Lukewarm Domestic Water From a Tankless Coil
A family complains that the shower starts hot and turns lukewarm after about five minutes in winter. The boiler is an oil-fired unit with a tankless domestic hot water coil rated 3 gpm at a 100°F temperature rise. The aquastat high limit is set to 180°F, the low limit to 150°F, and the differential to 10°F. The boiler cycles between roughly 150°F and 180°F. The mixing valve at the coil outlet is set to 120°F. The incoming cold water is about 45°F in winter. The showerhead is a standard 2.5 gpm unit. The heating circulator does not have domestic hot water priority. The coil has not been descaled in several years, and the homeowner reports that the problem is worse when the heating system is running. The technician must determine whether the coil is undersized, the boiler temperature is too low, or another fault is responsible.
1. Calculate the domestic hot water load and compare it with the coil rating. Does the coil appear undersized for this shower?
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The shower load can be estimated with the formula Btu/hr = gpm × 500 × temperature rise. A 2.5 gpm shower using 45°F incoming water and 120°F mixed water has a 75°F rise. That is 2.5 × 500 × 75 = 93,750 Btu/hr. The coil is rated 3 gpm at a 100°F rise, or 3 × 500 × 100 = 150,000 Btu/hr. On paper, the coil has enough capacity for this shower, so the coil rating alone does not explain the complaint. However, the rating assumes boiler water is hot enough and the coil is clean. With the boiler cycling as low as 150°F and no domestic priority, the coil may not transfer its rated capacity, especially when the heating circulator is moving heat away. The technician should measure actual hot water flow and temperature at the fixture, not rely only on the rating, and should check for scale and a stuck mixing valve.
2. What changes to the aquastat settings and system operation would you recommend, and what cautions apply?
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The low limit of 150°F is too low for reliable tankless coil performance in winter. The boiler water surrounding the coil should be maintained hot enough to heat domestic water to the mixed outlet temperature. A common approach is to raise the low limit to about 170°F to 180°F and raise the high limit to about 190°F to 200°F, provided the boiler and aquastat are rated for it and the relief valve and controls are in good condition. The differential should be set so the boiler does not short-cycle. If the system has a domestic priority control, enable it so the heating circulator pauses during a draw. Do not exceed the appliance manufacturer’s maximum temperature or the relief valve rating. Also verify the mixing valve is functioning and set to a safe delivery temperature, usually around 120°F, to prevent scalding. Raising the low limit increases standby losses, so the technician should weigh fuel use against hot water performance and discuss the tradeoff with the homeowner.
3. The settings are corrected, but the water is still lukewarm after a long draw. What other faults should be investigated?
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The next suspect is a scaled or fouled coil. Scale insulates the coil surface and reduces heat transfer, so the domestic water leaves cooler even when boiler water is hot. The technician should check the coil inlet and outlet temperatures and compare them with the boiler temperature. A large approach temperature difference suggests fouling. Descale or replace the coil according to the manufacturer’s procedure. Also check the mixing valve for a failed element or debris that allows cold water to bypass. Verify the domestic flow rate; a higher-than-rated flow will reduce the temperature rise even with a clean coil. Check the heating circulator for continuous operation during a draw and confirm priority is working. Finally, inspect the aquastat sensing bulb for contact and calibration, because a boiler that reads 180°F but is actually cooler will not satisfy the coil.