How are we going to make the water warm?"
The control then determines, based on its programming and the current outdoor temperature, what fluid temperature will be needed in the system. P5 then fires, circulating glycol through the snowmelt side of the heat exchanger. (P5 is the injection circulator and will run faster or slower based on the required fluid temperature versus the actual fluid temperature.) The control also uses a snowmelt return sensor, S5, to monitor the return-fluid temperature. This is done to prevent thermal shock to the snowmelt slab.
In this type of a system, the snowmelt heat exchanger should always be piped last in line on the primary circuit, since it usually requires the lowest fluid temperature in the system.
In some instances, when using a modulating-condensing boiler, a heat exchanger may not be required. Typically, a 30% glycol solution is run through the entire heating system as well as the snowmelt system. Steve David, commercial sales manager for Viessmann Manufacturing in Warwick, RI, recommends using a three-way mixing valve to control the snowmelt water temperature in this type of system.
"The goal is to get the return-water temperature at the boiler as low as possible. It's the best way to get maximum efficiency out of these boilers. The return temps will be much lower with a three-way mixing valve than with injection mixing. This is especially important at higher altitudes, when the actual dew point for condensation in the boiler is much lower than the standard 135
Condensation ConcernsIf the snowmelt system is particularly large, such as for a driveway or parking area, then the system will require a dedicated boiler or boilers. If a cast iron, non-condensing boiler is used, then steps need to be taken to keep the boiler from condensing. A heat exchanger, as already described, can be used. The heat exchanger prevents exceptionally cold fluid from returning directly to the boiler, while allowing the designer to specify non-barrier PEX tubing for the snowmelt system. This can economize the system design, since non-barrier tubing is generally less expensive than barrier tubing. By isolating the system with the heat exchanger, potential oxygen-diffusion corrosion issues caused by the non-barrier tubing are eliminated.
A motorized four-way valve also protects a non-condensing boiler from flue gas condensation. The advantage is there is no need for an additional air separator and expansion tank (not to mention a heat exchanger). PEX with an oxygen barrier, however, is mandatory in this application.
First of all, many mod-con manufacturers recommend primary-secondary piping, with the boilers piped as a "secondary gozinta."
SIDEBAR: Where Do We Put the Equipment?What do you do when the boiler and related snowmelt equipment is too large for the mechanical room or the garage on the project you are working on? What do you do when the driveway is too long to reach the farthest manifold with the distribution piping? One solution is a bunker or an outbuilding.
By Clay Thornton
In the mountain resort areas of Utah (and other areas I am sure), very long, wide driveways are common and are often too long to conveniently locate the snowmelt mechanical in or near the building. Shorter distribution runs are desirable to keep pipe and pump sizing smaller. Locating a snowmelt mechanical bunker or outbuilding midway along the driveway is often the most reasonable solution.
The first comment you will likely hear from your client is, "I don't want a building like that on my landscape."