Hot Water Pipe sizing for domestic and industrial systems

[Engineer_X]

Hi,

I'm new to this forum and this is my first post. I could not find a detailed answer to my question and I'm posting it so that someone can help me out.

Currently, I'm performing supply water pipe sizing calculations for a large villa using loading units and I have a doubt regarding the hot water piping. The plan is to use individual electric water heaters in every bathroom and feed them from the cold water lines.
My question is whether I need to double the fixture loading units wherever both hot and cold water is used or does it depend on the type of fixture? In addition, are the hot water and cold water pipes designed to be the same size? If not, what does the hot water pipe sizing depend on?

I'm aware of the pipe sizing standards and methods for cold water distribution pipes. I'm just not sure how to apply that when a hot water supply is integrated into the system as well.

Thanks.

 

[PEDARRIN2]

For your future reference, there is a plumbing engineering forum for these type of questions.

Fixture load for cold water will only be increased upstream of the cold water branch to the water heater, but it is not double.

Lavatory cold and hot water is 0.5 fixture units each, but the value for cold water upstream of the water heater to feed the lavatory is 0.7 fixture units. So, if you were sizing the water for a single lavatory, you would size the cold water and hot water supplying the lavatory for 0.5 fixture units and the cold water supplying the water heater for 0.5 fixture units. You would size the cold water upstream of the cold/hot split for 0.7 fixture units.

Hope that helps.

 

[dbill74]

First, you will get more and better results if you move this to the Plumbing engineering forum.

Second, where is your mentor? He should be guiding you through learning how to design these systems and all the nuances that go with it. Not trying to be mean or unhelpful, just that a mentor can help you in ways we in this forum cannot, as much as we try.

In the meantime:

My question is whether I need to double the fixture loading units wherever both hot and cold water is used or does it depend on the type of fixture? In addition, are the hot water and cold water pipes designed to be the same size?

It depends on what you mean by "individual electric water heaters". What type of water heater and how many fixtures is each water heater serving?

If not, what does the hot water pipe sizing depend on?

Same thing all pipe sizes depend on: flow, pressure and velocity. For potable hot water piping, recommended maximum velocity is 4-5 FPS.

 

[bimr]

You need to contact the local building department and ask the designated plumbing expert for design guidelines.

You will be better served by using the flow capacity of the specific fixtures rather than the fixture units estimate.

 

[Engineer_X]

Thanks for the responses.

First, I can't seem to figure out how to move this thread to another forum. Do I have to create a new post there?

Regarding the problem,

The 0.5 units each for hot and cold you mentioned is if it's a mixer tap? What if the fixture has separate taps for hot and cold such as in some wash basins?

And yes I do need to ask my mentor all these things, but he's out of town these days and generally busy haha

 

[PEDARRIN2]

You are not the first to put a plumbing question in this forum. Often times you will get a more technical response here, if it is not too "plumbing specific".

You cannot move it and forum "rules" frown against having multiple threads for the same topic. Next time you have a plumbing question, put it there.

If you have separate taps, I would likely split the difference and use 0.6 since you would have more flow through separate cold/hot taps than if they were flowing from a single tap, but don't think you would have the "full" 0.7 amount. This is just my opinion. The difference is fairly inconsequential.

 

[dbill74]

You will be better served by using the flow capacity of the specific fixtures rather than the fixture units estimate.

Not always. If you have a bunch of fixtures you will end up with oversized piping.

Keep in mind that sizing with fixture/load units takes into account diversity. The published FU curves also take into account increasing diversity as the number of fixtures increases. With very few fixtures, diversity will go to 0 and then yes, sizing for fixture flow is appropriate.

When to use HWFU, CWFU and TFU. That depends on the service of the pipe. If the water is going to hot water side of fixtures typically to/from the water heater, it is appropriate to use HWFUs. Likewise, if the pipe is going to cold water only (with hot water present), then CWFU. If the water will be used for hot and cold water service, i.e. building service main, then TFU is used.

For fixtures such as lavatories that will have a CW connection only I will typically use the TFU (0.7) for it.

Which sizing techinique is best? It depends on the pipe being sized and it is up to the designer to use his/her best judgement. I frequently use fixture units and fixture specific flows on a given project. Which method is most appropraite for the OP's situation? I don't know, he hasn't provided enough information.