Why is there no hot water unless the recirc pump is running?

There are two common designs, and the diagnosis differs.

If you're not sure which you have, look under the farthest sink. A small valve body with hoses connecting the hot and cold supplies means a bypass system. We explain the warm cold line on whether a recirculating pump heats your cold water lines.

### A flow check valve that's stuck, missing or backwards

A check valve is a one-way door. Taco's Hot-Link instructions say the valve has an internal flow check that prevents cold water from backing into the hot line. If that check fails or is missing, cold water can flow backward from the return path into your hot line whenever the pump is off, and the hot water you're waiting for gets diluted. With the pump running, hot water is being pushed through the loop, so the faucet turns hot. That's the typical pattern, though the exact behavior depends on how your lines are laid out.

Taco also describes a flow check built into some circulators. Its 0012 and 0013 circulators have an integral flow check, and Taco publishes steps for removing and replacing it, which makes it a serviceable part. Taco's SmartPlus-e sheet stresses installing the circulator with the arrow on the casing pointing the right direction. For a pump on the return line, the arrow must point toward the hot water source. For a pump on the supply line, it points away from it. A check valve or pump installed backwards is a classic cause on a system that never worked properly after installation.

### A bypass valve that isn't closing

Grundfos's fault-finding chart addresses this exact symptom, "no hot water at faucet or too much hot water on the cold water side." Its test for a bypass valve is:

1. 1Close the cold water angle stop valve below the sink.
2. 2Open the cold water faucet.
3. 3Water should flow slowly until hot water reaches the bypass valve.
4. 4As the valve closes, flow from the cold faucet should fade to a trickle and then stop.

If flow doesn't stop, Grundfos says to review the bypass valve installation and, if necessary, reinstall the valve. A bypass valve that stays open lets water pass between the hot and cold lines all the time, which can leave the hot side weak when the pump is off. Check that the hot and cold ports are connected to the correct hoses, since the valve is marked for hot in and out and cold in and out.

### Valves and air

• A closed or partly closed isolation valve. Both Taco layouts show isolation valves around the pump and the check valve. If one was left partly closed after service, it can make the system behave oddly, so it's worth a look, though on its own it restricts flow whether or not the pump is running.
• Air trapped in the line. Taco's instructions call for purging air from the system before running the pump.
• A missing second bypass valve. Taco's Hot-Link instructions say a home with more than one run may need another valve at the end of each run. A fixture on a run without one can behave differently from the rest.

Start with what's visible, with the pump unplugged if you're going to touch the piping.

1. 1Find the check valve and the pump. Look for an arrow on the pump casing and on any check valve. Confirm the arrow points the way the water is meant to move.
2. 2Check the isolation valves. Handles should be in line with the pipe when open.
3. 3Feel the pipes with the pump off, after a few minutes. The hot supply near the heater should be hot. If it's cool close to the heater with the pump off, that can point to cold water getting into the hot line. The first few feet of pipe right at the tank can be warm on both sides for another reason. The Department of Energy says heat traps prevent heated water in a storage tank from mixing with cooled water in pipes, a process called thermosiphoning, and that some new water heaters have built-in heat traps.
4. 4Try the Grundfos bypass test at the far fixture if you have that system.

After that, it's usually a plumber's job. Replacing a check valve or bypass valve means shutting the water off and draining part of the system.

You can, but it's a workaround, and it has costs. The U.S. Department of Energy's Building America program says recirculating systems generally have much higher distribution losses than non-recirculating ones, though they reduce wasted water and hot water wait time. The same guideline cites the International Plumbing Code (IPC 2009, 608.3) as requiring recirculating pumps to have controls that shut them off automatically, or a manual shutoff. Running the pump 24 hours a day to cover a hidden fault wastes energy and wears the pump.

Our page on whether a hot water recirculation pump is worth it covers the energy trade-offs. If your hot water is slow to arrive even with the pump on, see why you might have no hot water, and consider whether insulating your hot water pipes would help.

Tell the plumber when the problem started, whether the pump was ever serviced, and whether the pump runs on a timer or a demand control. A plumber will confirm which system you have, then work through the same list: check flow direction on the pump and check valve, test the bypass valve, confirm the isolation valves are open, and purge air. Taco's steps for its integral flow check show what a repair looks like. Disconnect power, shut off the water, let the system cool to prevent scalding, loosen the flange bolts slowly to relieve pressure, remove the circulator, and take out the flow check with needle nose pliers. Then clean the pocket, press the new check into place until it snaps, reinstall with new gaskets if required, refill, and confirm the system works.

HQ Plumbing and Air is licensed and insured, takes emergency calls 24/7, and gives free estimates with an upfront price before any work starts.