Hot water won't drain

I’ve been inspecting for almost 18 years and haven’t seen this. Today I did see it in a multi million dollar condo. It happened in 2 of 4 bath sinks. Cold drains great, runs right down. Hot stops draining almost completely, builds up in the sink as fast as it comes out of the faucet. Reactions are immediate. Add cold to the hot setting in the sink and it flows right down. Showed the agent so she would believe the report. Checked a few times during the inspection and it didn’t change. Looking for an explanation. Thanks.

I have been inspecting 20 + years…
I have no idea what you are suggesting
just report and defer for further review
that is what we as home inspectors do…

Thanks Jeff, sounds reasonable, but not really convinced. One sink had the small holes in the drain cover, the other had a regular pop-up. Irregular situation, so I am curious.

I believe the problem is caused by a combination of three things:

  • No secondary overflow in the sink
  • Plate over the drain with the small holes
  • Vapor pressure of hot water versus cold water

First the holes in the plate covering the drain are considered an orifice. There are equations that calculates the flow rate through a round orifice. The equation says the smaller the hole the lower the flow rate and the higher the pressure difference between the inlet and outlet of the orifice the faster the flow. If you had a bucket of water and you punched a small hole in the bottom the inlet pressure is the height of the water above the orifice. The pressure on the outlet side of the orifice is zero, because it’s draining into open air. As the water drains out the inlet pressure goes down which reduces the flow rate.

Now sit this bucked on a second bucket partially filled with water. Now this is where the vapor pressure of the water in the second bucket comes in to this problem. If you put 57 degree water in a bucket and seal the lid the vapor pressure in the bucket would be approximately 0.23 psi. Put 130 degree water in the bucket and the vapor pressure goes up to 2.22 psi. That’s a ten fold increase.

Stay awake were about done… So our drain pipe with water in the trap is the lower bucket and the sink bowl with the plate with holes is the bottom of the first bucket. Now turn on the cold water, if the flow rate out of the faucet exceeds the combined flow rate through the holes in the plate water will start to fill the bowl. As the cold water flows through the plate there will be a point where the air pressure above the cold water in the trap will rise to a maximum of 0.23 psi. However one inch of water in the bowl creates 0.43 psi of downward pressure so the water over powers the air pressure trying to get out, i.e. the water drains. Now turn on the hot water and the vapor pressure above the hot water in the trap will rise to a maximum of 2.22 psi. Now it takes over 5 inches of water to overcome the air escaping. But there is one more factor, the higher vapor pressure of the hot water reduces the flow rate through the orifice. So in theory there will be a point where no water will flow creating a vapor lock. The secondary overflow in older sinks provides an escape route which prevents the build up in vapor pressure.


very good explanations, thanks

Ok, ok. Convincing on the sink with the small holes, not so much on the sink with the standard pop-up drain. Thanks.

Very enlightening. Thank you for the analysis.

I thought that the primary influence was surface tension and the small orifices, though I never observed the effect of water temperature on the drain rate. You can bet I will be experimenting with this the next time I encounter a suitable vessel type sink.