Hot and Cold Water

House was re-piped with CPVC. Bathroom faucet has dual handles. When running full hot with no cold, if the toilet is flushed, the hot water will be interrupted with about 3 seconds of pure cold water, then revert to full hot. The bathroom is about 80 feet from the hot water heater. If the hot and cold lines are separated, how can this happen? Same thing happens with the shower which uses a continuously variable temperature, single handle faucet. Thanks in advance for any advice.

hambone said:

House was re-piped with CPVC. Bathroom faucet has dual handles. When running full hot with no cold, if the toilet is flushed, the hot water will be interrupted with about 3 seconds of pure cold water, then revert to full hot. The bathroom is about 80 feet from the hot water heater. If the hot and cold lines are separated, how can this happen? Same thing happens with the shower which uses a continuously variable temperature, single handle faucet. Thanks in advance for any advice.

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From the start it sounds like the water for the toilet was cross-piped...the feed is hot water, but....
Is there a tempering valve on the toilet, mixing hot & the cold to keep the tank from sweating?
Also...is the lav faucet single or two handle?

Additional info: Toilet is TOTO fast flush and is new. Lav faucets are two-handle style. This problem did not occur until house was re-piped. However, there is another complication in that there also is a water hammer problem the plumber has NOT been able to solve, even with air chambers and hammer arresters. Don't know if the two problems are related.

Turn the hot water off at the water heater. Then see if you can get any flow from the hot side of the lav and shower, and see what happens when you flush the toilet. Is the plumbing accessible, so you can physically trace the pipes?

Do you have a recirc system??

water

We would have to be there to check exactly when and how the water problem occurs and if the water hammer is connected to it. One possibility is that the plumber did not compensate for the fact that CPVC is smaller than copper tubing so the water velocity, and flow resistance, can be higher which could cause both problems.

Additional info. The original piping used 1/2 inch copper. The 80 foot run from the water heater to the back bathrooms was seen as a problem, so the re-piping used 3/4 inch. This increased volume may be causing the water hammer. The house has a crawl space and access to the two bathroom bath/shower tubs is accessible through access holes in the wall. All pipes can be inspected for entire house. Each bath/shower and each toilet and the clothes washer have the small 6-7 inch water hammer arresters installed, plus a large air chamber is installed on the cold water side of the bath furthest from the supply. Water hammer now is present when water is turned ON as well as turned OFF (bang, bang, bang reverberation). Bleeding the water lines does not change the situation. Pressure is nominal at 54-55 psi. Closing the cold water faucet handles SLOWLY does NOT cause water hammer. There is no hammer with the hot water supply.

Does the flushing/hot-water problem occur in both bathrooms? Is it peculiar to the flushing of one toilet, or do both toilets cause the problem?

To check the water-hammer problem, crawl under there and feel the long cold-water pipe while your lovely assistant operates the faucet(s), and see where it's banging against the building structure. That is a long slug of water to accelerate and decelerate, and you're limited in your ability to fasten the pipe firmly to the building structure due to the higher expansion rate of CPVC (cf copper). You might find that the pipe is banging around in such a way that you could cushion the blows with soft insulating material in a few critical spots.

Pipes have been observed for water hammer both inside the wall and under the house. The cpvc pipes are not hitting the wall. It appears that the hammer effect happens when the slug of cold water hits the constant temperature, single handle, shower handle mechanism for both tub/showers. The only copper remaining in the system is that which goes from the single handle mechanism up to the shower head. The toilets do not have any metal piping except for the stainless steel braided pipe from the above floor shut-off valve to the bottom of the toilet tank. There is no noise coming from the vicinity of the toilet or from the vicinity of the washbasins.

A common shortcut on a shower valve is to not anchor it properly. Normally, a water hammer is defined as occurring when a quick-acting valve is closed such as a solenoid valve in a washing machine, ice maker, washing machine, or toilet. If you get banging in your pipes when you open a valve, this is less common. You mentioned you have air columns and hammer arrestors. An air column normally becomes useless after as little as a few months as the air is absorbed in the passing water. A true hammer arrestor has a seal between the trapped air and the water, preventing that from happening.

If there is air somehow getting into the piping system, as that gets purged from the pipes, that can simulate rapid on/off symptoms. When you do turn on a valve that causes banging, do you notice any air or spurts, or does it flow smoothly?

Plastic pipe expands and contracts and if it passes through a tight opening can generate some popping or snapping noises as it expands then contracts back to its quiescent state.

There is no release of air from the faucets, shower/tub, or toilet tanks. While I bled water from the cold water lines, including from the outside faucets, I did not drain water from the gas water heater. Since I have water hammer only on the cold water side, I assumed there was no problem with the hot water side. I understand the suggestion about loose fitting tub/shower faucets, but they do not appear to be loose. However, if you hold your hand on the faucet assembly, you can feel the vibration from the water hammer, even if it was the sink faucet that was turned on and off, not the tub/shower assembly. The sounds point to vibration within the tub/shower faucet assembly itself, which is strange bathroom sink faucet can "cause" the hammer effect, and the pipes are not really that close in the bathroom. I estimate that the two are separated by a distance of 12 feet and several right angle junctions.

try turning off the water to the toilets one at a time and see what happens.

sounds like the mixing valve.

Well, the water hammer problem has been solved. It was caused by a defective temperature control valve in the tub/shower of the bathroom at the end of the water line. Replacing the valve also appears to have lessened the influx of cold water into the hot water stream so that it now is a barely recognizable nuisance. I'm quiting while I'm ahead. Thanks to all for the suggestions.