Really interesting post. I've been at war with standby losses on my own indirect, I've blanketed it, insulated with 3/8 poly and used both loops and traps nipples to cut the standby. My next project is to put in a programmable thermostat that has a wider hysteresis than the stock Triangle one . Despite my efforts it keeps cycling the boiler on summer days with no HW demand at all.
The part I'm not getting about the vent losses is wouldn't the losses up the pipe be the same whether it's used for heat or not? is it just the loss proportional to the higher BTU's burned that makes it more efficient?
When the flue is actively burning, there's no standby loss, but that flue loss accrues to it's raw combustion efficiency- it's not a standby loss. When standing-by the flue temp is much lower and it's losing far less heat per minute, but the center-flue heat exchanger is still creating a convection force pulling cold air in at the bottom, passing warm air out the top.
But with the added duty cycle of space heating comes lower absolute standby loss:
Just for yuks, say the burner the tank is 35KBTU/hr in, at 80% efficiency it's delivering 28KBTU/hr to the water in the tank, the other 20% is going up the flue.
Let's say the design condition heat load of the small rental is about 15KBTU/hr, give or take. The average winter load tends to be about half the design condition load, so you're looking a 7-8KBTU for an average load, which is (7/28=) 25% duty cycle on the burner. An 80% (steady-state) atmospheric-drafted burner running at 25% duty cycle will be delivering more than 75% efficiency.
When in hot-water only mode the duty cycle on the burner is about 4-5%, call it 5%, so add another 5% to the on-time duty cycle, you're at 30%, that's usually good for about 78% average efficiency.
The standby period has gone from being 95% of the time to 70% of the time, with active burn time rising from 5% to 30%. This reduces the absolute standby loss, since the standby TIME has shrunk from 3420 minutes/day to 2520 minutes/day, a ~26% reduction in standby time (and standby loss). You still have ~74% of the original standby loss, but the magnitude of that loss is being distributed over more fuel use- the as-used efficiency numbers continue to climb as the on-cycle climbs. The absolute standby loss is actually smaller by a good bit, but the loss as a fraction of the total fuel-us is
much lower. So while it's the proportional loss that's responsible for most of the efficiency uptick, the absolute number does come down.
Still, flue losses are only about half the total standby loss of a typical gas-fired tank heater. Insulating the plumbing (and sometimes additional tank insulation) is economic, even at buck-a-therm gas prices.
With indirects it's important to insulate the boiler loop plumbing as well as the potable plumbing to cut standby loss. Heat traps stop convection (or at least should, if they're working correctly), but that doesn't touch the conducted loss up the pipe, which is still substantial. The sidewall insulation on most indirects is pretty good, and most of the loss is plumbing related. You could triple the R-value of the tank and it won't affect the heat loss up the plumbing. The cheap 3/8" closed cell foam pipe insulation is only good for R1.5-R2- (better than nothing, but not huge.) It's worth getting 5/8" or 3/4" wall goods, which will cut that portion of the loss in half.
How often does the boiler actually cycle just for temperature-maintenance burns? What's the storage temp? If you've been keeping it at 180F (to keep up during winter demand with colder incoming water) you may be just fine in summer with 130-140F water which would inherently cut the standby loss by more than a third.
If you're a showering (rather than tub-bathing) family, a decently sized drainwater heat recovery heat exchanger would add enough apparent capacity to keep it at 130-140F (or lower) even in winter. (My entire system runs on 130F water, and with a
4" x 48" DWHR we can still take pretty much endless 105F showers with less than 50KBTU/hr of boiler output, even in January.) As long as the indirect has about the same capacity as your largest tub you'd be able to handle tub-fills with a 130-140F storage temp too, but there would be a real recovery time after a tub fill, and would get zero back from the heat exchanger (since for the heat exchange it needs concurrent drain & potable flow.)
