Radiant Heat Boiler

Ron Root_2

A friend is replacing a boiler for a residential multi zone radiant heat system. He mentioned mixing valves to reduce the temperature to the zones from 180 to 140 degrees because the (Weil McLain)literature stated a minimum of 140 degrees return temperature or you may damage the boiler. I'm not that well versed in radiant heating so I went through all of Dan's books (got em all) and the Weil McLain web site to get more info. Where I'm having a problem with what my friend told me, I thought the 140 degree return temperature was based on 180 degree supply temperature though I can't find a chart or anything on it. Why can't a boiler's supply be say 140 degrees as long as the return temperature is within the T/D recommended by the manufacturer (cast iron/fin & tube)?
I would appreciate any info or were to obtain it on this.
Thanks Guys or Ladies!

John_82

the mixing valves are used to temper the water down to whatever the radiant floor was designed to. The manufacturer probably doesn't want to see return temps below 140 because they are afraid of something bad happening... Thermo-shock, condensing, etc.

TomStayer

to Ron

Ron,

Monitor's MZ loves radiant. Drop me a note or call 717-235-8007 and I can help you select a boiler.

have a great day,

Tom

Constantin

Since you're looking at radiant heat...

... you have a lot of options. Ideally, consider a heating plant that is optimized for the low water temperatures of radiant systems. If you want to choose a gas-fired system, a low mass, condensing, modulating boiler makes a lot of sense. If you're into oil heat, choose either a noncondensing boiler that can handle very low return temperatures or a condensing boiler.

Many boilers will not tolerate very low return and/or internal temperatures due to condensation issues either inside the boiler or inside the flue. The Buderus G215, Burnham MPO, and Viessmann Vitola all offer low return temps, with the Vitola allowing the lowest continual return temps (50°F), IIRC.

Our Vitola oil-fired boiler has gone through 2 winters so far, and our radiant floor heating system sends back water in the 70's regularly. No sooting or other serious issues. The only reason for the boiler to be run at temperatures above 120°F is to heat the indirect domestic hot water tank.

Considering where natural gas and LP prices have been going, a low-mass, condensing, modulating boiler is the best way to go when you're dealing with year-round occupancy and domestic hot water production in areas that require a lot of winter-time heating. Gas prices are unlikely to drop much, so the 30+% savings that many happy homeowners report as a result of converting to a high-efficiency boiler add up to significant savings in short order.

However, also do not overlook the easiest way to conserve the $$$ in your wallet, which is properly insulating and weatherizing the home. Another thing to consider is the water temperatures needed to heat the home. Radiant floor heating can be done many different ways and ideally, the tubes are installed above the subfloor to ensure the lowest possible water temperatures. If you're into new construction, look into using 6" studs on 24" centers and Warmboard.