Matt Cooper’s 1,800-square-foot woodworking shop sits on a 6-inch concrete slab heated with a radiant-floor system. Unfortunately, the on-demand water heater that Cooper uses to heats the water for the in-slab tubing isn’t performing well.
“I’d like to figure out a better way to heat the water,” he says, “probably a direct-vent system so that the wood dust doesn’t create problems with the burner (just one part of the TK Jr. issues). It seems that my options are either a better version of the current on-demand hot water heater/boiler, or a tank-style water heater.”
The building and the slab seem to be well insulated, but Cooper’s hot-water options are limited to propane (at $2.25 per gal.) and electricity (12 cents a kWh).
Cooper’s options are the topic for this Q&A Spotlight. (Neil B. posted a similar question about heating a workshop elsewhere on the Q&A Forum.)
Tank-style heaters are simple and reliable
A.J. Builder recommends a tank-style water heater for Cooper’s system. “Tank units rarely have issues,” he writes. “Simple installs available from manufacturer.” He says he’s dealt with similar heating systems for decades, and would recommend installing the water heater in a dust-free utility closet.
“Standard tank units are bullet- and idiot-proof,” Builder writes. “I just installed an [A.O.] Smith 50 that cost $1,000 to the customer. PVC vent and intake. Four hours — done.”
Other suggestions from A.J. Builder include turning down the thermostat, burning wood pellets if he needs a cheaper fuel, or raising his prices.
Woodworking shops are notoriously dusty environments, and Cooper is on board with A.J. Builder’s suggestion to locate the water heater in a dust-free space. “The dust thing is a real issue,” Cooper adds. “I’ve been assuming that I’ll need to install some kind of direct-vented unit that draws combustion air from the outside. It’s really the only reason that electricity started to seem attractive.”
Tankless heaters may be susceptible to short-cycling
David Meiland says that a tankless heater is generally capable of putting out much more hot water than a tank-style water heater, but he suggests Cooper talk to the manufacturer if that’s what he intends to do. “As far as tankless for floor heat,” Meiland writes, “I’ve checked out a couple of systems recently that used various brands, and they seemed to me to be short-cycling. That’s probably a control issue, but would result in high fuel use.”
In Meiland’s locale, electricity is much cheaper than Cooper’s quoted price, making electric boilers common. “It’s like owning an old VW Beetle,” he adds, “very simple and reliable.”
GBA Senior Editor Martin Holladay is another who’s heard of problems with tankless water heaters are used for radiant floor systems. “Tank-style water heaters and boilers are more dependable ways to provide hydronic space heat,” Holladay says. “Ductless minisplits are also an option. I agree with your conclusion that, if dust has been a problem in the past, any combustion appliance should use ducted outdoor air for combustion.”
If Cooper is budget minded, Holladay suggests the least expensive sealed-combustion water heater with an output rating that meets his needs.
Invest in a direct-vent conversion kit
The least expensive way to solve the dust problem is with a direct-vent conversion kit for the Takagi water heater Cooper already has, says Dana Dorsett.
When set up properly, the heater “should be able to deliver reasonably good performance for at least a decade,” Dorsett says, adding, “but some of the ‘one size fits all’ Takagi based slab heating systems are extremely abusive to the water heater.”
He says it would be a good idea to add controls which report the temperature of outgoing and returning water, as well as flow rates.
“If you know the actual heat load of the place…it’s possible to optimize both the tankless and radiation flows so that the unit spends most of its life modulating rather than cycling,” Dorsett says, “which is the only way to get the total ignition cycles per year numbers and burning firing rates down to something reasonable. If it’s always blasting away at 75%+ of max-fire with 10 cycles/hour, the odds of it lasting a decade are pretty remote.”
One burn per hour would be about the same as a domestic hot water application, Dorsett says, and that should be possible with the high thermal mass in Cooper’s 6-in. thick slab.
Our expert’s opinion
Here’s how GBA Technical Director Peter Yost sees it:
I have a bit of a gut reaction on this one: Getting high performance from a system with a combustion unit designed for on-demand hot water (tankless) and a high-mass, slow-to-respond distribution system (radiant floor) just seems a set-up for control issues and field performance inefficiencies.
And when I decided to ask GreenBuildingAdvisor Michael Chandler to weigh in on this one, he went straight to solutions using a tank-type heater, not a tankless heater. Here is what Chandler, who has a background as a designer, builder, and master plumber, had to say:
“I used to use the State 40-gallon direct-vent GS6-40-HBDS water heater. It’s 62% efficient and rated at 38,000 BTU/h input (times .62 = 23,560 BTUh output) and takes a 3/4-inch gas connection. My local supplier, Fergusons Enterprises, has the natural gas model for $676.
A propane-fired unit is a special-order item which usually costs 15% more, so call it $780.
“We used to use those State direct-vent tanks for combo heat and hot water back in the old days (for ‘potable radiant’ systems) and the water heaters were trouble-free but burned a LOT of propane at 62% efficiency. There is no electrical connection. It uses a 7-inch-diameter concentric vent with the out-going air in the center and incoming on the outside.
“A more efficient solution would be the A.O. Smith 50-gallon Vertex power direct-vent condensing water heater (GDHE-50-LP). It’s 96% efficient and rated at 100,000 BTU/h input (times .96 = 96,000 BTUh output). It’s a special order at Ferguson but Pex Supply has the propane model for $2,383.
“The bottom line: the $1,600+ upcharge would get Matt the condensing tank with a 50% efficiency improvement and four times the BTU/h output as compared to the budget direct-vent tank (96 kBTUh @ 96% vs. 23.6 kBTUh @ 62%). If he’s burning $1,500 in propane per year with the buggy tankless I would expect him to get it down to $1,000 per year with the Vertex. I think it would be a very good choice but the State direct-vent water heater gets him up and running for a lot less money upfront.”