The Glitch

The system shown was installed to provide domestic hot water to a high-end home with six bathrooms. The installer selected the largest 119-gallon indirect water heater available from his local supplier. The tank’s internal heat exchanger was rated to transfer 250,000 Btu/hr when supplied with 180º F water from the boiler. 

The installer piped it in with 1-in. copper tubing because that was the size of the piping connections on the tank. He used three identical “zone” circulators, one for each boiler and one for the indirect tank. He used a larger circulator for flow to the space-heating portion of the system. 

Being concerned about a potential complaint over not enough hot water, the contractor convinced the owner to double-up on boiler capacity, and thus installed two boilers, each rated at 250,000 Btu/hr. He also set the internal high-temperature limits on each boiler for 200º during a call for domestic water heating (because that’s as high as the control would allow). He figured that 200º will significantly increase the heat transfer through the tank’s coil heat exchanger.

When put in operation, the system experienced serious problems. The 15-gpm “tropical downpour showering experience” in the master bathroom was only comfortable for about five minutes, followed by a rapid drop in water temperature. The boilers also short-cycled and emitted knocking sounds during the DWH mode. Even worse, the house quickly overheated each time there was a call for domestic water heating.

Can you spot at least five details in the Glitch drawing that are incorrect and also recommend how to fix this situation?


The Fix

Doubling the boiler heat output and setting the boiler limit to 200º will produce slightly better heat transfer in the storage tank (relative to the rated heat transfer based on 180º water from the boiler). My calculations show the increase to be about 25%, assuming a cold water inlet temperature of 50º and a hot water delivery temperature of 120º. Under these conditions, the indirect heater was capable of transferring about 312,000 Btu/hr to the domestic hot water.

This is still far short of the full boiler capacity of 500,000 Btu/hr. This system is bottlenecked by the performance of the internal coil heat exchanger. The remedy is to select an indirect tank with an internal heat exchanger that can transfer the full 500,000 Btu/hr. output of the boiler system or use an external brazed-plate heat exchanger that can do the same.

No means of hydraulic separation exists between the boiler circulators and the circulator supplying the indirect tank or the space-heating circuit. This puts the boiler circulators in series with the other circulators. And this piping, combined with the lack of a check valve, also allows hot water to flow backwards through the space-heating distribution system when there is a call for domestic water heating, but no call for space heating. 

Other errors include:

1. The 1-in. piping and zone circulator supplying the indirect tank will not provide sufficient flow to convey 312,000 Btu/hr. (much less 500,000 Btu/hr.) to the coil.  Assuming a nominal 20º temperature drop across the coil, the flow needs to be about 50 gpm. That’s a lot more than a typical zone circulator can provide and certainly more than a 1-in. pipe should be carrying. That flow requires a minimum 20-in. pipe.

2. The placement of the boiler supply temperature sensor; it will only accurately sense the temperature of the boiler on the right side.

3. The boiler circulators should push flow into the low-mass boilers. This keeps the pressure inside the boiler elevated, and thus widens the margin against steam flash.  This is especially important because the boiler water will be in the range of 200º during a call for domestic water heating.

4. The check valve on the boiler inlet piping is much too close to the underlying tees.  This is likely to produce rattling sounds. There should always be at least 12 diameters of straight piping on the inlet side of any check valve. This helps smooth out turbulence caused by upstream devices.

5. There is no central air separator in the system.

6. There is no anti-scald ASSE 1017-rated thermostatic mixing valve on the domestic hot water delivery piping.