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keithb
Oil boiler + tankless water heater - boiler runs when not in use?

Bought a house with a 20+ yr old oil boiler and attached tankless water heater. (Needs replacing for efficiency reasons, but that's another story...) Every few hours, the boiler will turn on for somewhere between 30 sec and a minute, even though it's not in use (no water running, too warm for the heat to be on)

Any ideas what this could be? I figure it must be either a faulty thermostat, or the sensor for the tankless heater thinking that water is running.

How would I go about debugging this?

Thanks for any help!

i.m.plummin
Re: Oil boiler + tankless water heater - boiler runs when not in use?

So which is running for no reason the boiler or the tankless water heater. The post reads kinda crossed up.

keithb
Re: Oil boiler + tankless water heater - boiler runs when not in use?

The tankless water heater is attached to the boiler - the boiler runs to make hot water.

NashuaTech
Re: Oil boiler + tankless water heater - boiler runs when not in use?

keith,

That sounds like normal behavior for this particular type of boiler.

What you have there is a "domestic coil" of copper tubing inside the boiler that holds a gallon or two of hot water to supply the taps.

The coil depends on the boiler water being constantly hot enough to satisfy the internal thermostat, so that when you open a faucet, hot water will come out.

This avoids having to use a separate hot water heater, but the downside is that the burner must come on every hour or two to keep the tap water hot.

This is more noticeable in the summer, since in the winter the boiler fires so much that hot boiler water is usually present to satisfy the coil.

The burner nozzle usually burns a gallon or less of oil in an hour, so it's not really that much oil being burned in the summer months.

The long-term solution is to replace the present "triple aquastat" boiler with a "cold start" boiler with an indirect hot water heater connected to it; this would eliminate the periodic short-cycling you're getting now and provide 30-40 gallons of hot tap water.

For now, you can lower the "low limit control" on the aquastat down to 140 degrees to save some oil until heating season starts.

Your aquastat probably looks like the one pictured in the site below; turn off the boiler switch and remove the little screw holding the cover plate.

Inside, you'll see a hi limit knob and a lo limit knob (see Fig. 7); turn the "Lo limit knob" down to 140 degrees, replace the cover and screw and turn the boiler back on.

http://www.exqheat.com/images/aquastatL8124_honeyw.pdf

Yeardleigh
Re: Oil boiler + tankless water heater - boiler runs when not in use?

n many countries the most common energy sources for heating water are fossil fuels: natural gas, liquefied petroleum gas, oil, or sometimes solid fuels. These fuels may be consumed directly or by the use of electricity (which may derive from any of the above fuels or from nuclear or renewable sources). Alternative energy such as solar energy, heat pumps, hot water heat recycling, and sometimes geothermal heating, may also be used as available, usually in combination with backup systems supplied by gas, oil or electricity.

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