Improving efficiency of FHW tankless hot water?

Jeff_104

As Eric suggested I think I am leaning heavily toward an electric unit.
With a cost of 8.5/KW it is certainly the most economical choice.
Propane is $2.90/gallon so that takes it out of the equation.
2 adults and 2 kids (5 and 2) so that eliminates an on demand system.
Storage and the heating unit all in one place for about $35/month
I'm burning over a tank of oil a year and thats only for hot water. An electric should pay for itself inside of a year.

Jeff_104

The only time my oil burner runs if for hot water. The house is heated 100% with the wood stove.
It is a tankless system with an expansion tank and we have baseboard heat.

Would it be more economical to have storage for the hot water? The boiler runs during the day just to maintain hot water and then runs again for showers etc.
I am planning building something solar this winter or at least getting it laid out and finish gathering materials but there has to be a fairly simple solution to at least assist the boiler in heating the hot water. As simple as a well insulated 55 gallon plastic barrel?

Brad White

DWH Storage

Storage is always a good idea for high mass systems such as you have. The idea to run bang-bang on-off is hard on equipment compared to a long burn which allows stable combustion.

The first minute or two of combustion during a cycle, especially from a cold start, is the worst part of efficiency. It may never reach "steady-state" which is the basis of efficiency ratings. You are perpetually below that point.

I would NOT use a home-made storage tank. It is probably not acceptable by code in your area (or any area that has codes). You want some vessel that is NSF-61 rated for potable domestic water use. This means passivated welds, glass coating or welded stainless steel among other features. Above all it has to have an ASME rating for 150 PSIG with few exceptions.

Might I recommend a Heat Transfer Products "SuperStor" coil booster? That among other devices is a well-insulated storage tank (no heating coil, just storage). A shuttle pump moves water from the heater to the tank to charge it.

I can also suggest that you get a regular Super-Stor and hook it up as a zone off of your boiler. This will allow you to heat the water for longer periods, given that the immersion coils tend to get mineral build-up and require more frequent replacement. You can do this and still use the immersion coil or the boiler although I am at a loss as to why you might do both :P

Eric Johnson

Electric HW heater?

Do you think he might be better off to idle the boiler and put in an electric hot water heater? Seems to me when you add it all up, that's the more economical alternative. Or maybe an oil-fired hot water heater?

I know that's not the original question, but it's the first thing that comes to my mind.

jp_2

usage

got to look at usage too, if real low an on-demand electric might be economical