Hello fellow home performance folks!

I am looking to heat a single room with a hydronic coil that I can tie to an existing, domestic, hot water heater.

Does anyone know of a small, hydronic, baseboard unit with a built-in recirculation pump and thermostat that I could use to heat a single room? A built-in fan would be pretty awesome too? Am I asking for too much? 

I haven't done the load calcs yet, but I'm guessing around 5000 btuh.

I have also looked at direct vent gas heaters like the Rinnai https://www.rinnai.us/direct-vent-wall-furnace/product/ex08c_rhfe-2.... I've also looked mini splits, and hotel room style through-the-wall heat pumps. 

But I would prefer a hydronic solution due to the adjustability of water temperatures by changing the rate of hot water flow. I don't want to over heat the room.

I install a fair number of First Co VHBQ hydronic air handlers for central forced air heating systems. Being able to fine tune the delivered BTU's is fantastic for comfort and even temperatures.  But I need something smaller for a 150 sf bathroom.

I seem to run across this situation with some regularity - that one room that the homeowner wants to control separately, or that cannot be added on to the existing central system.

Any suggestions would be much appreciated.


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This looks something where you borrow from solar heating installers. Most codes require a heat exchanger in the system -- you aren't supposed to run potable water in non-potable plumbing. Nor should you have potable water in a system where it might sit, unheated, for weeks on end -- and breed Legionella.

Taco Solar Pump Station, Heliodyne HPAK, PAW Solex, or Purist SPS-2-AC are all little plate heat exchangers with built-in circulator pumps -- just made for a job like this, IF they can deliver enough flow.

How much control do you have over the distribution? A fan-coil unit will require high flows of pretty hot water, to get any reasonable heat delivery. A domestic hot water heater that has to run 150 degree water to satisfy an air handler is going to be an outright hazard to the rest of the house. That's one extreme.

At the other extreme -- If the room is still being remodeled, I'd put a radiant loop in the floor and drive it at 110 degrees or so. That's a truly AWESOME system for a bathroom, where people are often barefoot.

But the best solution is probably a hydronic panel-type radiator, driven at pretty typical household hot water temps. If the room gets too warm, drape your bath towel over it -- thermal control and towel heater all in one!!  

The heat plate exchangers with circulator pumps sound promising. If the unit has a timer that circulates the water at a regular interval, I believe that it is okay to be coupled with the domestic hot water. This is how the First Co VHBQB units handle the Legionella issue. 

I don't think that we need super-hot water. We heat entire houses with a First Co VHBQB using 120 degree water. The timer circulates the hot water a minimum of 4 times per day.

I like the radiant loop idea. I do have access to to the sub floor area via the crawlspace. 

I see lots of hydronic panel-type radiators out there, but they all seem to be designed to operate as part of a whole house heating system. I'm wondering if anyone is manufacturing a single point unit with built in thermostat, pump, timer and maybe even a blower. I'd take even one of those options. 

I guess that a fall-back solution would be to install a standard hydronic radiator and then add the t-stat, pump and timer separately. 

Don &  Bachi ,

  Storing at higher temps in tank type units and using a domestic hot water mixing valve is a tried and true defense against Legionella .  Legionella bacteria have a hard time at 150* ( 90+% are killed at 140*) , this also allows many to install or use smaller tanks with similar delivery and performance results .

  As long as one has the capability of exercising the pump once a day for 2-3 minutes clearing the HX or tubing of whatever type heating is used there is usually an allowance in most jurisdictions .  In Mass for instance one must guarantee a circ cycle every 6 hours I believe , AHJ should be consulted regardless .

  A radiant loop is a great idea for a bathroom .  I do have questions about a 33.3 BTU s f requirement , is this an outhouse ?  Taking into consideration that this number is legitimate , one may want to look to the ceiling as a radiant panel since surface temps can be higher without causing damage to the material and/ or the occupants feet .  

   There are many water heaters that are set up for exactly this situation and are designed to perform just like this .

It is an attached room. I was overestimating the load. I just ran the calcs and the bathroom is at  3009 btuh. Or 19.67 btuh/sf.

K120 heater

You would need to provide your own pump though.

This looks like a sweet, little unit. The K84 would match my load nicely. I could wire the pump to a line-voltage thermostat.

  If running another line voltage circuit , hearing a bit of noise and using more power than needed is Sweet than the kickspace heater is certainly for you . Remember that it only matches your load at design .

If what you want is comfort , efficiency and no noise I would go with the radiant panel .  Sunboard has a product called Sunfoam .  It is a 1" thick foam panel with graphite coating that is pre grooved to accept 1/2" tubing .  On the floor you would use hardi backer and thinset .  Ceiling though will still warm the floor but is easier to install .  Attach right to the bottom of your ceiling joists and attach drywall right to the bottom of the board . Heat does not rise , hot air does .

100% conduction to the sheetrock and full contact with every object in the room , you included . You would only require average 105* water at design , would hardly cost you anything to operate and only 1 thing powered by electric , the circ .   The circ needed for this type application can be very small also .  If you really want to save energy you could include an outdoor reset mixing valve to make the room feel exactly the same no matter the conditions outside .

  What type of water heater do you currently use ?



No pricing or BTU specs posted on the website.

  Thats correct .  I only make suggestions based on real world performance , knowledge and science .

   Pricing can be found or gotten by interested persons .

The client has an atmospherically vented, 40 gallon, 40 kbtu water heater.

Look into a very little strip heat in the floor and put it under tile.  Google "under tile radiant heat"  IF just to bump up a little heat short time, not so bad.  But I see that's a hole house heated this way, is way out of line.   I saw a passive solar house heated with just tile floor heat cost $2200 yr and burned out the cast in floor strip heat in 4 years.  I just low use will last yrs and yrs.  

With Untreated city water in loop that is left in line below 120' will make some very nasty Legionella and other stuff.   Just heat up the water to 130' one a week for one hour.   

Thanks Eric. This is existing construction, so I won't be able to tear up tile for the install. But the hydronic strip heat might be an option to place under the floor deck in the crawlspace.


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