I'm looking into getting a Weil-McLain Ultra Boiler installed this
Spring. I've done research on it, and came across a quote that
bothers me a little: Someone saw it at a home repair trade show and
said that it never shuts down, it modulates up and down. Now, does
that mean it never shuts down AT ALL, even when the temp gets to what
the thermostat is set at; or that it never shuts off while the heating
system is running, and goes off when the thermostat doesn't need
anymore heat? I've assumed the latter is true, but it wouldn't hurt
if someone who knows this system confirms that. Thanks.
It will shut off when the demand is low enough. There is a minimum safe
flame level on any burner system below which the gas valve must shut
down. During periods of high demand, e.g. cold outside, it may very well
run continuously. The Ultra regulates air/fuel mixture to maintain high
efficiency burn. It's actually higher in efficiency at low fire, which
is one good reason to have a modulating valve.
Just so I'm clear on this, "demand is low enough" means "the temp gets
to the setting on the thermostat," right?
Sorry for seeming to somewhat anal, but this thought came to mind: The
Ultra's fuel efficency wouldn't do me much good if it runs 24/7!
The t-stat doesn't operate the boiler. The boiler will maintain an a
preset outlet water temp to the best of its abilities via an adjustable
internal temperature control. The internal control modulates the gas
valve in response to changes in water temp. The colder the water gets
the more the valve opens. The valve will thus raise or lower the burners
in response to the demand put on the unit. This is, believe it or not,
much more efficient than simply cycling the valve on and off with a
conventional thermostatic on/off control.
It won't run constantly all the time, only when there is a high demand
put on it, but still no worries. Depending upon water temperature
setpoint, the burner may or may not cycle completely off between calls
for heat at the t-stat. There are several factors involved, including
whether or not the boiler is being used for central hot water in the
house, whether hot water is being used to take a shower, wash clothes or
dishes, etc during the heating system off cycle. The boiler system is
designed to modulate the flame height in lieu of cycling off. Excessive
cycling kills burner efficiency, as the air/fuel mixture and stack flow
attempt to stabilize. In addition at low fire more of the heat generated
gets transferred to the water. Modulating the flame to a lower level
prevents cycling for the very purpose of *increasing* efficiency. Though
it may run longer at low fire, it is using less gas while it is running.
The end result is a net fuel savings.
Right. actually, it is more complex than that but no, it will not be
running 24/7. You will probably see less fluctuation in the temperature
also as the boiler modulates and decreases the output at times.
Why not? Why do you care how long it runs if it is saving you money. A
roaring forest fire will put out enough heat in a minute to keep your house
heated for a season. Do you think that is more efficient than running a
Modulating is the way to go. Most boilers fire the burner at a pre-set
temperature, the water temp comes up and the burner shuts off until it drops
to another pre-set, the starts up again. Modulating boilers work by closing
down the gas and air to make a smaller flame once a certain temperature is
reached and opens up more as the temperature of the water drops. This is
more efficient and keeps a more even temperature.
Once the temperature gets to high in very mild weather, it will shut the
At works, we have three modulating boilers for high pressure steam that work
the same way. Works well and I'd not want it any other way.
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