It all boils down to how the piping is laid out in the house and the type of shower valve.

You are suffering a pressure drop when the toilet or washer create an instant huge demand for water when the valves are opened.

GOOD NEWS - all modern shower/tub valves are constructed so that this cannot happen any more. Replace your shower valve and you won't have this happen to you again.

The two homes you describe have different shower fixtures.

The first that is more user friendly has a newer style fixture that reduces the pressure of the supply lines so the fixture gets the same amount of water. When someone flushes the toilet the hot water supply is restricted to the same flow rate as the cold. And vice versa when some one turns on the hot water in the kitchen.

The second house has the old style fixture. These fixtures can't adjust the pressure or flow and you will get extremes. This is much funner fixture for naughty little boys who like to play jokes on their sisters. Not that I have much experience here running hot water taps when my sister takes a shower .... OK I do....

But that was a long time ago.

Here is a link that might be useful: Pressure balanced shower valve

This thread is running currently and will give you some brands. It's a very nice change to the old plumbing!

Here is a link that might be useful: Thermostatic Shower Valve Recommendations

Code now requires anti-scald protection on all shower valves.

Look for either a pressure-balanced valve which will maintain the temp roughly +/- 3 degree, or a thermobalanced valve, which has a tighter tolerance, roughly +/- 1 degree.

Most will also have a high temperature restrictor that can be manually adjusted. In some valves it's simply a set screw. Turn the shower to the max temp you'd ever want, then use a screw driver to turn the set screw until it turns no more. It becomes a mechanical stop of sorts, limiting how far the knob can be turned. It doesn't sense temperature at all, it's simply a mechanical limiting device.

In your shopping you might also see valves that are listed as having "stops". "Stops" are water shutoff valves located on the valve itself. Shower valves don't normally have a water supply turnoff, like the on/off valve behind your toilet, or the shutoff valves under a sink.

Stops allow you to use a screwdriver to "stop" the flow of water into the valve. It allows you to make repairs to the valve without having to shut off water to the entire bathroom. Or worse, the entire house. They are normally required in apartment buildings, etc.

Good point Mongo. Many fixtures do not come with built in integral stops. Your plumber or you can purchase them. I prefer to use the threaded ones if your fixture allows for it. If you have to solder on Brass integral stops make sure the plumber removes the cartridge and any O rings from inside the mixing valve before installing the stops. They can deform under the heat of the torch and cause leaking in behind the wall. It is a good idea to have the plumber install the trim at the same time if possible to check the set backs.

Make sure you provide solid blocking or at least some material for your man to install it nicely. Decide on your tile layout so you can center this fixture on a full or half tile.

Every fixture we have installed also recommends removing the cartridge and flushing the valve. Make sure this is done. If not some debris from the waterlines can ruin this cartridge. I recommend to all my clients that they purchase a second cartridge to have as back up. If not this at least record the replacement part on file so you don't pay a plumber to drive about town finding a new part.

Most companies will air freight a replacement if it is not readily available in your town for free - make sure you are not charged for this replacement cartridge if in fact is under warranty.

Well, when I look at the thermostatic things, I see the faucets (?) with just one handle, the swivel type of thing, and I never liked those. But, anyway, our shower has the two handle faucets, and it comes thru the tile. I don't think we want to modify the tile, as it's fine as it is, just needs some scrubbing and such, and that would be an extra expense as well. The bathroom itself is very tiny and badly designed, so I am already looking at knocking out the closet and moving the sink to a different location. I do plan to get a corner tub or something in the future, as the tub itself is really badly worn/messed up.

But, like I said, this temperature problem is a big pet peeve and I'd like to fix it asap if I could.

My husband thinks that the problem can be solved by some sort of valve adjustment or something on the kitchen sink, but that probably won't solve the issue of not being able to wash clothes at the same time someone is showering (not to mention a dishwasher which is in our future).

Anyway, thanks for the advice so far. A lot of it is over my head, but I'm having my husband read it too. At least it doesn't sound like a problem of having to re-do the plumbing in the whole house or something like that.

Here's a pic of it:

Here is a link that might be useful: faucets

They make these thermostatic device thingies in all shapes and sizes. You can get one that is not in the wall, but in a metal body outside the wall (like a wall mounted faucet). It will use the same water supply holes in your tiles, so no damage and repair is needed.

Oh, thanks, David, I'll keep looking and see if I can find one that would fit the water supply holes! If you know of any good links for me, could you please post them?

I think that in order to have a thermo or pressure balance valve it is a single handle, but I don't know this for a fact.

It's not uncommon to upgrade plumbing in this way, and they have made trim to cover the extra holes. I do not believe you have a household plumbing problem, just a change in pressure as different water demands are put on the supply. Just a little physics going on.

I know you mentioned you like the 3 handle thing, but in case you want to look into it further...

Here is a link that might be useful: How to replace an old 3 handle shower valve with a single handle

trilliant it sounds like each of your taps is a hot or cold valve, and only that.
This means without any ability to regulate its flow.
To verify this, turn the hot valve on to get a partial flow (not full blast), and see if its flow diminishes when you turn on another large flow somewhere else, and comes back to normal when that other flow is turned off.

For your web search, here are key words.

Here is a link that might be useful: wall-mount bath shower mixer (keywords)

"The two homes you describe have different shower fixtures. "

ONCE AGAIN mr. WHIPPLE IS NOT ENTIRELY CORRECT !!!

John you have no idea what the posters homes had in terms of shower valves.

Once again, you are assuming and making a broad generalization which is not universally true. You know the problem with assumptions, right???

It is possible to plumb a house without pressure balanced, thermostatic (old style non code compliant) valves and not get scalded when a toilet is flushed or the washer or dishwasher or other high demand units open up.

It is also very trick to design and not easily scalable, hence the movement towards a more FOOLproof system.

It should also be noted that the integral stops or add on ones at the valve usually reduce the amount of water that can pass through the valve. Not a problem for most people , but some may want higher flow rates for multiple features or quicker fill times.

I definitely don't want a solution that would decrease the water pressure in the shower. It's barely enough to rinse well as it is...

At this point, I'm pretty lost (totally a layman here, I just don't have time/brains to figure out all the terms and specs and whatever), so I guess my best bet is to wait for several months until the plumber comes to install a dishwasher, at which point he can hopefully tell me what to do. I guess my "solution" for now is to TRY to remember to put cups or something on the kitchen sink to visually remind whoever may decide to use the kitchen when I'm showering, so I don't get scalded (sometimes twice!)

le sigh....

Thanks anyway, y'all, and sorry I'm not able to understand this stuff; it's obviously not very simple. :) I'll come back in a few months and post what worked, in case anyone else finds herself in my position.

P.S. Just FYI, I've been reading this forum for a month or two now, looking for solutions/ideas for this horrid bathroom, and the whipple war is very distracting and unpleasant.

trilliant - those stops don't reduce the pressure , they can restrict the flow. I.E. a valve that flows 9 gallons a minute without the service stops might only flow 7.5 with them.

Replacing your existing valve with an anti-scald one may or may not reduce the flow. It all depends on the on the flow rates of the valve you have and one you are considering replacing it with. You may also have a pressure problem with your system, clogged or restricted or improperly sized supply lines, or just a poorly designed system.

Only an onsite inspection by a qualified AND knowledgeable plumber will get you headed in the right direction. There are simply too many things to speculate in a forum like this.

Thanks, antss, that's what I was thinking.

Here's another question: how can you find a very good plumber? We have one that we use for our rental properties who gets the job done, but it's mostly minor repairs and things like that. I don't know how good he'll be with doing things like moving a sink from one place to another, or finding out if we have a poorly designed pipe system, things like that. I think we may have found him on craigslist or something.

Trillant,

Reduced flow could be from poor plumbing layout...or from restrictions in the valve...or even mineral build-ups in the shower head.

First thing I'd check is for any valves in your plumbing supply lines (in a mechanical room, a basement, or crawlspace) that might not be fully open.

If this is a tub/shower, I'd recommend turning on the water full volume with the water coming out of the tub faucet. time how long it takes to fill a 5-gallon drywall bucket. Convert that to GPM.

Then I'd unscrew the shower head from the shower arm and do the same to see what flow you're getting out of the shower arm.

Give the shower head a good look-see. Is there any debris inside the head? Any mineral deposits clogging the debris screen or holes in the head?

Sometimes the pressure in the house can be low. Depends if you're on city or well water. But usually there is a pressure regulator of some sort where the water line enters the house.

The thermobalanced valves David linked to perform well. Several hotels I've stayed in use those exclusively. It took me a little while to get used to them, but they perform fine.

It is true that there are a large number of possible situations and that going through them is too much work for the internet helpers here.

But...

The "verify" test I described above is just one of many that you can do too.
Do some tap-turning and spot what influences what.

Report back and share.

By a gradual process of elimination, you will reach a better understanding of what IS working right, what is configured right, and what might need doing.

If you are unable to spot correlations all by yourself ((and you have unlimited time and lots of self-interest)), then a repair plumber will certainly be unable to help you!!! A Master Plumber would then be more likely what you need, to figure out what you will need to have done. Trilliant before you go hiring plumbers, you may be able to have one or two over for a look see at no charge. Some of them like to operate this way. What they tell you on the spot is free of charge.

A Master Plumber (and maybe a very good and experienced repair plumber) will know how to spot what happens when he turns taps on, so that he will know swiftly whether the problem is here there or even outdoors. ((Not everything can be spotted by turning on taps, btw)). A repair plumber shouldn't be expected to understand everything in a DWV system, or else he/she would be a Master Plumber.

The diagnosis will be part of the solution; one site visit and fix might only be a couple hours work. : - ) But do your own homework first. At least it will help you get a handle on what is not required and what the possibilities are. It will help you direct the Master Plumber. They have been known to make mistakes too. I have paid thousands for work not required, all because Master Plumbers thought it WAS, in their instant diagnosis kind of way. It works both ways. Sometimes they save you tons of grief, and sometimes they cause you grief.

Go turn on taps.

Okay, Mongo, I will have the husband check that when he gets home. The pressure in the whole house is low, kitchen sink, washer, etc. It is city water. I wonder if that could actually cause the hot/cold situation, is that possible?

Before we bought the house, we noticed that the drainage from the sinks/tub wasn't very good, so the original owner had that "taken care of" by a plumber (the bill looked like he probably just snaked it), and we've had to snake the tub since. I don't know if that has anything to do with the pressure or the hot/cold thing, but I figured I'd mention it. :)

It would be nice if the husband could increase the pressure household-wide, though, so that will be good to try. Thanks!

Okay, David, I will do that when he gets home. It's just me and mom (she is disabled) right now, so I can't really tell if the flow reduces in the bathroom by turning on something in the kitchen, because they are too far away. I will report back, promise! :)

David, the flow definitely diminishes in the tub when the water in the sink gets turned on (and vice versa).

The kitchen sink is 2 gallons per minute. Tub is almost 3 gallons per minute. Shower w/o showerhead is faster, a little over 4 gallons per minute. This is all cold water.

The hot water in the tub diminishes when the cold water in the sink is turned on. But the cold water in the tub does not diminish at any point, or not enough that I can tell.

So, husband went under the house, to see if there were other things that he didn't see out front. Out front, it seems that the valve or whatever that goes from the city water to the house doesn't work. However, it Ts off to either side, on which there are valves that work. Husband found this out when he was trying to turn the water off for the house when he was trying to trouble-shoot kitchen sink issues early on. So that is how it is laid out: The one coming to the house is broken or something, but it Ts off from there and there are valves on each side which are both cold at that time.

After that, one side hits the hot water heater, and the other side supplies the cold water.

Does this make sense? :)

Not really - are you saying that you have two supply lines coming into your house?

DO you have a pressure reducing valve just inside the wall where the line(s) enter the house. It'll usually be a brass fitting with a cone shaped top that has a screw on the top to reduce / increase the pressure.

It's one supply line, but it branches out into a T, and one goes to the hot water heater, and the other goes to the other cold water throughout the house. There's not a pressure valve like you described, that we can find. The one that leads from the city water is just a cut-off valve (that doesn't work).

"trilliant it sounds like each of your taps is a hot or cold valve, and only that.
This means without any ability to regulate its flow.
To verify this, turn the hot valve on to get a partial flow (not full blast), and see if its flow diminishes when you turn on another large flow somewhere else, and comes back to normal when that other flow is turned off."

Did I verify this, and if I did, what does that mean? :)

I think what you have is a very basic plumbing layout. As the water comes into the house, it's sent in two directions, one hot and one cold. Simple. Then you have different flow rates at different fixtures, for instance, the kitchen sink has a slower flow rate, which is typical (often around 2.5 gpm) and a shower can be typically around 4gpm.

There seems to be enough pressure coming from the city water to keep your cold water pressure constant. But it seems that there is some pressure drop across the hot supply- which I don't think is that unusual. That is your rate limiting step- your hot water supply.

So each valve simply translates this balance, or unbalance, each time you use more than one fixture.

Have you ever been lightheaded when you stand up too quickly? Well, it's kind of the same; your body has to close off all the open "faucets" to your lower extremities in order to supply more pressure and blood to your head when you change position.

This is very much how newer balance valves correct this change in supply pressure to your fixture. When there is a quick change in pressure because you have opened up another outflow of water elsewhere, the valve is meant to keep the balance of the hot and cold water the same so you don't get scaled or frozen. I don't know exactly how the valve does that.... but it does work well.

That is my understanding of your situation. You may want to post over in plumbing, they are very helpful over there, in order to verify all this. I have gotten some really great advice from that gang with some non-novice stuff.

My non-professional DIY understanding, hopefully put into understandable terms. Welcome to stand corrected :-)

Ditto.
It means the most recommended solution will be to install a balancing thingie.
It will either have pressure-balance or thermostatic in the name.
There is a difference, but it's not a big deal to you right now.
Ask a plumber to sell you something that he will install too.

I searched using a few word combinations like
-- exposed tub-shower thermostat
-- thermostatic shower wall
-- wall-mount shower mixer

f.y.i.
To look at an image, search for
"Ecostat Exposed Tub-Shower Thermostat"
(It's made by hansgrohe, and it popped up in my image search).

All of the above assumes you will install something that sticks out from the wall, in order to save money.

f.y.i.
Here is another device, which goes inside a wall:
KWC # Z53292093122
KWC Sentro Remote Pressure Balance Valve

f.y.i.
using the keyword "commercial" along with your other keywords you will see the specs and prices of heavy duty thingies designed to do the same function.

Make no buying decisions or irreversible action based on this post.