Shower mixer gone bad?

[rattler]

I am re-piping my house, replacing galvanized pipe with PEX tubing. I reused a shower mixer that is just on the other side of the wall from the hot water heater. The mixer worked perfectly before, but now it acts strangely. When I turn on the hot side, it becomes hot fairly quickly. But then...when I turn on the cold, it becomes cold fairly quickly. I have to keep turning the hot up in order to balance the water to warm/hot. The really weird part is, there comes a point at which turning the hot side up does no good, i.e., doesn't make it warmer/hotter. At that point, I have to turn down the cold in order to achieve a warm/hot balance.

Once the balance is reached, the hot shower is really nice. But when I turn off the shower, the cold side requires less than a full turn (if that much) whereas the hot side requires many turns to turn off.

The lavatory mixer, which is less than 2 and half feet down the tubing, works perfectly. Also, the HW heater is set not quite to the hottest setting. I had it set to halfway between hot and warm, but had to turn it up because I could never achieve a comfortably warm/hot balance.

The mixer was working great when I took it out. What happened? Initially, I wanted to replace the mixer because the house is older than I am. But the guy who's working with me says they never go bad.

Should I replace the mixer or is there something I'm missing with respect to achieving a nice balance?

TIA,

r.

[Phelps]

You have very common problem when dealing with an old house with galvanized piping when you have 'disturbed' the plumbing.

By simply shutting off the main valve and back draining water is all it takes. Then when you turn the water back on, the rust scale lining the inside of the pipe breaks loose and travels up the pipe and lodges in elbows, couplings and other places where rust usually develops at the inside of those threads, or at a valve.

Try doing this test: Hook up a garden hose to your water heater drain valve and open it up into a pail and see how fast it comes out. Then dump the pail and repeat the step, only this time, open up the cold water faucet of the shower while you do this test.

Then publish your findings here. This will help hone in on where the restriction is.

[Phelps]

Scratch my post above. I can see what I said does not apply after rereading your post.

Do you have the problem now AFTER your new tubing is ALL in place? Did you remember to use trunk lines with branch lines? For example, most houses are built with 3/4 inch trunk lines, and 1/2 inch branch lines that head towards each fixture. For tubs, they remain 1/2 inch. And for sinks, the 1/2 inch pipe is reduced to 3/8 inch tubing. Did you plumb in said mannor?

If you didn't you could be getting too much cold out of the cold faucet and robbing the hot water.

[rattler]

I can guess what you mean by trunk and branch lines but I'm not sure what you're getting at here.

The entire house isn't finished yet, but I'll try to explain what I've done so far:

1. I have 3/4" PEX tubing coming into the house.

2. it tees at the water heater with a 3/4" branch supplying the water heater and a 3/4" branch going on to supply the cold for the rest of the house.

3. the cold 3/4" branch going on to the rest of the house is almost immediately reduced to 1/2" and sent down a common wall between two bathrooms.

4. This single 1/2" cold tubing is then teed in several places in the wall to supply cold water to the fixtures within the two bathrooms.

5. All sink and toilet branches in these two bathrooms end with a copper elbow stubout that is connected with a PEX crimp ring.

6. in the kids' bathroom, the sink and toilet stubouts are not yet fitted with angle stop valves and the PEX tubing going to the shower currently ends in a temporary plug.

7. in the master bath, The elbow stubouts are fitted with angle stop valves that have 1/2" on the stubout side of the valve and 3/8" on the fixture side. NOTE: the angle stop valves are connected to the stubout via compression fittings.

8. the shower in the master bath simply has a 1/2" branch going directly to the cold side of the shower mixer.

9. the hot water supply follows the same procedure as above except they obviously don't supply the toilets.

10. no lines are being run to the kitchen yet.

So, to summarize, 3/4" tubing is reduced (via a PEX brass tee) to 1/2" which then goes down the inside of a common wall to supply the needs of two bathrooms that share the wall. The total length of this tubing doesn't exceed 15' (probably less).

It is the shower in the master bath that acts goofy now. It simply seems to be feeding too much cold water, i.e., the hot side has to be turned way up whereas the cold isn't turned up much at all to produce a nice hot shower.

And like I said earlier in the thread, the master sink mixer doesn't seem to have this problem.

Doesn't make sense to me.

I just re-read your last post. Are you saying that I should have a 3/4" line going down the inside of this common wall?

Sorry for such a long post. I sometimes do that as I'm sorting out a problem; and I don't have a cool program for drawing out my situation.

r.

[LazyPup]

What is the diameter of the house "Main water supply" line?

[rattler]

A few years ago, the city moved the water meter. I have no idea how large the line is from the meter to a shutoff valve they installed in my yard (underground). But from there into the house, it's 3/4". The supply goes under my house to the backyard then comes out of the ground (years ago it went through a water softener at that point). Then the line goes back under the house to provide the supply to the inside.

My pipes burst in the line that went back under the house (concrete slab). So I cut the line just before it went back under the house and connected 3/4" PEX tubing to the side that came from the meter. The other end of the PEX tubing goes to the roof and down inside a vent. This is a temporary situation and all outside PEX tubing is currently inside rubber insulation. When I finish the job inside the house, then I'll dig a trench from the shutoff valve the city installed to the house and enter through the (not so big) crawl space above the house (i.e., not underneath the house).

[LazyPup]

We just discovered the cause of your problem. Your new PEX lines are too big.

While we commonly think of the water distribution system as just being one system, in fact, once the main water line reaches the water heater the distribution system divides into two separate but equal system, the Hot and the Cold.

The line from the meter or main water shutoff valve should remain equal to the diameter of the supply line entering the house until it reaches the water heater. Once the line reaches the water heater and the system divides into the hot and cold distribution systems the hot and cold trunk lines should be sized so that each will carry 1/2 the volume of the main supply line. This insures a balanced volume and pressure on both the hot and cold sides.

If the hot and cold trunk lines are continued past the hot/cold junction either side is now capable of carrying the full volume of the supply line at the expense of the opposite side, which explains why your hot and cold pressure is erratic at the shower mixer.

When comparing the volume of a line we cannot use diameter, but rather we must use a formula that compares the cross sectional area of the two pipes in question.

The formula for comparing volume is Large Diameter Squared divided by small diameter squared.

As a basic rule of thumb, increasing a line by one nominal trade size will roughly double its volume. Doubling the diameter will increase the volume by a factor of four.

You stated that your main supply line is 3/4" therefore the proper size of the hot and cold trunk lines shoud be 1/2".

3/4 squared divided by 1/2 squared equals:
(.75 x .75) / ( .5 x .5) =
.5625 / .25 = 2.25

This means a 3/4" line is capable of carrying 2.25 times the volume of a 1/2" line while maintaining equal pressure.

Not only will increasing the line size downstream of the water heater cause erratic pressure, it also causes the velocity of flow to drop, which means it would take longer for the hot water to get from the water heater to the point of demand.

[Phelps]

I read the posts above and it be neat to see if this is all rectified in the end. Keep us posted.

[rattler]

I was sick for more than 2 weeks so I'm only now replying.

Sounds like the inverse square law to me and I understand. However, I don't think it applies in this case. The 3/4" tubing coming out of the hw heater is only about 4' before it's reduced to 1/2" diameter which then runs the length of the house. The same goes for the cold water side. That is, from the tee that comes from the meter, the 3/4" line has about 18" of tubing going into the hw heater and about 4' coming out (the other side of the tee) before being reduced to 1/2" to run through out the house.

OTOH, either the problem fixed itself or we've just learned how to live with it because we don't seem to be having the problem anymore. It might be related to shower head we put in. The water coming out doesn't seem to be as strong as it was with the old shower head. So maybe it just took a little time for my wife and I to get used to it.

At any rate, thanks to all for replying.