Adding hot water gravity flow to my kitchen sink.

[Qmavam]

I'd like a hot water gravity flow to my kitchen sink. The hot water heater outlet is a little over 5ft above the water heater drain, that I would use as the cold water return. Is 5 ft enough for a hot water gravity flow system to work. The sink is physically only 7 ft from the water heater, but I have to draw 10 cups of water (22 seconds) before the temperature is reasonably warm. (that seems long, physically it looks like it should be just a straight line down the wall into the concrete floor and up at the sink) The volume of the 13ft of 3/4" pipe calculated to 4 cups of water, so I don't understand the 10 cups. I was thinking I would add both a hot water supply pipe and a cold water return pipe. But then wondered, can I get by with just the cold water return? Water heater is in the garage, which is connected to the kitchen.
Also what diameter pipe should I use for supply and return? Or just the return?
Thanks, Mike

 

[jkl 2455]

I'd like a hot water gravity flow to my kitchen sink. The hot water heater outlet is a little over 5ft above the water heater drain, that I would use as the cold water return. Is 5 ft enough for a hot water gravity flow system to work. The sink is physically only 7 ft from the water heater, but I have to draw 10 cups of water (22 seconds) before the temperature is reasonably warm. (that seems long, physically it looks like it should be just a straight line down the wall into the concrete floor and up at the sink) The volume of the 13ft of 3/4" pipe calculated to 4 cups of water, so I don't understand the 10 cups. I was thinking I would add both a hot water supply pipe and a cold water return pipe. But then wondered, can I get by with just the cold water return? Water heater is in the garage, which is connected to the kitchen.
Also what diameter pipe should I use for supply and return? Or just the return?
Thanks, Mike

I'd like to understand your question, but I can't make heads or tails about it...

 

[Qmavam]

Well, hmm. Hot water gravity flow--cold water falls in a pipe pushing the warmer water up. You can use this effect to make water circulate. I want hot water to flow to the faucet at my kitchen sink. So my question, I have 5ft drop from my highest hot water pipe to the lowest cooler water pipe which is the drain faucet on my water heater. I would remove the drain faucet, add an extension and a T for the cold water to flow back into.
Does that help?

P.S.
I previously had a system like that in a home and it worked great, but it had more like a 10ft fall for the cold water.

 

[jkl 2455]

Well, hmm. Hot water gravity flow--cold water falls in a pipe pushing the warmer water up. You can use this effect to make water circulate. I want hot water to flow to the faucet at my kitchen sink. So my question, I have 5ft drop from my highest hot water pipe to the lowest cooler water pipe which is the drain faucet on my water heater. I would remove the drain faucet, add an extension and a T for the cold water to flow back into.
Does that help?

P.S.
I previously had a system like that in a home and it worked great, but it had more like a 10ft fall for the cold water.

I get it now, you are using head pressure for water flow.
No doubt you've done calculations on the subject, undoubtedly the more head pressure the better.
Are you limited to just 5' drop?

 

[Qmavam]

Yes, basically the fall is the highest part of the hot water pipe out of the heater to the lowest part where the water heater drain faucet is.
I'm not sure I have the knowledge to do much in gravity flow calculations.
I don't know if head pressure is the correct term, the system pressure is about 50psi, but that is not the pressure pushing the hot water up.

 

[MicEd69]

Head pressure isn't involved here. You are basically just using the hot water line as an extension of the water heater volume. The only motive force in these lines would be from the differential temperature of the water in the line, as you indicated, but I don't really see how this would actually effectively work. The pressure in the water heater and the lines connected to it is not changing at all when everything is stagnant. So, when the water in the piping cools, there could be a slight flow of cold/hot water to move, but I don't see how there would ever be an appreciable flow of water around the water heater kitchen faucet loop.

If you connect two lines to your water heater, one at the hot water outlet and one at the water heater drain, I don't see how you would be doing nothing more than drawing water through both lines when you turn on the hot water faucet.

 

[Qmavam]

Good point, about drawing water from both lines! A one way valve is in order, but I doubt gravity flow is enough to make a one way valve function. Looks like I'm stuck using a pump. I see two types, one has a switch that turns on the circulation pump until faucet temperature reaches the temp wanted. then you turn on the water. (I don't like that idea) The other runs the pump every 15 minutes for long enough time to get the faucet sensor up to temp and then the pump shuts off.
Thanks, Mikek

 

[MicEd69]

Good point, about drawing water from both lines! A one way valve is in order, but I doubt gravity flow is enough to make a one way valve function. Looks like I'm stuck using a pump. I see two types, one has a switch that turns on the circulation pump until faucet temperature reaches the temp wanted. then you turn on the water. (I don't like that idea) The other runs the pump every 15 minutes for long enough time to get the faucet sensor up to temp and then the pump shuts off.
Thanks, Mikek

I like to keep things as simple as possible and reduce the number of times my pump runs. My system is simply based on a start button and a timer. I start my pump a couple of minutes before I need hot water and it turns off after 90 seconds. My hot water loop is insulated, of course, and running the pump for 90 seconds is plenty of time to get hot water to all my fixtures, except my washing machine which I don't care about having hot water there instantly anyway.

 

[wpns]

I’m still confused. Is the system under pressure? Are you trying to get convection to circulate the water for you? Pictures or drawings might help

 

[Qmavam]

Yes, it's under pressure, standard city water, Florida house built on a slab. Yes, I want convection to circulate the water.
I basically want to circulate water out the top of water heater down to the bottom of the heater by convection, however it will take a 7 ft detour out to the kitchen sink and back.
Thanks, Mikek

 

[wpns]

Yes, it's under pressure, standard city water, Florida house built on a slab. Yes, I want convection to circulate the water.
I basically want to circulate water out the top of water heater down to the bottom of the heater by convection, however it will take a 7 ft detour out to the kitchen sink and back.
Thanks, Mikek

Through a virtual trap under the slab? That's not going to work. Many water heaters have anti-siphon fittings, traps, etc to prevent convection from wasting your hot water. Also, as MicEd pointed out, unless you have a one-way valve, you'll be drawing hot and cold water when you turn the faucet on. And _if_ you have a one-way valve, it's cracking pressure is probably greater than your convection will generate.

5/8 of a gallon of water isn't much waste, how long does it take for the water to get properly hot? Not only is the hot water mixing with the cold water, but it's heating up the pipes, so I'd expect it to take more water than just the volume of the pipes.

I run the shower while I'm brushing my teeth, which 'wastes' probably 4 gallons of water, and eliminates all the problems with recirculation systems, much less the hassles of installing them post-construction...

 

[Twowaxhack]

I'd like a hot water gravity flow to my kitchen sink. The hot water heater outlet is a little over 5ft above the water heater drain, that I would use as the cold water return. Is 5 ft enough for a hot water gravity flow system to work. The sink is physically only 7 ft from the water heater, but I have to draw 10 cups of water (22 seconds) before the temperature is reasonably warm. (that seems long, physically it looks like it should be just a straight line down the wall into the concrete floor and up at the sink) The volume of the 13ft of 3/4" pipe calculated to 4 cups of water, so I don't understand the 10 cups. I was thinking I would add both a hot water supply pipe and a cold water return pipe. But then wondered, can I get by with just the cold water return? Water heater is in the garage, which is connected to the kitchen.
Also what diameter pipe should I use for supply and return? Or just the return?
Thanks, Mike

A gravity recirc system isn’t going to work for you if your water heater is on the same level as your kitchen sink. You need a basement or equivalent, water heater lower than the kitchen.

The easiest way for you to get hot water quicker is to increase the flow volume from your kitchen faucet. Maybe change the aerator or remove some flow restrictors.

Purge volume is always more than pipe capacity. The water mixes in the pipe, it’s not a slug of hot water replacing cold

I warn people about over sizing hot water lines for this reason. Long purge times.