# Max Distance from heat source?

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#### JoeJee

##### HVAC Contractor
When designing a hot water system, specifically one with a hot water loop, what is the max distance for each type of fixture to still be considered instant hot water?

Looking at the EPA water sense guide they tell me no more than 0.5gal of water from source to fixture. They then give us a nifty chart that says how much water is in a foot of x pipe. Great, but that means I can have 50’ of 1/2 pex. I know It takes a long time for hot water to travel 50’ just to wash my hands. Is there a better max length for each type of fixture and pipe size?

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#### breplum

##### Licensed plumbing contractor
You say "hot water loop", which typically means a recirculation loop and 99% of the time, we pump that...ideally with some control involved.
When we add a branch on that loop, the shortest distance possible is the rule of thumb. Two or three feet on a branch is no big deal.

#### JoeJee

##### HVAC Contractor
You say "hot water loop", which typically means a recirculation loop and 99% of the time, we pump that...ideally with some control involved.
When we add a branch on that loop, the shortest distance possible is the rule of thumb. Two or three feet on a branch is no big deal.

Yes, a hot water recirculating loop with a pump. So I can run off the branch for 2-3’ and have instant water. What about 5-10?

Is my math correct?
A 1.2g/min bathroom faucet flows max 2.56 ounces per second. A 1/2 pex line holds 1.18 ounces/foot. It takes 0.461 seconds to clear 1ft of 1/2 pex. At 100% hot we then get 4.61 seconds to clear 10’ of 1/2 pex. We get 6.91 seconds to clear 15ft.

If you have or build hot water recirculating systems, would 5 or 7 seconds be acceptable? Do you prefer 4ft max for less than 2 seconds to clear the line?

#### JoeJee

##### HVAC Contractor
Anyone able to check my math from above or if a customer would complain about a 6-8 second wait for hot water?

#### Reach4

##### Well-Known Member
3/8 pex to the lavatory hot would cut the wait almost in half. That would be plenty big to supply a lavatory.

I see there is 1/4 inch pex available. I did not see pressure drop numbers.

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#### Tuttles Revenge

##### In the Trades
People will complain about 12 seconds if you tell them 10.

The first thing I do when I am selling someone on a hot water recirc system is to set the expectation. I generally start with determinig what their current wait time is. They claim 2 minutes, but in actuality its 45seconds. I mention that for work contracted for State run projects, their goal is no more than 30 seconds wait time to cut down on energy loss. But that our goal is to get water to their shower within 5 seconds (actual is 2 or less) and that typically their sinks would benefit from that loop and be within 10 seconds (actual is more like 5). In this way I'm setting up the scenario where we are meeting or beating their expectation.

That being said.. I haven't had to jump through the new hurdle where we aren't allowed to set recircs on timers or aqua stats.. Only user activation is allowed now. Less standby loss is the new goal. So larger diameter loop in order to install a higher velocity pump.

#### JoeJee

##### HVAC Contractor
3/8 pex to the lavatory hot would cut the wait almost in half. That would be plenty big to supply a lavatory.

I see there is 1/4 inch pex available. I did not see pressure drop numbers.

3/8 would certainly be an option but what might make that difficult is the lack of local supply and the inspectors not liking anything smaller than 1/2 except for dishwasher, ice maker/fridge.

People will complain about 12 seconds if you tell them 10.

The first thing I do when I am selling someone on a hot water recirc system is to set the expectation. I generally start with determinig what their current wait time is. They claim 2 minutes, but in actuality its 45seconds. I mention that for work contracted for State run projects, their goal is no more than 30 seconds wait time to cut down on energy loss. But that our goal is to get water to their shower within 5 seconds (actual is 2 or less) and that typically their sinks would benefit from that loop and be within 10 seconds (actual is more like 5). In this way I'm setting up the scenario where we are meeting or beating their expectation.

That being said.. I haven't had to jump through the new hurdle where we aren't allowed to set recircs on timers or aqua stats.. Only user activation is allowed now. Less standby loss is the new goal. So larger diameter loop in order to install a higher velocity pump.

We are still allowed timer and aqua stats here. You would have to have a big pipe yo make sure the higher velocity doesn’t eat a pipe, especially copper… what max length do you try to stay under for lavs and tubs?

#### Tuttles Revenge

##### In the Trades
We are still allowed timer and aqua stats here. You would have to have a big pipe yo make sure the higher velocity doesn’t eat a pipe, especially copper… what max length do you try to stay under for lavs and tubs?
Honestly I've never gotten into the weeds on specific lengths. But when we're pulling a recirc loop through a bathroom that leaves everything within 10 ft of that loop. Now I'm going to have to learn some math.. or build another spreadsheet.

#### John Gayewski

##### In the Trades
3/8 would certainly be an option but what might make that difficult is the lack of local supply and the inspectors not liking anything smaller than 1/2 except for dishwasher, ice maker/fridge.

We are still allowed timer and aqua stats here. You would have to have a big pipe yo make sure the higher velocity doesn’t eat a pipe, especially copper… what max length do you try to stay under for lavs and tubs?
Gpm your lav is probably .5 gpm. You'll have to figure the lengths and time based on the pump gpm and the fixture gpm. How much 1/2"pipe hold . 5 gallon? And so on and so on. You'll figure the fixture branch from the hot water loop to the fixture has cold water in it and it needs to dump xgal of water to get fresh hot water. At these lengths and these velocities how long will it take? No one usually designs these systems with these exacting specs because it's mostly a waste of time unless it's a heating system. A domestic water system is just run with sort lengths off of the hot loop. Just stick with that. For a frame of reference figure out how much water 1/2" pipe holds per foot, pretty easy to get a decent system just guessing and giving a little effort.

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#### Tuttles Revenge

##### In the Trades
While researching some other unrelated thing.. I found this link to an app.. might be useful. I didn't have time to look at it yet. Probably other calculators like this too.

OK.. looked for a couple seconds and they do have a free downloadable spreadsheet. I'll be downloading it next week and seeing if I can get some numbers to jive.

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