Tankless Coil Priority on a Triple-Aquastat Oil Boiler
A house has an oil boiler with a tankless coil, a triple aquastat with high limit 190°F, low limit 160°F, and 10°F differential, and one heating circulator. The thermostat is a simple 24 V heat-only type. The homeowner complains that domestic hot water turns lukewarm whenever the heating circulator has been running, especially on cold mornings. When the heat has been off for an hour, hot water is fine. A technician finds the circulator powered from the aquastat circulator terminals and running whenever the thermostat calls. The tankless coil is rated for 3.0 gpm at a 90°F rise with the boiler at 180°F, and the home has a 2.5 gpm shower. The boiler is in a cold basement, and the tankless coil has no tempering valve. The aquastat low limit is set at 160°F. The homeowner has not noticed any relief-valve discharge or pressure loss.
1. Trace the control sequence and explain why hot water suffers when the circulator runs.
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The triple aquastat maintains the boiler between the low limit and high limit. When the thermostat calls, the circulator terminals energize the heating circulator, which moves boiler water through the radiation and returns cooler water to the boiler. The tankless coil is immersed in the boiler water, so its capacity depends on boiler water temperature and on how much heat the heating loop is removing. With the low limit at 160°F, the boiler may satisfy the aquastat but still be too cool for a 90°F rise at 2.5 gpm. Once the circulator starts, return water can drop the boiler below the effective coil temperature, so the shower goes lukewarm. The boiler is not necessarily failed; the control scheme gives heating demand priority over domestic hot water.
2. What changes would give domestic hot water priority without creating a scald hazard?
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Raise the low limit to at least 180°F so the boiler is maintained hot enough for the coil, and verify the high limit and differential are appropriate. Wire the heating circulator through a zone control or priority relay so a domestic-hot-water flow switch or aquastat de-energizes the heating circulator whenever the coil is in use. That gives the coil the boiler’s full heat output. Install a tempering valve at the tankless coil outlet to limit domestic hot water to a safe temperature, because 180°F storage can scald. Set the priority timer so a long shower cannot leave the house without heat indefinitely, and check that the circulator is not wired to constant power. Finally, confirm the coil is not scaled and that the boiler has enough capacity for simultaneous demand.
3. How would you verify the diagnosis with measurements before changing wiring?
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Measure boiler temperature at the aquastat well while the heating circulator runs and while it is off; a drop toward 160°F during a heating call supports the diagnosis. Measure domestic hot-water temperature and flow at the shower or a nearby tap, then repeat with the heating circulator off. Compare the observed temperature rise with the coil rating. Check the aquastat low-limit setting and observe whether the burner fires soon enough to hold temperature. Verify circulator operation and confirm it is powered whenever the thermostat calls. Check the tankless coil for scale or restricted flow, and confirm there is no mixing valve already installed. If hot water recovers when the circulator is disabled, the problem is control priority and boiler temperature, not a failed coil.