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Would 3/4" pex from a tankless water heater 25' up in the attic get hot water to the kitchen sink on the 1st floor faster or slower than 1/2" pex?

isherwood
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James
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    If you add insulation to the pipe it won't take so long if you draw water twice within, say, half an hour. – Andrew Morton Dec 05 '23 at 17:02
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    @Ecnerwal, it's not likely the OP has a choice where the water heater needs to be located WRT the faucets. My own tankless is in my garage and I have no attic to put it in. My kitchen sink is probably also about 25' away, due to how the house was built in the 90's. Fortunately, that's the furthest faucet in my house, but "why would you..." is easily answerable with "because I didn't have a choice" or "because I put it where the old water heater was". Unfortunately, your question is fairly condescending. – computercarguy Dec 07 '23 at 21:18
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    There's a question of English language here: "Faster" can equally mean both "sooner" and "at a higher rate" (and slower can mean the opposite in both cases). It seems reasonable to assume that 2sooner" is the desired interpretation, but really it should be clarified – Chris H Dec 08 '23 at 10:24

3 Answers3

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Slower - 3/4" pipe has more internal volume, (approximately double) so when the water in the pipe has cooled down and you call for hot water, you have to flush out about twice as much water before the warm water gets to you.

  • 1/2" PEX ID 0.485"
  • 3/4" PEX ID 0.681"

Volume/100 ft

  • 1/2" 0.92 gallon
  • 3/4" 1.83 gallon

Specs source page no affiliation

Volume/25ft

  • 1/2" 0.23 gallon
  • 3/4" 0.46 gallon

With 2.2 GPM faucet,

  • 1/2" 6.3 seconds
  • 3/4" 12.5 seconds
Ecnerwal
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    The OP my be confusing the time it takes hot water to get to the delivery point with the flow rate (gal/min) that can be delivered at the use point. A 3/4" line would deliver a higher flow rate than a 1/2" line IF the water heater can supply that flow rate and if a restrictor at the faucet isn't limiting. Usually a 1/2" line is sufficent for a single use point. And the time delay is half of that for a 3/4" line. – Jim Stewart Dec 05 '23 at 16:17
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    The restrictor on the endpoint is almost always the limiting factor, and the question was about time, not flow rate. But the flow rate will not likely change a whit, as the restrictor on the endpoint (or even just the orifice on the endpoint valve if the restrictor is removed) will rule. – Ecnerwal Dec 05 '23 at 16:36
  • However the hot water line to the kitchen would normally supply both the kitchen sink and a dishwasher. Plumbing with PEX might sometimes be done with a 3/4" line from the water heater that divides into two 1/2" sections at the end. But I would think a standard setup would be a 1/2" line all the way from the water heater to the kitchen. If there is a recirculation system the time delay is abolished. – Jim Stewart Dec 05 '23 at 16:37
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    Some kitchen designs that are at commercial/industrial level with a large sink might have special faucets that don't have the normal flowrate restrictor. In that case a 3/4" PEX line might give improved performance, but I don't think that is the case for this OP. – Jim Stewart Dec 05 '23 at 17:07
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    And since the question is about time to hot water, they will still get hot water twice as fast with 1/2" PEX. – Ecnerwal Dec 05 '23 at 17:22
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    @Ecnerwal Not super relevant, but the flow through the pipe wouldn't just be limited by the greatest resistance in the line. Every restriction in the pipe adds up. – JMac Dec 06 '23 at 15:30
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    And just like electrical resistors in series, the most resistive (or most restrictive) rules the sum of them. But feel free to write another answer rather than bringing up vanishingly small factors in comments that are in no way relevant to the question, the pair of you. – Ecnerwal Dec 06 '23 at 17:01
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    Not only that, but after the flow has stopped, twice the amount of heat will be wasted when the pipe cools down.That might not matter in winter, as the heat lost will still be within the building, but in summer it's undesirable. – Weather Vane Dec 06 '23 at 17:25
  • @Ecnerwal Do you have any numbers on the relative resistance of a faucet compared to lengths of pipe? I couldn't find them, but I really doubt that a single faucet is orders of magnitude more restrictive than typical lengths of pipe leading to it. I'm not sure why you would expect me to write an answer when it's a specific thing you said that could be misleading, not the whole answer. I dont see what is wrong with trying to give correct information when possible. Quite frankly, I dont get the tone of your response. I was not hostile. – JMac Dec 06 '23 at 20:29
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    @JMac it is approx. an order of magnitude. 23 GPM @ 3/4" vs a 2.2 GPM faucet. https://resources.hy-techroof.com/blog/how-much-water-can-flow-through-a-pipe – evildemonic Dec 06 '23 at 22:29
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    @JMac It's you not understanding how plumbing actually works. And asserting things that not true as a result. Let's say 50 PSI supply pressure. 25 feet of pipe. 2.2GPM faucet. 1/2" pipe, 49.425 PSI at the input, 0 at the output. 3/4" pipe, 49.855PSI at the input, 0 at the output. No Practical Difference https://www.engineeringtoolbox.com/pe-pipe-pressure-loss-d_619.html – Ecnerwal Dec 06 '23 at 23:07
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    For this example, the faucet is TWO orders of magnitude more restrictive than the pipe supplying it, as the 1/2" pipe drops just over half a PSI to supply the faucet, while the faucet drops 49+ PSI to supply the sink. – Ecnerwal Dec 06 '23 at 23:23
  • @Ecnerwal The faucet itself doesnt drop all the remaining pressure... There is some pressure drop through the pipe, and then the orifice, and then the pressure remaining is the pressure that it has when it comes out of the faucet which gives it velocity. – JMac Dec 07 '23 at 13:00
  • @evildemonic Going from 3/4" to 1/2" also drops it from 23 GPM to 14 GPM according to that chart. So it's closer to cutting it by 40% instead of 90%, but not orders of magnitude different (also, the chart doesnt mention the length of pipe, which will change the impact of pipe size but not the faucet). – JMac Dec 07 '23 at 13:13
  • If the pipe isn't insulated (or insulated well enough) and this is in a cold climate, getting hot water to the faucet may take longer in 1/2" just because the weather is cooling the water faster than in a 3/4" pipe. Just as it takes longer to heat water in a larger volume, it takes longer to cool it, too. And a 3/4" pipe will take longer to freeze in winter than a 1/2" pipe because of the increased volume. – computercarguy Dec 07 '23 at 21:48
  • I have always thought it would be interesting to put 1/8" piping and jack the pressure way up so you get the same flow - hot water would be very fast to get to the faucet! – Michael Dec 08 '23 at 10:40
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Let's talk about extremes to discuss the meanings.

Let the thick tube be 1" and the thin 1/4".

If you consider "slowness" as ammount of cold water needed to be dumped before the hot appears, then thicker tube is slower. The thick one is having 4 times bigger diameter and 16 times bigger cross-section. In other words, every meter (inch, feet,...) of thick pipe holds 16 times more water tahn the thin one.

If you consider "fastness" as ammount of water that flows through the pipe per second (minute, hour,...) - flow rate - then the situation gets a bit tricky. The flow rate is driven by pressure differences and obstructed by pressure losses. The thicker the pipe is the lower pressure loss in the pipe. On the other hand, if you have 1/128" faucet, no matter how big the pipe is, the flowrate will be limitted. Also straight pipe will have higher flowrate than crazed labyrinth of pipes.
Rule of thumb here is "from thicker to thinner". Here the conduit on the heater sets the upper limit for your gauge.

The other point, not asked by OP but by Ecnerwal in comments is the ammount of wasted water by such a design.

What is actually wanted is shortest reasonable piping from the heater to the faucet. Short pipe reduces both waiting time for the hot water and water waste while waiting for it.

Crowley
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To answer the unasked, but very related question, of how to get hot water faster to your faucet(s), you could consider a recirculating pump. These are usually for very large houses and increase cost for heating water, but if you absolutely demand nearly instant hot water at the faucet, a recirculating pump might be your ticket. You'll also likely need to add plumbing in your house to make it work, so it's definitely not cheap and likely costs more than just the pump and labor to install the pump.

Take this example, for instance:
https://fasterhotwater.com/Tankless%20circulation%20pump.shtml

Or you could consider under sink tankless heaters. These can work to heat water until water from your main water heater arrives. These can still increase the cost of heating water, but don't require the kind of replumbing the recirculating pump does. And most of these are DIY ready, so you only need to buy the equipment, if you are handy with plumbing and electrical.

The trade off with this solution is that you do this for a single faucet at a time, instead of the whole house benefitting from it. But maybe only your kitchen needs it, so why spend more if you don't have to?

For example:
https://www.homedepot.com/b/Plumbing-Water-Heaters-Under-Sink-Water-Heaters-Under-Sink-Tankless-Water-Heaters/N-5yc1vZckrd

(Links are only for example. I'm not promoting brands or stores.)

And let's face it, the real/root question is how to get hot water to your faucets faster than it's currently happening. The asked question is an implementation question that can may not actually have an answer that addresses your issue.

computercarguy
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  • This doesn't actually answer the question verbatim. It tries to help a situation which OP probably can only alleviate with pipe diameters. – Tim Dec 08 '23 at 14:36
  • @Tim, there's nothing in the rules that state answers have to address the question verbatim. In fact, many of the best answers on all Stacks don't address the question verbatim. BTW, the answer by Crowley doesn't address the question verbatim. And I showed that the OPs issue could be solved by other means than pipe diameter, so no, your assertion there is also false. – computercarguy Dec 08 '23 at 17:16