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Hot water heater for radiant heat

hot_rod Member Posts: 21,792

The RETScreen is a good place to start.

Solar domestic hot water SDHW make sense in most areas. Pay back can be in as little as 5-7 years. A fairly easy and affordable way to begin your solar voyage.

Solar radiant gets pricey fairly quickly. Large insulated storage, multiple panels, piping and control intergration... lot to think about.

Similar for wood/ hydronic. I don't know your locatiojn but guesstimate about 20- 25 BTU/square foot?? Probably well below a 50K load?

Consider a passive solar design. that's the best bang for your solar buck. Then SDHW.

Nothing screams louder then a well constructed and well insulated and ventilated buiding shell. Windows, insulation etc. Spend as much attention and money on well researched products for that step of the project.

My vote would be a HTP Phoenix with the solar option and a small HX to drive the radiant or panel radiator portion.

A one footprint, multi purposed appliance. Solar coil helps both DHW and heat, high efficiency, simple to service, plenty of back up power for large DHW loads, if needed.

hot rod
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream


  • Consumer
    Consumer Member Posts: 2
    Hot water heater for radiant heat

    I'm a consumer, not a professional, so please pardon my lack of knowledge! I'm planning to build a 1250 sf cape cod, with a passive solar design and a masonry wood stove as primary heat. I've been investigating radiant heat as back up heat for the house and possibly as primary heat for the 2nd floor. I like the economy of having the hot water heater also provide the radiant heat.

    Question: The open dual use system for hot water and radiant heat seems problematic. What's your view on a closed dual use system which keeps hot water separate from the heating loop?

    Question: Would the closed dual use system allow hot water input from solar collectors and/or masonry wood stove? I plan to have a storage tank on the 2nd floor for hot water generated by solar collectors and the wood stove. I'd like to have solar/wood pre-heated water feed the hot water heater in order to reduce propane use.

    Question: Is an on-demand hot water heater a viable alternative for radiant heat?

    Thanks for your advice!
  • scott markle_2
    scott markle_2 Member Posts: 611
    stay clear of vermont based co.

    High quality radiant heat and solar/wood integration are not simple DYI projects. I know of a company in Vermont which sells "systems" to people that I believe are poorly conceived. They are big on water heaters and open systems. The main argument against shared dhw and radiant is the risk of legionnaire. most radiant systems only require operating temperatures which are not sufficient to kill this organism. Even if higher than required temps are used (135 or so) there is the issue of this water (in the tubing)incubating bacteria during the summer months. Interestingly this companies price on non barrier tubing (the type they sell) was more than what I would have billed for major brand barrier tubing. (thats with a 10% mark up on my part).

    Active solar heating via collectors is probably not the best investment or even most ecological way to spend your money. If you size your collectors such that you have a meaning full winter thermal input you will have an excess of heat in the summer, bottom line is that maximum thermal demands for a structure coincide with the lowest available solar radiation. Some will say that this can be compensated for by using steep collector pitch , maximizing winter efficiency while shedding efficiency during the summer,This is true but if you take a good look at the economics you see that this is not practical for normal working people.

    Not to discourage you from solar thermal, go for it, but be realistic,the best application is supplemental DHW. Size for 100% contribution at peek insolation (july) and supplement as needed the rest of the year. In my opinion this is currently the most practical application for solar energy.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Answer 1: Please don't even consider a dual-use system that combines space heating and domestic hot water. You need at least one degree of separation between space heating and domestic hot water! One rather neat thing about separating domestic and space heating systems via a single-walled coil is that the pressure differential itself provides a second degree of separation. Domestic hot water systems have FAR higher pressure than space heating systems, so even if the there's a leak in the coil, domestic hot water will tend to leak into the space heating--not vice-versa--and as long as the space heating system has an appropriate T&P (temperature AND pressure) device, you'll get discharge before cross-contamination.

    Answer 2: If you want a combined appliance--such MUST--incorporate a heat exchanger between domestic and space heating AND you want an "alternative" source of heat, look into the Voyager or Phoenix. They have a HX coil at the bottom of the tank that is perfectly suited to this purpose. Control however can get very complicated--especially with wood. You have to absolutely, 100% guarantee that relief is provided BEFORE the system overheats. If the fire is not carefully attended the system will dump. Our wonderful Hot Rod has a good story about a rather lackadaisical customer with just that problem...

    Answer 3: On-demand domestic water heaters are not well-suited to providing space heating--especially low temp systems like radiant floors/walls/ceilings. While they are VERY similar to condensing/modulating boilers their control systems are NOTHING alike. It can be done, but by the time you've done it properly, you might as well have used a mod-con to begin with.
  • Consumer
    Consumer Member Posts: 2
    Thanks and another question

    Thanks for the advice. This will help me direct my efforts to plan the system. I've focused a lot of work into developing the best passive solar plan for my situation and am now planning the active systems.

    I looked at specs for the Phoenix solar water heater and the Voyager. The Phoenix looks especially useful.

    I've been thinking about building "slowly". Would it be possible to install the Phoenix and use initially for regular propane DHW alone? Then, when I can afford solar collectors, I could hook them up to the water heater. Finally, when I can afford the radiant heat system (having installed the tubing in my slab while building the shell) I could hook up the radiant heat system to the auxiliary connection. Does this sound viable?

This discussion has been closed.