12-07-2010, 06:42 AM
I just bought a small house that has Burnham V84 boiler installed.
It feeds a regular baseboard heat loop and a radiant floor loop. The hot water is from an electric water heater.
I had the boiler serviced and the technician could not answer my question so I called Burnham directly and they would not answer my question.
So I'm asking all of you professionals:
When there is no call for heat, the boiler will drop in temperature all the way to down to ambient temp.
I was taught that the aquastat on the boiler maintains the boiler with a low and high temp setting.
I was also taught that it is very bad for a boiler to drop low in temperature as all of the soot hardens and destroys the boiler.
I want to know if I am doing any damage to my boiler or is designed to operate this way?
In theory, in the summer, the boiler will not come on ever thus making it drop in temp and possibly sweat?
As long as the return water from radiation isn't running at temps lower than 140F for long periods of time you're not doing damage to the boiler by cold-starting it. The aquastat allows the burner to fire but inhibits the circulator from running when below the low limit. Running below 140F for extended periods would result in corrosive condensation on the heat exchangers and in the flue, compromising both. As long as you have the low limit set to 140F or higher, you're golden.
From p41 of the manual (http://www.heatinghelp.com/files/posts/4266/Burnham%20V8%20Series%20Instructions.pdf):
"A call for heat by the thermostat energizes the L8124C control which in turn energizes the primary control to turn on
the burner. The burner will operate in the following sequence: Prepurge for the first 10 seconds; fire until the thermostat
is satisfied or the limit setting on the high limit is reached; post-purge for the last 10 seconds. The circulator will
also operate when the thermostat calls for heat if the boiler water temperature is up to the setting of the low limit in
the L8124C control. If boiler water temperature is below the low limit setting the burner will operate but the circulator
will not, giving preference to the domestic hot water demand."
The only way for the boiler to sweat in summer would be if it's temp were to drop below the dew point of the surrounding air. This isn't very likely. Were humidity that high in the boiler room EVERRYTHING in the boiler room would be condensing water on the surfaces.
Soot should not form during normal operation, and soot deposited during the brief cold-temp operation of cold start would burn off during the course of a burn. If soot is forming throughout a burn the mixture is too rich- the burner needs a tune-up (by a qualified technician with the requisite flue gas analyser/combustion-efficiency measurement tools.)
Depending on your local utility rates you may/may not be better off heating your hot water with an indirect-fired tank (operated as a heating zone), not the internal coil. See: http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf (http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf)
If your boiler is 2x or more oversized for your peak heat load, you may be able to reap double-digit fuel savings by installing a Beckett Heat Manager or Intellicon 3250 HW+ economizer. They work by cutting out the burner as the end of the call for heat is sensed, purging heat from the boiler to finish. The result is much lower standby losses, since it'll leave the boiler at a much lower temp between burns. The burner on a V84 is pretty hefty, and unless you live in a very large conventionally insulated house (or some really leaky wreck) odds are pretty good you're way oversized. According to the chart on p5 of manual the output on different versions of the V84 range from 127-159MBH. For reference, the heat load on my ~2200' 2x4-framed not fully (but mostly) insulated home is about 30MBH at 0F outdoor temps- that boiler would be 4-5x oversized for my space heating loads. Your design temps may be lower and house bigger, but it probably never drops to -250F in your neighborhood, and you probably don't have 4x the exterior surface area of my place. Most insulated homes in NY (even in the Adirondacks) will have peak heat loads under 75MBH. On L.I. they'd be under 50.
The regression curves for non-heat-purging operation at 4x oversizing would take most cast-iron boilers from the ~85% range down to 80% on during the coldest stretches of the year, and would average something like 70%, since the average heat load will be less than half that of the design-condition load. Look at eh curve of Appendix 1, p 3 (p19 of the .pdf pagination) of that Brookhaven document. At 4x oversizing your peak heat load is at the 0.25 point, and the average somewhere around the 0.1 point- you'd be on the steepening part of the curve all season long. A Beckett or Intellicon moves the knee of the curve to the left, putting it on a higher-flattter portion, similar to (but not quite as good as) the curve on Appendix 3 page 3 (page 25 of the .pdf pagination.)
Downloading and using this handy li'l tool (http://nora-oilheat.org/site20/fsa/FSACalculator_1_1_0_8.zip) based on the Brookhaven Nat'l Lab boiler modeling is one way to tell just how oversized you might be with the V84. Read the plate on the boiler for the actual input/output numbers to insert into the tool. If you're at 2x+, it's worth installing a heat purging economizer.
12-10-2010, 06:23 AM
I would think that the high and low limts would keep it within range