We have a home built in the 70's with wall to wall carpeting, including the bathroom. There is plywood under the carpeting. There is a fully insulated basement. We wonder if putting in solar-powred radiant floor heating would be the right way to go versus a gas hot water system with a condensing boiler. Advice? :)

for full heat? you won't do it with solar in a 70's house. solar would, at best, supplement another primary heat source.

I use a condensing water heater with sub-system for radiant floors for smaller homes or zones. I also use Euro-panels in my design and installation business. If you really need heat, solar will be cost prohibitive.

camille2t, It depends on where you live but most likely will be costly both up front and high maintenance to have solar. I would look at geothermal or a radiant mod-con system...Eric

with 700 sq ft, it is also unlikely that a mod/con or geo will make any sense unless the heat load is very high indeed for the area. If it were high enough to justify a mod/con boiler, I would advise upgrading the envelope first in most cases. I'm not aware of any affordable 2-ton or smaller geothermal options.

presuming a "normal" or "good" heat load on the building, you may be in a situation where a combination heating/DHW water heater makes the most sense. with heat exchanger, definitely. but on 700 sq ft, high efficiency can take a very long time indeed to recoup costs. a cheap water heater, sized properly, with heat exchange can be extremely economical and run high 70's for heating efficiency... not great, but on a small volume of heat energy, not necessarily bad either. the warmer your climate, the more this would make sense.

that ALL presumes that cost is an issue and that the economics are important, of course. If not, great!

You could retrofit most 700 square foot houses in the lower 48 to near PassiveHouse levels for less than the cost of a typical low-tonnage geo system, eh?

Getting the peak heat load down to well under the thermal output of a cheap gas water heater would likely cost a lot less than a minimal-installation mod-con too. (Depending on location and how tight the house is you may already be there.)

When the load is low enough, the efficiency of the mechanical systems required to support that load become irrelevant. Insulation, windows, & air-sealing are a lot lower maintenance, with longer life cycles than mod cons or heat pumps.

Close, but no passive solar system will heat DHW in anything but a desert climate.

Retrofitting windows is the least cost effective (lowest return on invest) and low maintenance is a valid feature but serviceable life is about the same as a condensing water heater (20 years).

A conventional water heater with hydronic sub-system if you have a savvy hydronic designer and or installer will yield the best ROI on a small project like this.

Decisions in residential applications are not just about economics. Comfort, reliability, peace of mind, social conciseness etc play into the choice. If you have drafty windows that make you uncomfortable then replace them. If you have a low load and cheap electricity i would look at improving the envelope and the electric radiant panel of some type.....as usual it depends on where you live and the climate...Eric

If you have nat gas, I would use it. If you want AC, I'd look at forced air. Otherwise, baseboard radiant.

If it's 700 sq feet then congrats on resisting an energy sucking mega house.

Posted By BadgerBoilerMN on 09 Aug 2010 06:14 PM
Close, but no passive solar system will heat DHW in anything but a desert climate.

Retrofitting windows is the least cost effective (lowest return on invest) and low maintenance is a valid feature but serviceable life is about the same as a condensing water heater (20 years).

A conventional water heater with hydronic sub-system if you have a savvy hydronic designer and or installer will yield the best ROI on a small project like this.

Depends on the level of window retrofit. 

Fixing the drafts, re-working aging double-hungs with better weather stripping, and insulating where the counteweights used to be then adding quality tight-fitting exterior storms is usually cost effective, with a service life much longer than 20 years, yielding better total performance than U0.50 vinyl replacement windows (at 4x the installed cost) that fall apart in under 3 decades. Throw in some interior storms and you'll hit U values around or under 0.30, and still be cost effective on a 10year NPV analysis in many places.

As opposed to buying triple-glazed krypton-filled high performance windows, which is only cost-effective if it means that by going that route you can now skip the heating system entirely.  (And yes, gas charged windows need periodic re-glazing every coupla decades to perform to spec.)

With vintage 700' homes in sub-6000HDD climates you can still get close  to "no heating system necessary" for less than the cost of a geo system necessary to run the draftier less-insulated as-is home.  Getting there for less than a minimalist mod-con system might be possible in some instances, but that would be an exceptional case.

Building a combi system on a gas fired tank HW would be dead easy. Spend the difference in upfront cost on envelope upgrades can result in burning the same fuel or less as running a minimal-mod-con on the existing as-is building in many, if not most instances. (Depends on just how tight & well insulated it is starting out.)

Well, when you put it that way...I did my first one in 1984. Still, the water heater outlasted the windows hehehee.