Best hot water solution for radiant floor plus domestic hot water?
I have a 100 gallon commercial water heater (A.O. Smith) in my home that serves my radiant floor heating system (7,100 sq. ft.) plus hot water for two guest bedrooms and our kitchen. The water heater is now leaking (after only five years but, of course, out of warranty) and the radiant heating system installer has recommended against my replacing it in kind. He's suggested instead that I get either a 75 gallon water heater or a tankless water heater for the DHW and that I get either a high-efficiency boiler or a 75 gallon water heater for the radiant system. Now I'm trying to decide how to make the best decision based on economics.
My average annual gas bill is about $2,700 and it looks like about $1,700 of that is for the radiant heating expense, leaving $1,000 spent on DHW. The radiant guy says I'll be able to install everything but the boiler with fairly nominal costs for re-plumbing but that the boiler will require a PVC stack and, while the chase that currently encloses the water heater's stack could probably accommodate it, the piping would have to go up through through two floors and the attic and that would add to the cost.
And one last complication...I have an electric pump that circulates the hot water in the pipes to eliminate the long wait for DHW. Can that system work with a tankless hot water heater?
Based on this, can you help me decide:
a) tankless or hot water heater for DHW?
b) hot water heater or high-efficiency boiler for the radiant system?
I plan to stay in the house for another 20 years (if I live that long!).
Thanks for any advice you can offer.
OK, let me see if I've got this right...
GrumpyPlumbr, first let me explain that my radiant installer did suggest getting a boiler as an alternative, although only for the radiant system (relying on a WH or tankless WH for DHW). My system is gas so I assume that any discussion of an anode doesn't apply; is that right? And you mentioned the need for a "sanitary drain" in conjunction with a boiler's exhaust. Since my equipment room is above grade but below the level of my sewer system, I assume that I'd have to somehow connect the boiler's exhaust to the sump pump that pumps from the sink in that room up to the sewer line. Is that right?
And Jim D, it sounds like you're recommending that I get a boiler with a) an indirect tank for DHW and b) a separate pump with a mixing valve for the radiant system. But from your description, it doesn't sound like an outside reset valve would make sense in my climate. With mid-to-high 20 degree temperatures being the lowest we get and with a need to reduce the boiler output with a mixing valve on the radiant system, I assume that I could permanently set the boiler to the lowest setting to ensure a minimum amount of on-off conditions. If I did that, how would it effect the efficiency? And can you explain how I'd figure the sizing on the boiler and the indirect tank? I assume I have to start by figuring the maximum demand for both the radiant system and the DHW. But then what?
Maybe I'm trying to figure out too much of this on my own and should just call in some vendors to get their proposals. But I get nervous when I rely on folks intent on making money at my expense. And I've been living with a system that, although professionally installed, probably wasn't optimally designed to begin with.
Thanks for the help, alternety.
Here's more background in response to your questions:
House is 7,100 square feet, not including the four-car garage that's also heated. The only rooms on slab (and with no insulation beneath) are the garage, a guest suite, an exercise room and a project room; all the rest have PEX imbeded in gypcrete. The slab rooms are infrequently heated or seldom to much more than 50 degrees. The radiant system has what appears to be two pumps, 14 solenoids, three green control boxes (Taco mfr.), an expansion tank and a number of manual valves. Each zone is controlled by its own thermostat. About 4,000 sq. ft. is limestone and the rest is carpeted with a special radiant pad beneath (not slab foam but the next best). The heating system, relying on the 100 gallon water heater shared with DHW, seems to have worked fine, although we've had some shortfalls on hot water in the guest room during winter (weird...enough hot water in the bathroom sink but not in the shower).
Since the Buderus boiler was recommended by several of you, I called two of their distributors in my area and asked for installers who've bought that boiler. I've put in two calls, both to plumbers who specialize in radiant system repair/installation, but haven't heard back yet.
Based on the replies to my initial question, it looks like I want a high-efficiency SS boiler with an indirect SS tank for DHW, an outside reset and a mixing valve for the radiant (if it doesn't have one already or unless the "smart" modulating boiler makes it unnecessary).
The one thing that's still unclear to me is how my DHW recirculation pumps will work with the indirect tank. It sounds like I'll have to keep the indirect tank supplied with 120 degree hot water 24/7, since the recirc pump will pull hot water 15 minutes every hour (except 11:00 pm to 5:00 am) to heat the pipes and eliminate the wait for hot water in remote locations. I guess that's still better than heating a 100 gallon WH 24/7 like I've been doing.
Dang, this stuff can get complicated! Thanks again for the many insights.