Boiler efficiency and water temp

mikeg2015

Was searching the web trying to find some info on boiler efficiency vs. return water temperature and noticed this.

Keep in mind that radiator output drop by almost 1/2 going from 180 to 140, and almost another 1/2 to 120 and again to 110. Put another way, if your boiler output refectory match your radiator at 180F, and it has burner a turndown of 5:1 out would never cycle on water temp, only on a call for heat. Efficiency, assuming a 20F delta T at full rate and 4F at min fire (constant flow, respectively would increase from about 86% to 93%. A variable flow primary, might get you to 95%.

https://www.google.com/search?q=cast+iron+boiler+effeceincy+chart+return+water+temp&rlz=1C9BKJA_enUS780US780&oq=cast+iron+boiler+effeceincy+chart+return+water+temp&aqs=chrome..69i57.12857j0j7&hl=en-US&sourceid=chrome-mobile&ie=UTF-8#imgdii=Nm9ue2T6Ou0HAM:&imgrc=Nf6FtF6phuxpDM:

https://www.expressradiant.ca/pdfs/product_classic_sizing_how_to.pdf

What I haven’t been able ot find is boiler efficiency of a steam boiler running an indirect. I’m guessing it only gains maybe 1% combustion efficiency because it’s only 2/3 full of water and so there’s little heat transfer the last 3” of the sections. I figure standby loses in off-season are about the equivalent of 10% Efficiency. So net economy is probably around 70-73% so still a lot better than a gas fired tank water heater at around 58-60% with standby losses factored in.

The Steam Whisperer

The efficiency of the boiler will probably vary dramatically with the type of boiler and the amount of load. Probably a low mass copper tube or low mass condensing will be most efficient if not too oversized. A high mass power burner boiler with a damper on the burner air intake that closes on the off cycle is probably next most efficient followed by a power burner with no damper. At the very bottom is probably a high mass atmospheric with no stack damper. I attached a chart that I have used for years for determining potential savings from various configurations and upgrades of existing heating plants. It has proven to be quite accurate. You'll see this same chart in Burnham's literature for instance. If you are looking at typical atmospherics with no stack damper., just find out the typical load on the boiler and plug it in to the table to see your efficiency. As you can see as the load drops below about 1/3, efficiency begins to plummet. I have found the typical oversized atmospheric commercial steam boiler running about 56% Efficiency for the season.

hot_rod

Actually a large mod con on lowest turndown would be most efficient as you expose a lot of HX surface area to those ideal condensing conditions.

The Steam Whisperer

I agree HR, like you said.....just have to be careful of sizing to make sure it doesn't short cycle under light load.

mikeg2015

> @hot_rod said:
> Actually a large mod con on lowest turndown would be most efficient as you expose a lot of HX surface area to those ideal condensing conditions.

Lochinvar has some published curves showing both RWT and burner %. It’s a pretty big difference. The heat exchanger efficiency becomes oversized at low fire so the approach temp drops.

Some commercial mod cons can have approach temps around 1f. Compare that to my atmospheric steam boiler with a 250f approach temp.

mattmia2

this is missing something:
"Keep in mind that radiator output drop by almost 1/2 going from 180 to 140, and almost another 1/2 to 120 and again to 110. Put another way, if your boiler output refectory match your radiator at 180F, and it has burner a turndown of 5:1 out would never cycle on water temp, only on a call for heat. Efficiency, assuming a 20F delta T at full rate and 4F at min fire (constant flow, respectively would increase from about 86% to 93%. A variable flow primary, might get you to 95%. "

In order to get the 95% you need a return water temp around 130 degrees to get it in the condensing region. There is a tremendous amount of energy in the heat of vaporization of the water in the flue gas and if your return water temp isn't low enough to make much of it condense your efficiency is going to fall significantly.

mattmia2

The steam boiler with the indirect can be made more efficient by running on an aquastat when it is a dhw call only and running the boiler at a lower temp, the lower the temp of the water and boiler, the more heat will move in to the water instead of the flue. As mentioned on another post, you can flood the boiler in the summer so more surface is in contact with the water when it is only serving the indirect.

mikeg2015

> @mattmia2 said:
> The steam boiler with the indirect can be made more efficient by running on an aquastat when it is a dhw call only and running the boiler at a lower temp, the lower the temp of the water and boiler, the more heat will move in to the water instead of the flue. As mentioned on another post, you can flood the boiler in the summer so more surface is in contact with the water when it is only serving the indirect.

I get that. Just not sure how much you really gain. I’m guessing maybe 1-2% at most.

What I found interesting is that, on my hydrotherm at least, the sections only connect at the bottom rear and at the steam drum at the top. If the water line is near min level where the auto fill comes on, the drum is dry is not connected by water and heat transfer drops dramatically with the indecent pumping across the boiler. You mostly lose the center sections. Further compounded by the compromise location of aquastat resulting in measuring return temp when the pump is running. But it overall works fine. Indirects have so much capacity that I’ve found 150-160f water is adequate for our use. To reduce short cycling but good response, I’m using the high/low gas valve setup. High stage maintains A min of 110-140 all the time and low stage comes on with a call under 150 (RWT). Tank temp is set at 140. So boiler water needs to get to at least 145 to satisfy a call.

I’m still getting about 16000BTU in that situation which is equal to one element of a electric water heater coming on.


One harsh reality is that standby losses are very high. I have 500lbs I’d iron and 8 gallons of water that lose probably 5f per hour. That’s 632 BTUs assuming 500lbs cast iron and 8gallons of water. If the boiler runs for 20 minutes total a day for dhw the that 27% in thermal losses. So NET efficiency of 60% in summer. Still better than a standard tank. In winter standby losses are irrelevant as it’s hot anyway for CH calls and would actually reduce those losses a little as it consumes residual heat and lowers the boiler temp.

Solid_Fuel_Man

Or get a solar indirect with a coil (winter use) and an electric element (summer use). Or an add-on heat pump water heater for summer use and just use the indirect as a storage tank.

Running a high mass boiler in the summer for DHW is a huge waste and heats up the boiler room compounding hot weather.

unclejohn

The most efficient setting for any boiler is off.

The Steam Whisperer

High mass is only a problem efficiency wise if you let the heat leak out. I've been around some newer small commercial power burner boilers and the boiler room is down right cool. These boilers have a damper to close the air intake into the boiler and about 3 inches of jacket insulation so they'll stay warm for a couple days after they have been fired.

mikeg2015

> @Solid_Fuel_Man said:
> Or get a solar indirect with a coil (winter use) and an electric element (summer use). Or an add-on heat pump water heater for summer use and just use the indirect as a storage tank.
>
> Running a high mass boiler in the summer for DHW is a huge waste and heats up the boiler room compounding hot weather.

Have you priced those dual coil tanks?. No way I can recover $1000 in fuel any time soon. Probably 10 year payback.

Heating the boiler room I agree could cost you. But in my case it’s a damp stone foundation so a little heat just helps it the dehumidification anyway. Right now it’s only warming the boiler room 2-3f.

I’ll have to verify the rate if heat loss. But a full heat up of 100f rise is 16500btus. So if it lost 100f in 24 hours total and my AC downstairs absorbed 100% of that, it would run 45 minutes longer. That’s 1.2kwhr electricity. Or around $0.15. Do that for Let’s say 150 days a year, that’s $22. I’m saving that much each month I estimate with a gas indirect over electric tank.

So it’s a lot of heat in a small space and size able compared to Total DHW use but not that much heat overall.