Editor's Note: Figures 1-3 could not be reproduced for online use. Please see the June PME print edition for reference.
At the 2002 International Plumbing Code hearings in Pittsburgh, PA, an interesting code change was passed that adds piping requirements for thermostatic mixing valves used in a system with a circulating pump. The new code section is now published in the 2003 International Plumbing Code as section 607.3.7 and has the following language:
607.3.7 Re-circulating Pump. Where a thermostatic mixing valve is used in a system with a hot water re-circulating pump, the hot water or tempered water return line shall be routed to the cold water inlet pipe of the water heater and the cold water inlet pipe or hot water return connection of the thermostatic mixing valve.
The new paragraph requires circulated tempered water return lines to be split and routed back to the water heater and the cold water side of the mixing valve, so there can be hot water flow from both sides of the thermostatic mixing valve. When there is usage or a draw from the tempered water system, hot and cold water are drawn through the mixing valve and exit at the temperature setting of the mixing valve. When there is no draw from a fixture in the system, the piping must allow hot water to mix with cooler tempered return water to provide tempered water.
There is only one correct way to pipe a thermostatic mixing valve when there is re-circulated tempered water through the thermostatic mixing valve. The new code language mandates what many manufacturers have been suggesting in their literature for some time now. This new code language will allow the plumbing inspector to red-tag a project if it is not piped properly to prevent the possibility of scalding. Any mixing valves used as a master mixing valve in a tempered water system or hot water system should be of the thermostatic type, and they should meet the requirements of ASSE 1017.
Hot Water Return Pump SizingThe ASPE Data Book gives a sizing method for sizing domestic hot water circulation systems that allows for a flow rate that will give a 20-degree temperature drop through the hot water supply and return piping circuit. It is critical to have balancing valves installed in the hot water return piping system that can be set at actual flow rates. If only ball valves are used for balancing, it is very difficult to know what the flow rate is. Temperature gauges should be installed in the system at the outlet of the water heater, at the outlet of the mixing valve and in the hot or tempered water return line prior to the circulating pump. The circulating pump should be installed in the return line near the water heater, because that is the coolest place in the recirculated piping system, and lower temperatures will extend the service life of the circulating pump. In all cases, a domestic hot water return system should be balanced to give the proper flow rate to maintain the desired temperature differential across the system and to prevent short cycling across branches located near the mechanical room.
Example System TemperaturesIf the thermostatic mixing valve were set at 120 degrees F, the farthest fixture in the system would ideally be receiving water at about 110 degrees to 115 degrees F. The hot water return temperature back at the circulator or near the water heater would be about 100 degrees to 110 degrees F. The temperature at the farthest fixture and back at the circulator depends on the temperature difference across the system (10-20 degrees F). Rada Mixing Valves have a unique feature in the "R" series valves. They offer a return temperature limiting valve that monitors the returning water temperature and diverts the water back to the mixing valve loop to cool down if the temperature starts to creep up. Several other manufacturers are offering pre-piped high-low master mixing valves, where the contractor only has to connect to the inlet, outlet and return connections, and all of the components are pre-piped.
Using the 20-degree temperature differential, in Figure 1 the return water at 100 degrees is piped to the cold water side of the mixing valve and back to the cold water inlet of the water heater. The mixing valve can then mix the 140-degree hot water from the water heater with the 100-degree hot return water during periods of non-use to give a blended water temperature of 120 degrees at the thermostatic mixing valve outlet. A 20-degree temperature differential is recommended by many mixing valve manufacturers in order to get proper mixing. Several mixing valve manufacturers can blend water with a temperature differential that is much less.
Periods of Non-UseThe problem is that most of the time (during night time hours and periods between uses) there is no draw or flow from fixtures in the system, and the mixing valve is simply blending tempered water return (TWR) with hot water (HW) to make tempered water (TW). The accuracy of the mixing valve becomes important to maintain temperature, and the design of the thermal motor in the mixing valves is critical when the return water temperature approaches the set point on the thermostatic mixing valve.
Regardless of the type, if the mixing valve is not piped properly, you can get scalding hot water in the tempered water system, or if piped the other way, the mixing valve will not be able to blend, and the system will return to ambient temperature over a period of time. Both piping configurations can be dangerous. Hot water can cause scalding and death, and ambient temperature water usually results in a sudden temperature rise during a shower that can lead to thermal shock and a slip and fall injury.
Common complaints when the recirculated line is piped only to the water heater include:
- 1. "The water is too hot."
2. "The circulator pumps are failing too often."
3. "The system temperature is rising during use."
4. "He/she was scalded in the shower."
If the tempered water return is connected only to the water heater inlet, the tempered water circulated path can only be through the hot water port of the thermostatic mixing valve. In this case, the mixing valve closes down the hot water side of the valve and opens to the cold water side. If there is no water flowing out of the system, cold water cannot enter the mixing valve to blend with the hot water. The hot circulated water will leak through the mixing valve because of clearances required for the piston or actuator movement. This causes the outlet temperature of the mixing valve to rise to or very near the same temperature as the water heater outlet temperature until there is a draw or flow from a fixture in the system that will allow cold water to be blended with the hot water. Figure 2 shows an incorrect piping arrangement that is often the culprit when there are complaints about a thermostatic mixing valve not maintaining the system temperature and temperatures are too high. Symptoms include failing circulator pumps caused by dead heading of the pumps during long periods of non-use and fluctuating temperatures in the system.
Tempered Water Return Piping to the Mixing Valve OnlyAnother problem can occur if the piping is only recirculated to the mixing valve. In this case, there is no path for hot water in the circulated path, and the water continues to cool to ambient temperature.
Common complaints when the recirculated line only goes to the mixing valve include:
- 1. "The water is too cold."
2. "The mixing valve does not maintain the system temperature."
3. "The system temperature rises after several minutes of use."
If the tempered water return is connected only to the mixing valve, the tempered water circulated path is only through the mixing valve, and there is no hot water introduced to the system during periods of non-use.
In this case, the mixing valve opens to the hot water side, but if there is no flow in the system, the hot water cannot blend with the tempered water return. The result is the system temperature drops to ambient temperature over a period of time. The piping configuration shown in Figure 3 does not result in scalds as often as piping only to the water heater, but it can lead to thermal shock if the water remaining in the tempered water line cools down to a comfortable showering temperature. A bather often adjusts the shower valve to full hot and begins showering as the hotter water is on its way to the shower from the mixing valve. Depending on the size of the tempered water mains and how far the shower is from the master thermostatic mixing valve, it could take several minutes before the 120-degree water gets to the shower for a thermal escalation that could cause a slip and fall, but not likely a scald if the thermostatic valve is set at a safe temperature below 120 degrees. Figure 3 shows an incorrect piping arrangement that is often the culprit when there are complaints about a thermostatic mixing valve not maintaining the system temperature and temperatures are too low with sudden increases in temperature. Often, the building occupants, maintenance personnel or the contractors blame the temperature problems on the mixing valve before realizing it is a piping problem.