Hello All,

I am preparing to install a hydronic forced air system for a variety of constraints, local code, space, etc.  The system I am planning on installing is a hot water coil with a forced air unit to service the main living area with a toekick heater for the bathroom and a toekick heater with a floor conversion unit for the single bedroom.  What are the drawbacks, for my situation,  to using a tankless water heater in an open loop system?  This is a weekend cabin that totals 640 SF.  My heat load calc is 36 MBTU/Hr at the design temperature.  My permanent home has in floor radiant heat (SOG) with a 90 MBTU/Hr boiler on a closed loop system.  A similar sytem for "the cabin" seems like overkill.

On another note.  I have a good friend that used to install hydronic systems in MA tell me that they sized boilers for space heating without accounting for domestic hot water and did not have any issues.  My current home has a 199 MBTU/Hr tankless for domestic and, judging by the flow charts, this doesn't ring true regardless of heat source.

Thanks in advance for any help.

First,
Why that high a heat load for 640 sf? That seems very high unless you are in a very cold climate.
As far as using a fancoil unit for the main part of the cabin, most of the ones sold are industrial units and they are not that quiet. Something like:
Mcmaster car product # 1836K81


Horizontal Flow Steam/Water Forced-Air Heater 115 VAC, Totally Enclosed Motor, 24000 Btu/hr
Why use a tankless instead of a small boiler like a munchkin t50 or triangle t60. The thing about the tankless units, is they are not meant for the duty cycles of space heating. And they usually are not as efficient. Plus there is the whole open loop issue.
How about Munchkin T50, + a 50 gallon indirect tank and a single zone for heating. Simple, reliable. Midweek you turn off the heat to the indirect tank if you like and turn it on when you get back on friday.
For that matter if you want super simple use a wall mounted direct vent gas heater.

Eric is right.

Tankless water heaters are not intended for combi duty.

I am the ModCon zealot but your load is too small, even if we accept the - likely inflated - heat load you suggest. The cost is also a concern as the ModCon/indirect will double the cost with labor of a tank-type water heater with fan coil.

I use condensing water heaters and fan coils made for the purpose. The stored water affords ample load to lower efficiency killing short cycles and affords standing capacity for bathing without heavy math of designing a system around an unfriendly platform such as the tankless water  heater. A close-coupled fan coil (designed and installed by a professional will make for a safe "open" system within certain accepted parameters but a tankless is not part of this method.

As for small fan coils the guys over at Energy Saving Products have known all about them for more than 25 years and the owner has been heating his own home with a water heater and fan coil for all of those.

http://www.hi-velocity.com/

Naturally every successful hydronic heating system starts with an accurate heat load analysis performed by an experienced professional using Manual 'J' certified software.


DIY design; always the first mistake.

Thanks to you both for your input.  I really need to have one source to supply both the DHW and space heating due to space constraints.  I am leaning more towards a boiler system as I read more about this subject.  Hot water coil air handlers are available for this purpose, I am a bit perplexed by that coment.  Arcoaire has one and I found another that is rated for the BTU output even at 140 degree water.  In so far as the heat calculation goes the outside design temperature is -10 degrees and I have a lot of glass in relation to the wall area.  I have one wall that is about 40% glass and another that is about 65%.  Do either of you know of a boiler unit that would serve this purpose and is self contained?  Internal expansion tank, internal heat exchanger to provide or a closed loop on the heating etc.?  My current boiler arrangement occupies 6' of wall almost floor to ceiling.  Some of this is due to the retrofit situation but most of it is in the expansion tank, backflow, make-up water, piping, etc.  Here's an  unrelated question for you, why are boilers so highly regulated as compared to tankless water heaters? 

Again, thanks for your help.

Condensing water heater with proper trim, 22" diameter, 6ft. high.

I design these systems for a living.

Did you actually find any good educational article on how to calculate heat load? I'm looking for a cheap way to heat basement and may be garage too. I want to use hydronic heating coils, but not sure how to properly size the unit.

example of hydronic coil heat exchanger that I was looking for:
http://cgi.ebay.com/16x16-Water-Air-Heat-Exchanger-Furnace-80-000-BTU-hr-/250808961155?pt=LH_DefaultDomain_0&hash=item3a65610883#ht_1524wt_905

in case this link doesn't work, ebay item id #250808961155

would it be enough for 800 sq.ft. basement? or 2-car garage?

thanks

Posted By peter_ac on 23 Apr 2011 10:49 AM
Did you actually find any good educational article on how to calculate heat load? I'm looking for a cheap way to heat basement and may be garage too. I want to use hydronic heating coils, but not sure how to properly size the unit.

example of hydronic coil heat exchanger that I was looking for:
http://cgi.ebay.com/16x16-Water-Air-Heat-Exchanger-Furnace-80-000-BTU-hr-/250808961155?pt=LH_DefaultDomain_0&hash=item3a65610883#ht_1524wt_905

in case this link doesn't work, ebay item id #250808961155

would it be enough for 800 sq.ft. basement? or 2-car garage?

thanks

The first step is determining the heat load for the given space. After that is known then the temperature of the heating source and flowrate through the heat exchanger will determine the size necessary to heat the space.

The optimum radiant floor design in terms of material cost and performance is gained through an independent designer. The next best thing is to use an experienced supplier for a proven package system. You will find two of them haunting this blog with me.

As Dana would say, design by internet is not optimum.

I agree - ask some questions, learn some things, get some ideas and some questions to ask - but ultimately, find a good local guy.