Indirect Hot Water v On Demand Hot Water

Bruce M.

I continually see posts concerning the benefits of an Indirect Hot Water heater as the most economical method for DHW. It seems to me that this is not as economical as an on-demand oil fired hot water heater. For one thing, the boiler must be oversized and during the non-heating season it is even more oversized. What if anything, am I missing from this theory.

Joe Brix

Oil fired instant hot water maker?

I've only seen one marketed: Toyostove. I guess it would suffer the same issues as gas tankless instant hot water makers: restricted flow. Seems to me you can't beat storing the hot water so you have a buffer for when demand kicks in. A sealed insulated tank will always have less standby loss then any appliance with a flue on it, so in cold weather, the tankless's heat exchanger is stone cold.

With priority controls, why would the boiler have to be oversized? Now if you're heating load was only 50K and you needed 100K BTU for the indirect, you might be right. I still would not want to have two burners to maintain though. But extensive users of hot water like a restaurant with a comercial dishwasher unit might be better off with a dedicated hot water maker.

I've never seen an instant oil fired tankess water heater in use. Anyone have experience with one?

Uni R

On-demand...

My only experience with on demand water heaters has been in Europe and it made me give thanks for tanks.


An oiler burning device should be maintained at least annually. Why maintain two burners when you could get by with one? Same thing with flues and having to provide for make-up air. Redundancy isn't much of a factor either since you need to get either fixed pronto if they are pooched. Having your boiler run occasionally but regularly during the summer would be argued my many as being a good thing. Boilers are tremendously efficient water heaters. What the on-demand folks like to say is that because they have standby losses through the tanks that the on-demand is more efficient. In the winter, this point is pretty well moot unless the water tank is outside of conditioned space. I think on-demands have their place and that is a situation where you have a weekend retreat where the water doesn't need to be heated until you actually need to use it.

Bruce M.

Toyotomi Hot Water Heater

I installed a Toyotomi on-demand hot water heater. It is a direct vent heater that weighs 86lbs. It is 12-5/8" wide and 27-3/4" high. It is rated at 148,000 BTU. I can take a 12 hour shower and never run out of hot water. The heat exchanger holds 5.1 gallons of hot water. After it has been in use it will run for a maximum of 30 seconds. As far as I can see it is the greatest thing since sliced bread. It is rated at 88% efficiency and can use either #1 or #2 fuel oil. Here is a link to the specifications.
http://www.toyotomiusa.com/products/OM-148.html