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Posted by dana on November 13, 2008 at 15:37:22:
In Reply to: Lochinvar Knight posted by Ken on November 11, 2008 at 18:48:57:
The affects of altitude on the ability of the boiler to actually condense the flue gases (in the heat-exchanger, not the flue) are significant. Most boilers are set up with about 10% CO2 content in the flue gases at sea level pressures, under which conditoins the flue-gas dew point is ~130-135F. Whenever the return water from the radiant is 10F below the dew point it's condensing sufficiently well to pull mid-90s combustion efficiency.
But that gets robbed by altitude in two ways- the CO2 content of the flue gases goes down a little bit which lowers the dew point by ~2F for every percent of concentration. But more significantly the lower atmospheric pressure lowers the dew point of the flue gases by about a degree-F for every 1000' of altitude.
So for you , at 10K' with maybe 9% CO2 in the flue gases your flue-gas dew point is somewhere around 120-122F, which means you won't hit 90%+ efficiencies unless the return water is under 110F. If the radiant is installed in a slab and the house is well insulated that will likely be most of the time.
But if it's wood floors...
...in a house requiring warmer water to keep up...
... you'll be lucky to see much benefit with a condensing boiler over a properly sized pretty-good mid-efficiency boiler.
But proper sizing is key. A significant benefit to the MOD/CON is the MODulation portion- cycling losses are minimized whenever it's cold enough that the boiler can track the load continuously. With an on/off boiler it's guaranteed to have more cycling, but minimizing the numbers of cycles makes a dramatic difference in it's actual efficiency. Oversized boilers are guaranteed to cycle more often, with shorter burn times eroding efficiency.
Do a manual-J heat loss calculation on the house and check that against the current boiler's output. If it's "in the range" for your design-day and a combustion-efficiency test gets you over 80% you may want to keep it. Don't assume anything over 90% true-efficiency on the mod/con unless the radiant system is designed to run with return water well under 110F most of the time. (A temperature that is destructive to non-condensing boilers unless boiler-bypass plumbing is installed.)
There is some loss of efficiency with non-condensing boilers with altitude as well, but it's very VERY slight compared to the impairment of condensing units. Once the flue gases are over the dew point temp you only lose about a percent for every 40F increase. But in order to get any condensing benefit you MUST stay below condensing temps, and those temps are lower for you than most installations, no matter how the boiler performed in an AFUE test.