I Want a Hot Shower
December 3, 2012 12:26 PM   Subscribe

When my furnace is running, I don't get hot water in my shower. What's going on?

This only happens during the winter when the furnace is running. It seems to be a crapshoot whether or not I'll have a cold to luke warm shower or warm shower all winter long; it's never hot. Other areas of the house also have trouble getting hot water during the winter, but it's most irritating in the shower.

The hot water heater is about 10yrs old and is inside the house in a closet with an outside facing wall. The furnace is about 20yrs old and shares a wall with the hot water heater, but is in a separate closet (the furnace was maintained this year and the plumber said it's running well and I don't need a new furnace). The both run on natural gas.

What could be causing this to happen?
posted by backwords to Home & Garden (25 answers total) 1 user marked this as a favorite
What kind of heating system is it — baseboard hot water? Hot water radiators? Steam radiators?
posted by beagle at 12:34 PM on December 3, 2012

Do you have a boiler? That is, does the furnace heat the water and dump it into the hot water tank for storage? Do you have forced hot water for heat?
posted by schoolgirl report at 12:34 PM on December 3, 2012

Call a plumber, that ain't right.

It could be that they're sharing a natural gas line, but that would be totally UNCOOL (and dangerous and a whole bunch of other unsavory stuff.)

If this is going to be a BFD thing (and even if they have to split the connection it shouldn't be) look into replacing your Hot Water Heater with a Tankless Hot Water Heater. I have one and between the gas company, the Feds and local rebates, I didn't pay anything for it.

Can I tell you how awesome unlimited hot water is? It's VERY awesome.
posted by Ruthless Bunny at 12:35 PM on December 3, 2012 [1 favorite]

Response by poster: The heating system is forced air. I don't have a boiler, just a furnace for the forced air and the hot water tank for hot water.
posted by backwords at 12:43 PM on December 3, 2012

Do you have a well with a pump, or city water?
posted by beagle at 12:55 PM on December 3, 2012

Can we assume that the times of no (or limited) hot water do not also coincide with the use of a dishwasher or clothes washer? Even if it's been a couple hours since you used either one, an older hot water heater (or even one with its thermostat set too low) can be slower to recover.

Do the hot water lines run through an attic or crawl space where they could be subject to especially cold temps?
posted by John Borrowman at 12:55 PM on December 3, 2012 [1 favorite]

And, what's the size of the tank on the hot water heater? Even if, somehow, gas pressure is dropping when the furnace is running so that the hot water heater can't do its job, there should be enough hot water in the tank to supply a shower. Unless it's very small, or tankless, or there are multiple showers being taken.
posted by beagle at 1:00 PM on December 3, 2012 [1 favorite]

I lived in an apartment once with what the landlord and plumber called an "Apollo heater" where there was no true separate furnace; Instead the 'heater / heat exchanger -- furnace-like-thing' used water from the hot water heater to heat the house. This meant that hot showers were limited when the 'heater' was heating the house.

Googling 'Apollo heater' shows no obvious similar designs. Are you certain that your furnace has a burner in it, or could it be the case that your furnace is just a heat-exchanger and a fan and the heat for your house comes from your hot water heater?

(The apartment in question was in North Carolina, built in the 1950s, not sure of the vintage of the heating system)
posted by u2604ab at 1:04 PM on December 3, 2012

Response by poster: I have city water.

The times when there it little/no hot water don't coincide with clothes washing or dish washer running (even within a few hours). I'm the only person in the house, so no one is hogging all the hot water before I can get to it (unless my cats are doing something I don't know about).

The house is built on a cement pad, I think the lines run through the pad, there is not crawl space (and I'm sure they aren't running the the attic crawl space). Could the ground be cold enough to suck all of the hot water out of the lines running from the water heater to the sinks and shower? The water doesn't travel very far, I think the distance from the hot water heater to the shower is less than 20ft.

Not sure the size of the hot water tank, whatever size is normal for a small house. Although, I'm sure there's mineral build up in there, hot water isn't an issue in the fall/spring/summer when the furnace isn't on. The furnace has a burner/pilot light.
posted by backwords at 1:12 PM on December 3, 2012

Could the ground be cold enough to suck all of the hot water out of the lines running from the water heater to the sinks and shower?

Sure, but then you'd have no hot water most of the winter, not just when the furnace is running.

Or are we misunderstanding your question, and you in fact have no decent hot water all winter long, ie., during furnace-running season? I think we understood it to mean that when the furnace is actually running, you've got no hot water, but at other times you do.
posted by beagle at 1:30 PM on December 3, 2012

Can you clarify if "no hot water when the furnace is running" means "during the actual times the furnace is actively heating air", or "during the part of the year which requires that the furnace be on"? Does this happen as soon as the furnace is on in the fall, or does the effect take a couple weeks of cold weather to happen?

I used to design plumbing, and in one case our plumber did not separate the hot and cold water when running pipes underground. Not a problem when the building was turned over in May, but it became a problem in November. The pipes being indirect contact, underground, meant that the heat in the hot water was lost to both the cold ground, and to the (much colder in winter) adjacent cold water pipe.

I would turn up the temp setting on the water heater and see what happens. Another thing that may or may not help, depending on your shower valve, is to set the flowrate of your shower lower, so that you're using less heat per unit time.
posted by notsnot at 1:36 PM on December 3, 2012

Is the furnace gas or electric? Hot water tank?
posted by WinnipegDragon at 1:36 PM on December 3, 2012

Response by poster: Only when the furnace is running (i.e., not just on, but blowing out hot air or getting ready to) is the hot water affected. During the winter, this active period is fairly frequent.

When the furnace is not running during the winter, the water is slightly cooler than during other seasons, but not as much as when the furnace is running. Turning up the temperature on the hot water heater makes the water hotter during this time, but not when the furnace is running.

Thanks for all the input so far.
posted by backwords at 1:48 PM on December 3, 2012

Man, this is tough one. Could you find out if there's a pressure drop in the water heater gas line when the furnace kicks on? If the heater is getting choked out, that could cause some of the problem...but then why would it not supply hot water if it's already got a full tank of hot? Is there any possiblilty that the water pipes run *in* a duct? If so, your 120F hot water might be getting "chilled" down to hot-air temperature (90F) via the pipes.
posted by notsnot at 2:00 PM on December 3, 2012

Only when the furnace is running (i.e., not just on, but blowing out hot air or getting ready to)

Considering that you also are saying,

During the winter, this active period is fairly frequent.


It seems to be a crapshoot whether or not I'll have a cold to luke warm shower or warm shower all winter long; it's never hot.

I think you're probably wrong about the furnace and water temperature being directly related. Instead, I think the pipes are being chilled somewhere between the water heater and shower.

If you turn on a faucet (any faucet, doesn't matter which or where in the house), to draw hot water, leave it running and go grab the hot side pipe coming out of the water heater, is the pipe hot? If it is, then heat is being lost along the way to the shower. If it's not hot at the heater, then the temperature is set too low or the heater is broken.
posted by jon1270 at 2:01 PM on December 3, 2012

Is this a gas furnace and a gas water heater? Are both electric?
posted by Thorzdad at 2:08 PM on December 3, 2012

Response by poster: Both are gas.
posted by backwords at 2:34 PM on December 3, 2012

Tangential: Do you have a carbon monoxide detector? If this problem involves bad venting and/or incomplete combustion, you could be swimming in carbon monoxide.
posted by qxntpqbbbqxl at 2:41 PM on December 3, 2012

Ok, I may have an idea for what is causing this. Your hot water delivery lines run through the wall at least somewhat in not totally. These are the 'out' lines from the water heater to the sink/showerhead/dishwasher/clothes washer/whatever. At some point they are running near or through an area that is drawing in outside cold air for the furnace and being cooled off.

This can be a very hard one to chase down since it may be inside your walls and not accessible. Start with where the furnace is drawing air from and see if it isn't just sucking air from a wall cavity somewhere. The try to chase where the out lines from the hot water heater are going and see if they aren't somewhat close to each other. The good news on this one is that you can fix it with some insulation once you find the location of the problem (assuming I am right).
posted by bartonlong at 3:26 PM on December 3, 2012

OH, another idea, is your shower/bathtub faucet on an outside facing wall? if so this may need some insulation and/or your anti-scald valve in the plubming for your shower could be bad and when it gets a little cold it doesn't mix enough hot water in. This valve is why when you are showering and someone flushes the toilet you don't get burned by the sudden decrease in cold water pressure to the mixing valve. This is easy to test, turn on the shower and let it run till it gets as warm as it can, then flush the toilet and see if the shower water tempature changes briefly. If so than start with that for fixing your problem. NOTE: both things can be going on simultaneously.
posted by bartonlong at 3:31 PM on December 3, 2012

Is the water heater a "tankless" type? If so, then the gas line might be undersized. In other words, when the furnace is running, there's not enough gas to let the hot water heater turn on.

This could be an issue with a traditional tank heater as well, but you should get at least some hot water out of it.
posted by Kadin2048 at 8:28 PM on December 3, 2012

Kadin2048 said exactly what I was about to write. The most obvious possibility is that your gas pipes are undersized to meet the demand. Do you definitely have any tank? The tankless kind of hot water heaters require much more heat and don't have a tank of water to supply any hot water during the times they can't get gas.
posted by slidell at 9:26 PM on December 3, 2012

Response by poster: It's a 67gal natural gas water heater. I tried some of the ideas people had yesterday. When I touched the hot water line in the evening it was warm to the touch and after running hot water for awhile it was hot. This morning, the line is cool to the touch. I'm thinking that is probably part of my problem.
posted by backwords at 5:55 AM on December 4, 2012

Do you mean the line was cool after running the hot water in the morning? If so, you may have an issue with the upper thermostat or element in the hot water tank.
posted by WinnipegDragon at 8:23 AM on December 4, 2012

Gas HWHs don't have an upper element (just one burner at the bottom, heating the whole tank), and at least mine only has a single thermostat. But gas HWHs do let the temperature of the water fluctuate rather widely. When the burner is on and heating, the water will get quite hot. It will then let the water cool down significantly before re-lighting the burner. The steady state is a sinusoidal pattern, where the temperature oscillates around some middle value.

Over a period of a few days, I'd pull some water directly out of the heater (via the drain valve or the closest faucet to the heater) first thing in the morning and measure the temperature. If it's unacceptably cold (not warm enough to shower in) then I'd raise the setpoint on the HWH.

This will consume more energy though, so you might want to consider an extra insulating blanket to keep heat in.

That's really about the only thing I can think of to do at this point. (I was pretty pleased with my tankless HWH theory... I hate it when reality gets in the way of a good guess like that.) There still might be an issue with the gas line sizing, but it would be much more subtle on a tanked HWH than on a tankless, where it really could lead to an immediate cold shower when the furnace clicked on.
posted by Kadin2048 at 11:13 PM on December 4, 2012

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