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downinflames68 · Nov 24, 2009, 09:45 PM

I live in an apartment with my own water heater. Whoever showers first has a cold lukewarm shower. Next person to shower in a few minutes has a nice hot shower.

What gives?

nonhuman · Nov 24, 2009, 10:08 PM

Is the water heater in your unit, or is it in the basement while you're a couple floors up or something?

Doofy · 10:44 PM

Is it one of those heat on the fly thingies? They sometimes take a bit of time to get out of bed.

Laminar · Nov 24, 2009, 10:34 PM

When not in use, the pipes between the water heater and the shower cool down to approximately room temperature (or less, depending on location). This means that the pipes act as a thermal mass (the characteristics of which are determined by material type, thickness, total volume, insulation, etc.) absorbing energy from the hot water as it travels through them.

The pipes gradually reach a temperature close enough to the temperature of the water traveling through them that the water loses very little energy on its way to the shower and you have a hot shower again.

This was especially bad in an old house I used to live in when the furnace went out during the winter. The inside would be around 50 degrees so the water on the way to the shower had to heat old pipes from 50 degrees to 110 degrees, and the water pressure was terrible so it took forever. It's no fun taking a cold shower in a cold house.

chabig · Nov 24, 2009, 10:38 PM

Ask your landlord! But it's probably an on-demand heater. Run the water at about half rate until it warms up and then you'll be fine.

reader50 · Nov 24, 2009, 10:46 PM

Originally Posted by downinflames68

I live in an apartment with my own water heater. Whoever showers first has a cold lukewarm shower. Next person to shower in a few minutes has a nice hot shower.

What gives?

Sounds like karmic justice for those of us who are not Type-A personalities. The early bird gets what's coming to him, for getting started way too early.

Oh, and the above technical answers are right too.

Andy8 · Nov 24, 2009, 11:57 PM

Is it an electric heater or natural gas?

smacintush · Nov 25, 2009, 12:39 AM

Maybe you need a gravity loop.

Would make your utility bill go up though.

imitchellg5 · Nov 25, 2009, 12:48 AM

My shower has this issue, but it was with the plumbing in the shower. My dad tried to fix it, but ended up calling the plumber.

downinflames68 · Nov 25, 2009, 02:08 PM

Hrm, thanks for the help so far guys, but no dice. The water heater is one floor below my bathroom, almost DIRECTLY below the shower. It is also a regular water heater with a big tank, not a "heat on the fly" type unit. Also, the basement isn't even cold.

So... it isn't the pipes.

The Final Dakar · Nov 25, 2009, 02:12 PM

Originally Posted by Laminar

When not in use, the pipes between the water heater and the shower cool down to approximately room temperature (or less, depending on location). This means that the pipes act as a thermal mass (the characteristics of which are determined by material type, thickness, total volume, insulation, etc.) absorbing energy from the hot water as it travels through them.

The water and pipes outside the water heater cool when not in use.

Originally Posted by Laminar

The pipes gradually reach a temperature close enough to the temperature of the water traveling through them that the water loses very little energy on its way to the shower and you have a hot shower again.

Hot water warms the pipes.

Originally Posted by Laminar

This was especially bad in an old house I used to live in when the furnace went out during the winter. The inside would be around 50 degrees so the water on the way to the shower had to heat old pipes from 50 degrees to 110 degrees, and the water pressure was terrible so it took forever. It's no fun taking a cold shower in a cold house.

FML

(Why yes, I am bored)

downinflames68 · Nov 25, 2009, 02:14 PM

It isn't the pipes. Water heater almost directly under shower. Not enough pipe to cool down the entire tank of water from the water heater

Laminar · Nov 25, 2009, 02:20 PM

In that case, it's Jesus punishing you for your sins.

Powerbook · Nov 25, 2009, 03:38 PM

Sounds like our type of heater controller (built 1991) :

Basically you have two temperature thresholds, one when in idle (during night, here ~25° celsius) and one when actually in use (or winter mode, here ~50° celsius). As soon as you run hot water in the morning, the heater switches to the upper temperature. This takes a time for all the water, so only the second person gets really hot water.
This could be combined with a timer function to start heating up anyway from e.g. six o' clock in the morning.

Check if you have any temperature gauges you can re-set.

PB.

hayesk · Nov 25, 2009, 05:02 PM

Originally Posted by Doofy

Is it one of those heat on the fly thingies? They sometimes take a bit of time to get out of bed.

If you mean a tankless, that doesn't make sense. The water doesn't stick around in the unit to be heated, so it has to be heated instantly or it won't get heated at all.

If I had to guess, it was either a dual threshold thermostat (i.e. what Powerbook said), or much simpler, you have really long pipes coming from your heater to your shower. This effect would be compounded if you have a low flow shower head.

Doofy · Nov 25, 2009, 05:05 PM

Originally Posted by hayesk

If you mean a tankless, that doesn't make sense. The water doesn't stick around in the unit to be heated, so it has to be heated instantly or it won't get heated at all.

All I know is that when I had one, for about five minutes you'd only get lukewarm water coming through. Maybe the heating element needs to be up to a certain temperature before the water passing over it is suitably heated?

Eug · Nov 25, 2009, 05:19 PM

Yes, I've read many times that so called instantaneous tankless water heaters are not actually instantaneous. Or at least, many of them aren't.

Why? Not sure, but:

1) What you said.
2) Need to expel room temp water in the pipes.
3) Some intentionally pass through cold water to gauge the water temp before kicking in. So, you get room temp. water, then cold water, then hot water. Some have called this a "cold water sandwich".
4) In low flow situations, some tankless heaters won't come at all. ie. If you turn on the hot water tap but leave it only at a very low flow rate, with some tankless heaters you'll never get hot water.

In fact, some advertise water heaters with tanks as true instantaneous hot water heaters, and market it as an advantage over tankless, at least if the tanks are near the spot where the water is to be used.

downinflames68 · Nov 25, 2009, 05:25 PM

Just checked. Only one knob on the thing. Ranges from 1-9, then goes to hot, then A,B,C, then VERY HOT. No dual knobs to control idle temperature. This doesn't make any sense. It's like the whole thing cools down over the night, so that the water inside it isn't very hot, but when it refills it is piping hot.

Again, it is NOT the pipes. They're insulated hot water pipes, and almost directly under the shower.

Eug · Nov 25, 2009, 05:34 PM

BTW, kinda on and off topic.

The house I moved in has three hot water heaters, in series. Two of them are electric, and are off, but the first one in the line is gas and heats the water for the whole house. Weird.

I guessing when they built the house they used one gas one for one part of the house, and one electric one for the other, but then decided the extra electric one wasn't needed so they just linked it up in series. So, why the third one? Hell if I know.

One interesting thing to note is that the electric heaters have a spout at the bottom, so I suppose if the water ever got cut off, they'd still have stored water from two water tanks to last for a couple of days. Otherwise I have no explanation for keeping this strange setup.

Hmmm... I wonder how much I'd get for two electric hot water heater tanks...

Gankdawg · Nov 25, 2009, 05:43 PM

I have no idea but I was happy to read 'Water Heater' and not 'Hot Water Heater' in the title.

reader50 · Nov 25, 2009, 05:57 PM

Originally Posted by Eug

One interesting thing to note is that the electric heaters have a spout at the bottom, so I suppose if the water ever got cut off, they'd still have stored water from two water tanks to last for a couple of days.

Drain plugs are usually for sediment removal, a problem in some areas of the country. It also lets you drain the tank if the area goes below freezing and the house was going to be empty for a time.

smacintush · Nov 26, 2009, 08:18 AM

Sticky thermostat.

When the water heats up to full temp it "sticks" at that high temperature. As the water in the tank cools, the thermostat is "stuck" thinking that the tank is still hot. Cooling to room temp is not enough to "unstick" the thermostat. Then, when you shower the tank drains and the colder-than-room-temperature water comes in and cools the thermostat enough to "unstick" it, heating up the water and starting the whole process over again.

downinflames68 · Nov 26, 2009, 02:20 PM

That makes sense, but how do I fix that? And what caused it?

sek929 · Nov 26, 2009, 02:22 PM

Corrosion most likely.

Messing with a water heater thermostat is not something I'd recommend, call your landlord.

hayesk · Nov 27, 2009, 01:10 PM

Originally Posted by Doofy

All I know is that when I had one, for about five minutes you'd only get lukewarm water coming through. Maybe the heating element needs to be up to a certain temperature before the water passing over it is suitably heated?

Now that I think about it a bit more, if you had an electric one, then yes, that could happen. I was only thinking about natural gas models. Instant heat.