I want to get in hot water!

November 19, 2007 2:13 PM

NoobHomeOwnerFilter: Where do I start troubleshooting water temperature issues?

Mrs. pdb and I bought a house in August. The two-story house was built in 2002, and has one utility closet on the main level that has a water heater and the heating unit in it. There is no wall or barrier in this closet separating the water heater and the heating unit; both units were inspected before we bought and found to be in perfect working order.

It's been getting a bit chilly lately, so we've occasionally had the heat on. We have noticed that, when the heat is on, our water doesn't get nearly as warm as it does when the heat is not on - my wife ran a bath yesterday in the upstairs bathroom that struggled to get to tepid by the time the tub was full.

The water heater doesn't have gradations for heat setting, just a dial that says "cooler" on one end and "warmer" on the other. I would estimate, on a scale of "1-cooler" and "10-warmer", the water heater is currently set to about a 3. This setting is perfectly adequate to heat the water - except, for some reason, when the heat is running.

Are these two things related somehow, or am I missing or not thinking of something else to check? The water heater is not wrapped or otherwise insulated, which I know it probably should be - would this help? What else should I be looking into?

posted by pdb to home & garden (21 answers total)

Just to confirm: Do you mean that the water has been less hot since the heat first started running, or that it has repeatedly happened that one day the furnace is running and the water is less hot and the next day the furnace is not running and then it's hot again ?
posted by winston at 2:26 PM on November 19, 2007

What powers your furnace and water heater? Gas, electric, oil?
posted by exogenous at 2:28 PM on November 19, 2007

Winston: The water temperature drops when the heat runs. When the heat is not running, the water is acceptably hot; when the heat is running, it is unacceptably not. We have a timed thermostat that, for instance, has the heat coming on between 4-6pm; during those hours the water is noticeably cooler than during other hours.

Exogeneous: both water heater and heat are gas. Heat is forced air, if that makes a difference.
posted by pdb at 2:36 PM on November 19, 2007

Are you sure that the device you describe as a water heater is, in fact, a water heater and not just a hot water storage tank? A storage tank is usually driven from a hot water coil in the heating furnace, which comes on under thermostat control from the storage tank. In such systems, if the furnace burner is turned on to supply heat for the house, a solenoid control blocks water circulation to the hot water storage tank, to avoid overheating of water in the storage tank, and to provide maximum heat from the furnace to the house. You may need to set up the temperature on the hot water storage tank a bit in winter to compensate, particularly if your furnace is on the small side for your house, and needs to run for long periods in cold weather to keep your house warm.

As for adding extra insulation around the water tank, if it is, in fact, a hot water storage tank, it's not particularly useful since your tank is in heated space anyway, except, perhaps, if you air-condition that space in summer, and want to save a bit of energy in summer.
posted by paulsc at 2:39 PM on November 19, 2007

My first thought is that the furnace is messing with the water heater's thermostat (i.e. heating the utility closet so much that the water heater shuts off prematurely). Perhaps there is a leak in the furnace's outlet that is heating the closet.

But that makes zero sense, considering how these appliances are usually well insulated, and they are both relatively new (i.e. built in 2002).

So, I'd lay money on the water heater just being faulty in general. A repairman should be able to replace the water heater thermostat relatively cheaply.
posted by Cool Papa Bell at 2:44 PM on November 19, 2007

On review, paulsc seems to be heading down the right path.
posted by Cool Papa Bell at 2:45 PM on November 19, 2007

Are you sure that the device you describe as a water heater is, in fact, a water heater and not just a hot water storage tank?

According to my inspection, which is all I have to go by, it is in fact a water heater - American is the brand name, not sure of the model number.
posted by pdb at 2:47 PM on November 19, 2007

Is it a tankless water heater? If so then the gas line might be undersized with the furnace using up most of the available gas and not leaving enough for the heater to function properly. This shouldn't affect a regular water heater as they heat and store a quantity of hot water. Whether the furnace was on or not would not affect the temperature of the water in the tank.
posted by caddis at 3:11 PM on November 19, 2007

Have you played with the thermostat settings on the water heater? On your 1-10 scale, 3 seems mighty low, to me. Push it up to 5 and see what you get. Also, is it a one or two element water heater? That is, is are there cover plates for element access on the top as well as the bottom? If it's a two-element heater, be sure to adjust both thermostats, with the top thermostat set slightly lower than the bottom thermostat.

Otherwise, I have no clue beyond, possibly, the furnace and the water heater are on the same circuit, and when the furnace runs, it's drawing power away from the water heater. But that sounds all kinds of wrong.

The two should be on separate circuits, though.
posted by Thorzdad at 3:13 PM on November 19, 2007

Your water heater should keep the water hot for hours, even with the burner off. The temperature won't change much over the 2 hours the furnace is running.

Water heaters don't keep the hot water at a constant temperature. There will be a difference in temperature between the time right after the burner switches off, to the time when the water has cooled enough so that the burner switches on again. You might just be pulling water from the WH when it's on the colder side of the cycle. It's possible that your hot water usage patterns have changed a bit with winter, so that this happens to occur.

The first thing I would try in your situation is turning up the WH.
posted by yohko at 3:15 PM on November 19, 2007

Caddis: It's not a tankless water heater.

Thorzdad: it's a one-element heater, as far as I know (not at home right now so I can't look, but I'm pretty sure there's only one cover plate). All summer, and even now when the heat isn't on, having the water heater set where it was (at that 3 setting, we didn't touch it when we moved in) provided ample hot water, but I guess I could try bumping it up a little and seeing what that does.

Interesting theory about the circuit, I'll have to check that when I get home. With this house, I'm learning that just because it sounds all kinds of wrong, doesn't mean it's not done that way - like the living room smoke alarm that we found to be hardwired into a kitchen electrical outlet, for some reason. So it's not out of the realm of possibility that the water heater/heater are on the same circuit.
posted by pdb at 3:20 PM on November 19, 2007

I suspect Caddis is right and they are both gas appliances sharing a fuel line that cannot supply them both with the gas they need at the same time.

Try this:

Open the space where the water heater's pilot light is. You should be able to see the burner burning. This is what heats the water. While it's burning, have Mrs. pdb turn the thermostat up so that the furnace kicks on. Does the burner still burn as bright? If not, you've found the problem.

Becareful not to asplode yourself.
posted by jeffamaphone at 3:28 PM on November 19, 2007

As summer comes to a close your city water temperature usually starts to drop so that you need more hot water to make things warm enough. If you have a two handle tap the hot water should still be just as hot. However, if you have mixing taps you will need to add in more hot water to achieve the same temperature. A setting of 3 seems low to me also. To save energy you want it as low as possible while still having a sufficient stock of hot water to bathe etc. That can mean turning up the heat a bit in winter.
posted by caddis at 3:29 PM on November 19, 2007

I'm reaching a little here, but is there a combustion air vent to the outdoors arranged such that cold outside air could be being drawn over the water pipes when the furnace runs, cooling the hot water after it's left the tank?
posted by jon1270 at 3:37 PM on November 19, 2007

First you need to determine for sure if your water heater (and furnace) are gas or electric because there are a different set of possibilities for each. You can tell for sure because a gas water heater will have a 3-inch diameter flue pipe on top that probably joins with the flue pipe for your furnace. When the hot water faucet is turned on full in the bathtub, you should see the water heater fire up within about 30 seconds to 1 minute. You can more easily see this if the utility room is dark. If you don't see flames when the water heater and furnace turn on then you have electric heaters. It would help if you can read the model number off the name plate on the front of the water heater.

Is it possible that you just have a long run of cold pipes to the bathtub and these pipes are colder when its cold outside and therefore the furnace is on?
posted by JackFlash at 3:40 PM on November 19, 2007

JackFlash: They're both gas. The pipe run, I think, goes up the outside wall for about 10 ft and then under the floor beneath the upstairs until it gets to the bathroom. I don't know enough to know whether this is a "long" run, or not, but there could be something to that theory.

Jon1270: I have no idea. I'll check when I get home.
posted by pdb at 3:46 PM on November 19, 2007

My hot water heater will let the water get epid if there's no hot water in use for a while. If the temp is below the 1/2 way point, there's not usually enough for a hot bath. I'd bump up the hot water temp a bit. You'll use less of the hotter water, so the increased usage isn't too bad.
posted by theora55 at 4:10 PM on November 19, 2007

It's possible that you have a faulty thermostat. Test the temperature setting of the heater when the hot water hasn't been used for at least an hour or two, like first thing in the morning. Run the water at the nearest tap for about 3 minutes into a water glass so that the glass is pre-heating and doesn't give a false reading. Use a cooking thermometer to measure the temperature. It should be 120 to 130 degrees. If too low, turn up the thermostat on the heater a little bit. Test again an hour later. If you can't get the temperature up to at least 120 degrees, you may have a faulty thermostat. See if you get any different readings when the furnace is on.
posted by JackFlash at 4:45 PM on November 19, 2007

Again, depending on the kind of water heater you have, it could be something as simple as a cracked or broken dip tube, which is letting cold water mix immediately with heated water in the top of the tank. Or, if you have a 40 gallon or greater electric heater, it could be that your top element is gone, as most water heaters of greater than 30 gallon capacity have multiple elements, for faster recovery. If you have a gas water heater, it could be that your tank hasn't been drained and flushed in a while, and built up scale over the burner area is forcing your burner to short cycle.

It would help us help you if you could post nameplate data from your water heater, so that we could talk about specifics.
posted by paulsc at 7:43 PM on November 19, 2007

Not sure how much/which nameplate data would help, but here's a bunch:

MFR: American Water Heater Co
MDL: G61-50T40-3N
PROD NO: 0720671
CAP: 50 gal
INPUT: 40,000 BTU/HR
SUPPLY PRESSURE: 4.5 WC (I have no idea what a WC is)
MANIFOLD PRESSURE: 3.5 WC
MAX SUPPLY PRESSURE: 10.5 WC

Is that enough data? Thank you all for your help, I'm really trying to learn a lot about a totally unknown topic here...
posted by pdb at 8:58 PM on November 19, 2007

Your installation and operation manual is here. It shows the thermostat adjustment on page 17. The 130 degree mark on the dial is a tick mark that is fatter and longer than the others. Turn the dial so that it is pointing straight up and in line with the index mark. (The 120 degree mark is slightly smaller and to the right.) Use the 130 degree setting and then test the water temperature as I described above. If the temperature of the water is in the range 120 to 130 degrees at the tap, then your thermostat is probably okay. Check to see if the flame comes on within about 30 seconds to 1 minute of when you turn the faucet on.

Fifty gallons is a pretty big heater and it also has a pretty big burner for a fast recovery time so you shouldn't be running short of hot water for one bath.

One possibility is a long, cold pipe run that I described above, but even that shouldn't make you run short of hot water with a 50 gallon tank.

Another possibility is that your gas pressure is too low or the pipe is undersized. The 4.5 WC means you need at least 4.5 inches of water column, which is a way of measuring the gas pressure. It is the gas pressure that would support a column of water 4.5 inches high in a vertical tube. 4.5 WC is equal to about 3 ounces of pressure per square inch, not very much. It is possible that when your furnace is on, there is not enough gas pressure left over for your water heater so the flame is too small. You might be able to call the gas company and ask them to do a gas pressure test of the water heater when the furnace is running.
posted by JackFlash at 11:51 PM on November 19, 2007