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Re: Hot-Water Boiler Temp Settings

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Posted by dana on October 07, 2008 at 11:36:23:

In Reply to: Hot-Water Boiler Temp Settings posted by Doug Criner on October 06, 2008 at 20:43:26:

: I have a 56-yr-old, gas-fired, hot-water boiler that works fine. I set the boiler aquastat manually, depending on outside temp and wind.

: During fall and spring, or during warm spells during the winter, I set the aquastat at about 145 deg. During winter cold spells, I raise it to 160 deg. In either case, the burner cycles - although in bitterly cold weather with wind, the cycle time lengthens considerably.

: Does this sound about right? Any comments or suggestions? Thanks.
: Doug

There's a rule of thumb that for every 10F you reduce the temperature you get about a 3% reduction in fuel use.

A lot of traditional hydronic heating systems are designed to be able to keep up on "design day" (coldest day of the year) with 160-180F water. If you've insulated & weatherstripped the place it will likely be able to keep up at lower temps, but if you're setting back the temp at night the lower temps increase recovery times to bring the place back up to temp (not a disaster, with programmable thermostats.)

Depending on how the boiler is plumbed, there may be issues with going below 160F (or not). The hydronic loops are typically designed for a 30F drop in temperature. If return water is entering the boiler at temps below 130F it is highly likely that exhaust temps are low enough to cause corrosive condensation in the flue, shortening it's service life. If the return water temps are below 125F there could even be condensation inside the boiler itself, causing it to rust-out in only a few years time.

This is easily remedied by a "boiler bypass" pipe that sips hot water directly from the boiler output and mixing it with the return water at the point where it enters the boiler, keeping it above 130F. (It may already be plumbed that way- often with valve to adjust the flow, and therefore the return water temperature.) If you measure the temperature of the return water pipe directly at the boiler 5 minutes or more into a burn it should tell you whether you need to bump up the temp a few degrees.

If the burn cycles are shorter than 5 minutes, or the duty cycle is less than 25% (15 minutes out of every hour) on an average cool heating-season day the boiler is probably oversized for the load and losing significant efficiency. On the coldest hours of the COLDEST days it should run pretty much continuously but still maintain temperature in the house.

But it's pretty typical to find boilers that a 2x, 3x or even 4x oversized, especially when a house has seen significant insulation and air-sealing upgrades. If it's 3x+ oversized that 82% AFUE boiler is acually delivering something like 60-65% actual efficiency (best case!) If it's short-cycling it only gets worse from there- take off another 10-15%. Getting the size right is critical for efficiency. Even using a standard ACCA Manual-J/ASHRAE heat loss analysis to size the boiler it still tends to be oversized by 20-40%, but that's still way better than the typical 100-200% oversizing.

Contractors will tend to oversize to avoid the call back, but the expense (both initial & operational) are then borne by the customer. In some places building codes are now disallowing oversizing by more than 10%.

It's probably worth having a burner-tech measure the combustion-efficiency of the burner- if it can't be tweaked up to 75% or better your actual operational efficiency may be in the abysmal range, especially if oversized for the load. There may be ways of de-rating it to more appropriate firing level, adjusting combustion & dilution air etc. to boost the efficiency short of a boiler replacement, depending on the actual design. State of the art in 1952 wasn't TERRIBLE, but there have been improvements along the way, and fuel isn't getting any cheaper.

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