Help me get INTO hot water.
July 23, 2015 6:39 AM   Subscribe

Does running the hot water on full help the hot water reach the faucet more quickly than running a hot/cold water mix?

When I am getting ready to shower I always run the hot water at full blast until it warms up. I assume that this allows the hot water to reach me more quickly than running a combination of hot and cold water. My reasoning is the presence of cold water means there is slightly less hot water running through the pipes, i.e. the hot and cold water are essentially competing for space to come out of the tap and so less cold water = less pressure pushing back on the hot water. But I realized I have no idea if this is true - could it be that running the hot and cold water at the same time results in hot water getting to the tap just as fast?

So plumbing experts of Metafilter: does turning the tap all the way to hot cause the hot water to reach me more quickly? Or would it reach me as quickly if I had both hot and cold running at the same time? Or maybe there's some nuance, e.g. "as long as the handle is at least halfway towards the hot side you're getting hot water at the maximum rate."

I’m mostly used to showers with a single handles (rather than separate controls for hot and cold), so if the answer depends on the type of handle I would be interested to learn that too.

This question is purely out of interest, I’m not making decisions based on this. So alternatives like “get a small tankless heater for your shower” would miss the point. I’m asking about the theoretical behavior of water flowing through two pipes and meeting in the faucet, not about how to hack my shower routine to get hot water faster.
posted by Tehhund to Science & Nature (6 answers total) 2 users marked this as a favorite
 
Best answer: You have some volume of water in your hot water line between your tap and your hot water heater that needs to come out before you get to your hot water.

Running the cold water does nothing to pull out that water.

The pressure in your hot water heater is driven by the overall pressure to the house, so running the cold water simultaneously may actually (by a small bit) lower the pressure into the hot water heater and make the hot water side run less quickly.

And to a lesser extent you've got a limited amount of flow that can come out of the faucet, so running both simultaneously limits the water from the hot water side that can flow there as well.

tl;dr: Don't run the cold water.

You might look into an on-demand recirculator. We've got one, installation was easy (under the bathroom sink), remote control button at the head of our bed means we press a button, wait just a little bit, walk into the bathroom and the water's hot.
posted by straw at 6:45 AM on July 23, 2015 [3 favorites]


With a single-valve faucet, turning it full hot gets hot water out of the faucet faster. With a two-valve faucet, turn the hot valve to max and the position of the cold valve won't appreciably affect the time it takes to get hot water unless you have a severely flow-restricted faucet.
posted by Johnny Wallflower at 6:47 AM on July 23, 2015 [1 favorite]


I agree with straw. The mixing of hot and cold happens at your shower valve. You've got a cold water line running from its source, where wanter enters you house, and a hot water line running from your water heater. The two meet at your shower valve(s) — whether it's a single control or two separate handles makes no difference. So, assuming enough time has passed since the shower was last used, the water in both lines is cold (ie., the temperature of whatever spaces the lines run through). Let's look at two options:

(a) If you open the single control to 100% (or open just the hot valve on a dual control 100%), let's say it takes 30 seconds to clear all the cold water from the hot line, and have the hottest possible water coming out of the shower. You're not ready to adjust the water temp to whatever you like via your shower control.

(b) Alternatively, let's say your preferred temperature requires a 50-50 mix of hot and cold, and you open your shower controls that way (ie., you set the single control to "warm", or you open the dual controls equally). The showerhead itself will constrain the flow to the same gallons per minute that it does with 100% hot water; that is you will not get twice as much water flowing as in (a). This means that under this option, the hot water will flow at half the rate it does under (a), because the showerhead now has to emit 50% cold, 50% hot. At half the flow rate, it will take 60 seconds rather than the 30 seconds of option (a).

With a two-valve faucet, turn the hot valve to max and the position of the cold valve won't appreciably affect the time it takes to get hot water unless you have a severely flow-restricted faucet.

If you're running water out of the tub faucet and then flipping the switch to the showerhead, this is true. But if you are running water directly to the showerhead during the warmup period, chances are your showerhead restricts flow significantly, relative to the faucet flow. (Bathtub faucet flow rates are something like 4 gallons per minute; showerhead flow rates are 2-2.5 gallons per minute.) But in any case, if you do the warmup via the faucet rather than via the showerhead, you should still run hot water only, because you would just be wasting the cold water during that time.
posted by beagle at 7:45 AM on July 23, 2015 [2 favorites]


Your assumption is correct, the presence of the cold water will cause significant back pressure on the hot water line. This will reduce the flow though the hot water line.

If you have both the hot and cold water valve fully open then you will have the same total flow rate through the valve that you would have with just one valve fully open (unless the valve is really oversized, which it isn't in this situation). Flow rate through a valve (or actually anything) is really a function of the pressure across the valve and something called 'valve coefficient' (Cv). Since the supplied pressure is the same from either the hot or cold water lines and your mixing valves Cv is constant then you will have the same flow rate. The pressures from the hot and cold lines don't add together since they come form the same place.

Play around with the Flow Coefficient equation to convince yourself of this. Remember that both Cv and Pressure (ΔP) are constants in your case.
posted by Confess, Fletch at 9:52 AM on July 23, 2015 [1 favorite]


does turning the tap all the way to hot cause the hot water to reach me more quickly? Or would it reach me as quickly if I had both hot and cold running at the same time?

This seems like a question that you could answer for your particular plumbing configuration with a stopwatch.
posted by yohko at 12:27 PM on July 23, 2015 [1 favorite]


I can't understand how doing anything but running only the hot water would be the fastest. I have a long run from my hws to the shower, and I habitually run two hot taps on full, turning off the basin when it starts to warm.
posted by GeeEmm at 3:42 PM on July 23, 2015


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