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Re: Boiler for a small heat load

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Posted by dana on June 24, 2009 at 17:57:07:

In Reply to: Boiler for a small heat load posted by Grinch on June 15, 2009 at 00:32:41:

: I'm looking at my options to heat one floor of a smallish house. Heat loss is 48 thousand BTU on the three coldest days of the year but is typically in the 10-15 thousand BTU range for the vast majority of the heating season.

: Can a mod/con boiler work in this application given that, for most of the year, the heating load will be below the boiler's lowest modulation step?

: I've been considering the Ti100, Alpine 080, and Solo-60.

: Thanks.

48K for three days? Try 48K for a total of ~30 hours, occurring on 3 different days. (And if you used Manual-J, to derive that number, it's 48K for more like 2 hours/year in an average year. There's significant cushion built in to any heat-calc.)

The fundamental issue with sizing is ALWAYS that the peak load is several times the average, which leads to cycling inefficiencies. Mod-cons can track-sorta, but still lose quite a bit of efficiency in multi-zoned (micro-zoned) systems due to cycle-losses. They can never track the micro-loads, not even close.

And outdoor reset is actually a very crude load model, since solar gain, wind, rain etc also have very pronounced effects on the load as well.

HeatPro's approach will mitigate cycling losses those effects far better than simple outdoor reset modulation. The reasons WHY can be found here:


Whether you use a mod-con or cast-iron or a tankless, a buffer-centric topology where the boiler remains agnostic of zone calls, and only serves the buffer, it's guaranteed to minimumize cycling losses by-design.

If your buffer-tank is a reverse-indirect (as in HeatPro's approach) it means you won't get return water temps back to the boiler low enough to make 90%+ efficiency until you're keeping the tank below 130F, but with outdoor reset control on the TANK (not the boiler) with a 130F temperature-floor you can have at least some of your cake: If you need 180F water on design-day, but 140F is more than enough most of the time, the fuel savings will be significant, even with standard-efficiency boilers. The rule of thumb is ~3% savings for every 10F degree drop in return-water temp. Tankless HW heaters & mod-cons buy you more, since they tolerate lower return water temps (and whenever the temps drop below 120F the mod-con's efficiency really takes off.)

The tank in a buffer-centric system is really a high-mass hydraulic-separator primary/secondary type system. When coupled with a low-mass boiler like a tankless it should perform similarly to the Energy Kinetics System 2000, but with much dumber controls. Unlike the Sys2K there isn't a boiler-heat purge at the end of every burn cycle to reduce standby loss, but the loss is already quite low, since the thermal mass of the water in the tankless is a small fraction of that in a cast-iron or steel boiler.

Low mass boilers (like tanklesses or mod-cons) ramp up to their full combustion-efficiency quite quickly (within the first 2-3 gallons of water pumped through them in a burn cycle). Having the mass of a buffer to work against, minimum burn cycles can be quite long, reducing the numbers of fixed losses that occur at every burn cycle (ignition & flue purges, etc.) But even with a cast-iron boiler this is rock solid, and providing more whole-system efficiency than doing the hot water as a separate "priority" zone (unless you add more controls to do boiler-purge heat dumps into the indirect, System-2000 style.)

In small heat load situations, combined heat/hot water is the "right" thing to do, always. Proof:


The steel oil boiler + indirect system #3 in the Brookhaven tests is a System 2000. Note that the approach meets/beats mod-cons in annual efficiency despite being a bang-bang on-off unmodulated boiler.

OK, too much to digest, and not a simple answer, but whatever... Just be aware that the system, not just the boiler determines the true efficiency. If you're making potable hot water in some standalone tank (or even a separate tankless), the combi-approach will make the hot water almost FREE, out of the improved system efficiency if designed right. The total number of burn cycles using a tankless as a buffered boiler is a tiny fraction of what a tankless sees as simply a hot water heater (firing at miserable efficiency for every 2 quart draw for hand washing/rinsing, etc.). The standby losses of a tank HW heater are atrocious- FAR more than an indirect, due to the necessary insulation gaps & constantly convecting flue sucking heat out of the tank when it's not firing.

Mod-cons are great if you have very low-temp radiation (like radiant slabs, or staple-up radiant floors in well insulated buildings), but even with extra baseboard it's hard to really make 'em pay. Without the very low temp radiation you'll so as-well(maybe better) with a well designed buffer-centric approach (even with a right-sized cast-iron beast.)

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