Home Water Line Question
April 15, 2022 6:04 AM Subscribe
The hot water water in our home is a single line that dead ends at a bathroom. This furthest bathroom is located only a few feet from the hot water tank in the cellar. Is there any reason why this bathroom hot water line could not be connected directly to the water heater, turning the hot water line into a loop? This would allow hot water to travel in the shortest line to its end use. The cold water lines would not be changed.
If you do 'close the loop' and connect to the hot water heater from the dead-end, you will get the behavior you expect -- when you turn on the hot water in that room, it will mix both water from the long loop, as well as drawing from the hot water heater directly. This will also have the added benefit that if, say, you're running hot water for laundry at the other end of the house, this bathroom will not lose pressure because it'll draw more directly from the hot water heater. This is often used in manifold construction -- inlets on both ends -- to keep pressure even to all branches on the manifold.
The benefit of the recirculating pump is that 'mixing' of long-pipe cooler water to direct-from-heater won't be as cold -- the recirculating pump will move water around to ensure there's always hot water in the long pipe. Note that this can be added without tying directly back into the hot water heater, and probably with minimal tools -- you go under the sink and bridge the hot and cold water connections where the sink hooks up -- the lukewarm water goes into the cold pipes, drawing hotter water from the heater through the entire system (note this doesn't waste water because back-flow into the cold ends up back in the hot water heater anyway, you don't pull fresh water from the outside but you do use more energy running the pump and re-heating the backflow lukewarm water).
posted by AzraelBrown at 7:22 AM on April 15, 2022 [1 favorite]
The benefit of the recirculating pump is that 'mixing' of long-pipe cooler water to direct-from-heater won't be as cold -- the recirculating pump will move water around to ensure there's always hot water in the long pipe. Note that this can be added without tying directly back into the hot water heater, and probably with minimal tools -- you go under the sink and bridge the hot and cold water connections where the sink hooks up -- the lukewarm water goes into the cold pipes, drawing hotter water from the heater through the entire system (note this doesn't waste water because back-flow into the cold ends up back in the hot water heater anyway, you don't pull fresh water from the outside but you do use more energy running the pump and re-heating the backflow lukewarm water).
posted by AzraelBrown at 7:22 AM on April 15, 2022 [1 favorite]
Or you can just run a short direct branch off the line that emerges from the water heater, to that bathroom. No reason the water needs to take a long detour, if I'm understanding you right. The loop idea can be done with or without a recirculating pump. If you put in a pump, be sure that the entire loop is well insulated, or it will increasing your water heating expense a lot. Plus the cost of running the pump motor all the time. Maybe put it on a timer so it only runs when you're likeliest to be using hot water.
posted by beagle at 7:23 AM on April 15, 2022 [1 favorite]
posted by beagle at 7:23 AM on April 15, 2022 [1 favorite]
Is there any reason why this bathroom hot water line could not be connected directly to the water heater, turning the hot water line into a loop?
You could do that.
This would allow hot water to travel in the shortest line to its end use.
Yes it would.
So would capping off the bathroom's existing hot water feed pipe, then using the same new pipe that would otherwise complete the loop to make a direct, dedicated feed from the heater to that bathroom. With a simple dedicated feed you get all of the hot water taking the shortest path to that bathroom, instead of having some of it drift around the long way and mixing with the cold standing water in a loop pipe.
The other way to avoid that long-way cold water mixing is to make a loop and keep it permanently hot with a circulating pump, but that's going to use a fair bit more energy. The pump itself won't use much, but insulation around hot water pipes isn't usually as effective as insulation around a hot water storage tank, so if you're continuously circulating hot water through a loop instead of letting the water in the pipes just sit there when no hot water's being drawn off, you're going to be losing heat from the loop 24x7 and the reheating required to cover that will cost extra.
By contrast, without the circulating pump you only lose heat from the pipework when you're actively drawing hot water from storage, and also from water that stays in the pipes when you stop. If the end bathroom had its own direct feed, there would be quite a lot less pipe sitting between it and the heater to hold cooling water and correspondingly less heat lost.
Water that's allowed to cool in the pipework does typically get run to waste, so a loop with a circulating pump to keep it permanently hot would indeed save a bit of water, but I can't see how it would save "over 10,000 gallons a year (the manufacturer claims up to 17,000)" as claimed by the site linked above.
10,000 US gallons is 38,000 litres. The cross sectional area of the water in a typical 12mm domestic water pipe is 6mm × 6mm × pi = 113mm2. Let's be super generous with pipe and put the heater 50m from the bathroom, which is just bonkers amounts of pipe reflecting horrible home design. That puts 50m × 113mm2 = 5.7 litres of water in the line. If all of that got run down the drain every time you used hot water, you'd need to do that 38,000l ÷ 5.7l = 6700 times per year = 18 times per day to waste 10,000 gallons. If the outlet was only 5m of pipe away from the heater you'd need to do it 180 times per day.
A short run of new pipe that gives the current endpoint bathroom its own dedicated feed, with no loop and no circulating pump, would therefore save water and energy and the purchase, installation and ongoing maintenance costs of a circulating pump.
posted by flabdablet at 8:12 AM on April 15, 2022 [4 favorites]
You could do that.
This would allow hot water to travel in the shortest line to its end use.
Yes it would.
So would capping off the bathroom's existing hot water feed pipe, then using the same new pipe that would otherwise complete the loop to make a direct, dedicated feed from the heater to that bathroom. With a simple dedicated feed you get all of the hot water taking the shortest path to that bathroom, instead of having some of it drift around the long way and mixing with the cold standing water in a loop pipe.
The other way to avoid that long-way cold water mixing is to make a loop and keep it permanently hot with a circulating pump, but that's going to use a fair bit more energy. The pump itself won't use much, but insulation around hot water pipes isn't usually as effective as insulation around a hot water storage tank, so if you're continuously circulating hot water through a loop instead of letting the water in the pipes just sit there when no hot water's being drawn off, you're going to be losing heat from the loop 24x7 and the reheating required to cover that will cost extra.
By contrast, without the circulating pump you only lose heat from the pipework when you're actively drawing hot water from storage, and also from water that stays in the pipes when you stop. If the end bathroom had its own direct feed, there would be quite a lot less pipe sitting between it and the heater to hold cooling water and correspondingly less heat lost.
Water that's allowed to cool in the pipework does typically get run to waste, so a loop with a circulating pump to keep it permanently hot would indeed save a bit of water, but I can't see how it would save "over 10,000 gallons a year (the manufacturer claims up to 17,000)" as claimed by the site linked above.
10,000 US gallons is 38,000 litres. The cross sectional area of the water in a typical 12mm domestic water pipe is 6mm × 6mm × pi = 113mm2. Let's be super generous with pipe and put the heater 50m from the bathroom, which is just bonkers amounts of pipe reflecting horrible home design. That puts 50m × 113mm2 = 5.7 litres of water in the line. If all of that got run down the drain every time you used hot water, you'd need to do that 38,000l ÷ 5.7l = 6700 times per year = 18 times per day to waste 10,000 gallons. If the outlet was only 5m of pipe away from the heater you'd need to do it 180 times per day.
A short run of new pipe that gives the current endpoint bathroom its own dedicated feed, with no loop and no circulating pump, would therefore save water and energy and the purchase, installation and ongoing maintenance costs of a circulating pump.
posted by flabdablet at 8:12 AM on April 15, 2022 [4 favorites]
Note that this can be added without tying directly back into the hot water heater, and probably with minimal tools -- you go under the sink and bridge the hot and cold water connections where the sink hooks up -- the lukewarm water goes into the cold pipes, drawing hotter water from the heater through the entire system (note this doesn't waste water because back-flow into the cold ends up back in the hot water heater anyway, you don't pull fresh water from the outside but you do use more energy running the pump and re-heating the backflow lukewarm water).
Hooking things up that way means that you'd need to run the cold tap to waste for a while before what comes out of it is actually cold, which is pretty exactly much what you used to do with the hot tap before the circulating pump went in so you'd probably not end up saving much water. Also, cold water lines are usually not insulated at all, so making an existing cold water feed line do double duty as a hot water circulation loop return is going to waste gobsmacking amounts of energy.
I have no doubt that installations plumbed this way exist, but if I owned one, I'd fix it.
posted by flabdablet at 8:21 AM on April 15, 2022
Hooking things up that way means that you'd need to run the cold tap to waste for a while before what comes out of it is actually cold, which is pretty exactly much what you used to do with the hot tap before the circulating pump went in so you'd probably not end up saving much water. Also, cold water lines are usually not insulated at all, so making an existing cold water feed line do double duty as a hot water circulation loop return is going to waste gobsmacking amounts of energy.
I have no doubt that installations plumbed this way exist, but if I owned one, I'd fix it.
posted by flabdablet at 8:21 AM on April 15, 2022
Finally, depending on the layout of the existing hot water feed line, then rather than capping off the existing feed at the endpoint bathroom you might be better off doing that somewhere else, and backfeeding more outlets from that bathroom after giving it its own new feed.
The best run of pipe to cap off would be the longest one that currently exists between any two hot water outlets.
posted by flabdablet at 8:28 AM on April 15, 2022 [1 favorite]
The best run of pipe to cap off would be the longest one that currently exists between any two hot water outlets.
posted by flabdablet at 8:28 AM on April 15, 2022 [1 favorite]
This would allow hot water to travel in the shortest line to its end use.
Well, no, it wouldn’t. The water isn’t going to make choices about which route it takes. The water will travel both routes, and while the endpoint temp might begin to warm up sooner, it will actually take longer (possibly much longer) to stabilize at full temperature because the cooled water will have to be cleared from both sides of the loop, and the longer side of it will flush through more slowly than it does now because half or more of the flow is taking the shorter, somewhat easier route.
posted by jon1270 at 9:35 AM on April 15, 2022 [2 favorites]
Well, no, it wouldn’t. The water isn’t going to make choices about which route it takes. The water will travel both routes, and while the endpoint temp might begin to warm up sooner, it will actually take longer (possibly much longer) to stabilize at full temperature because the cooled water will have to be cleared from both sides of the loop, and the longer side of it will flush through more slowly than it does now because half or more of the flow is taking the shorter, somewhat easier route.
posted by jon1270 at 9:35 AM on April 15, 2022 [2 favorites]
I’ll say that whenever I run into a house situation where I can’t figure out why on earth they did something a certain way, it becomes apparent why when I try to “fix” it. Learn from my woes and make sure you understand why it’s set up the way it is before you start mucking with the pipes.
posted by momus_window at 9:36 AM on April 15, 2022 [2 favorites]
posted by momus_window at 9:36 AM on April 15, 2022 [2 favorites]
One reason *not* to connect it in a loop in your situation involves potential "thermosiphoning".
A dead-end pipe above the water heater fills with hot water when you use it, and when you are finished using it, the water in the pipe sits there, slowly cooling, until it is filled with cold water, and it will stay like that without much change until you use the hot water again.
If you provide a short connection between the cellar to the dead end of the pipe above it, you've changed the flow situation. When you are not using hot water, you might find that water is rising from the water heater through the steep, short limb that you've just added, and that cold water from your presumably uninsulated pre-existing water line is sliding out of the way, aided by gravity, and slipping down into your water heater.
There is a counterflow situation sensitive to the pipe diameter around the mouth of the pipe where you tee into old water line, where cooled water flowing down crosses hot water rising: smaller pipes might thermosiphon less.
posted by the Real Dan at 5:17 PM on April 15, 2022 [1 favorite]
A dead-end pipe above the water heater fills with hot water when you use it, and when you are finished using it, the water in the pipe sits there, slowly cooling, until it is filled with cold water, and it will stay like that without much change until you use the hot water again.
If you provide a short connection between the cellar to the dead end of the pipe above it, you've changed the flow situation. When you are not using hot water, you might find that water is rising from the water heater through the steep, short limb that you've just added, and that cold water from your presumably uninsulated pre-existing water line is sliding out of the way, aided by gravity, and slipping down into your water heater.
There is a counterflow situation sensitive to the pipe diameter around the mouth of the pipe where you tee into old water line, where cooled water flowing down crosses hot water rising: smaller pipes might thermosiphon less.
posted by the Real Dan at 5:17 PM on April 15, 2022 [1 favorite]
« Older Am I an asshole if I block my ex? | What are the "rules" on painting over another... Newer »
This thread is closed to new comments.
posted by true at 6:16 AM on April 15, 2022 [2 favorites]