Hot water first then cold out of cold side of fixture

I had a pre-listing today and came across something I’ve never seen before. In one of the bathrooms, when you turn on the cold water, hot water comes out for about 5-10 seconds and then turns cold, and also when flushing the toilet the water is hot, then is cold. (Tested with Laser thermometer) Tub/shower is normal. Hot water tank is plumbed correctly.All other plumbing fixtures in the house were normal. Any ideas??? Its got me stumped.

John Berthiaume
JB Home Inspection Services


Only had this happen a few times and it turned out the cold water pipes to certain locations were run through a heated duct chase or along/against a section of hydronic heated floor.

This did not bother the clients that much because of our climate.

I have a similar situation in my own kitchen…the water is warm for about 10-15 seconds in the winter months as the piping runs along the heat duct that terminates under the kitchen sink base…could this be happening there ???..jim

I also have same in my house. Running along heat duct. Judging by previous replies, that seems to be the best answer.

Thanks guys, That is absolutely the cause. Their is a radiant heat system installed under the flooring system and there are heating ducts running up the wall on same side as supply. I never even thought about that.

John Berthiaume

I have a Grundfos hot water recirculation system at my house, and I get a splash of hot water before I get cold. This is normal operation for this system.

I was just looking at one today for my home.

Ditto here on the west coast (my house)

What about heat losses from pipes 24/7/365?? Might save some H2O but what about the energy?

I have also seen this caused by a cross connection.

What about it? Certainly there is some heat loss, but it’s something I’m willing to pay for. Others may not tolerate that. Fact is, the circulator just slowly “stirs” the water. It’s not really all that much flow while it’s circulating. Just enough.

Did a consulting call on a new supposedly highly efficient home last summer. Power bills were higher than expected(only 2 occupants). Found a recirculating DHW pump for instant hot water purposes (electric tank).

In the basement mechanical room, the temp was 25 deg C (78 deg F), warmer than outdoors that day!!! I’m going to monitor the electric usage of the tank without any hot water being drawn after some other legal stuff with the builder gets out of the way. I wouldn’t be surprised that 1,000 to 1,500 watts is a constant parasitic heat loss from the poorly/partially insulated pipes in the loop.

This house has a HP/AC system. The heat loss in the summer adds to the AC costs while in the shoulder seasons of spring and fall especially, the heat loss (resistive) to the house will cause the HP not to work as much in the times when the COP is highest! This is when the HP is supposed to earn its keep!!

These may not be big energy costs individually but if you save 2-3% 10 times in a house, it adds up!

It’s on a timer but yes there would be some heat loss. In the winter that’s not a problem but the pipes should probably insulated to reduce losses.

I found a small point of use water heater installed in series with the hot water source, in remote areas from the main hot water heater is more efficient than a circulating system if the building layout can accommodate the design.

There is a heat loss in hot water circulating systems, however you must consider the alternatives. The draw-down to get hot water to a remote faucet is a loss of water (several gallons) as well as energy .

The amount of water in the draw-down must be made up with cold water into the hot water heater initiating a recovery. Often the water need is small (1 quart just to wash your hands etc.) but the water usage (several gallons) and heat recovery is substantial in proportion to the need.

In my house, the water heater, laundry room, kitchen are on one side of the house and three bathrooms are stacked at the other side of the house. I installed a 4 gallon water heater below the bathrooms in series which provides near instant hot water. The cold water in the pipes to the main water heater is tempered in the 4 gallon water heater and you never feel it in the shower.
The cost of the additional piping as well as the ability to install a loop in existing construction must be considered.

The cost of a Grundfos pump (plus a timer) is as much (if not more) than a water heater. Its life expectancy is equal to or less than that of a water heater.


There was a period in history in which installers favored, due to lack of no other choice, higher volume Bell and Gosset type circulating pumps. They flowed so well, I’m sure the heat loss was much more noticible. Folks with hot water recirculation systems can normally afford to also either spring for the pipe insulation or the higher bills, so it seems like a non-issue to me. The losses from a modern, low-flow system, particularly those on a timer, are inconsequential.

I sure would like to know your source on that one.

Regardless, it’s a quality of life and comfort issue. From a pure cost standpoint, it can seem to make little sense. It’s just one of the pleasures of life.

As long as you’re willing to pay for it, no problem. Not many people see that side of it…like the designer and contractor for the house I mentioned earlier.

Would you drink from the hot water tap? sounds like that is what you would be doing, Contaminating the cold water in the system. Is there a mandatory back flow preventer installed on the Main? or should we have the whole town drinking the Micro Organisms in our hot water tank.

From my personal replacement of these things. It’s just an opinion. I determine this from the age of the equipment it is attached to.

Mechanical things do wear out.

They start leaking at the bearing and then short the internal motor.

The life of a water heater in your area may be different than here. But I often see the original water heater there when I replace the pumps.
Is this good enough for you?

Here we call that condition SUMMER. :smiley: