Cold side 8 gal/min but hot side only 3 gal/min - Plumbing Forum

pat72 · 04-30-2010, 11:59 PM

here is the problem:
Both bathrooms and kitchen have a very good flow of cold water (about 8 gal/min at downspout or with showerheads removed) but if I open the hot side only the flow drops to 3 gal/min at every fixture. I have 80 PSI at the water heater inlet, outlet and bathrooms. Also, since the water heater has 2 flex lines, I also checked the flow of hot water (outlet side) and I get 8 gal/min so no problem there. What could create such a drop from 8 gal/min at the outlet of the water heater to 3 gal/min at the showers and downspout?
The condo has copper water lines so unlike galvanized piping I don't think this has to do with corrosion restricting the flow in the hot side.
Thanks for your help.

**havasu** · 05-01-2010, 03:22 AM

I wonder if the size of piping is different from hot to cold?

**Reedwalker** · 05-01-2010, 04:52 AM

Curious situation you are depicting.

Modern fixtures are required to limit flow by design. I do believe a shower can allow no more than 2.2 gpm (I could be swapping that with the lav faucet numbers but I,m too drunk to look it up). The flow restrictor in the heads are not the only form of restrictor. These fixtures are also restricted in the cartridges.

Im thinking the difference has something to do with the engineering of flow restriction melding with the pressure balancing safety engineering.

Effing tree huggers and engineers......

**havasu** · 05-01-2010, 05:14 AM

Quote:

Originally Posted by Reedwalker

Curious situation you are depicting.

Modern fixtures are required to limit flow by design. I do believe a shower can allow no more than 2.2 gpm (I could be swapping that with the lav faucet numbers but I,m too drunk to look it up). The flow restrictor in the heads are not the only form of restrictor. These fixtures are also restricted in the cartridges.

Im thinking the difference has something to do with the engineering of flow restriction melding with the pressure balancing safety engineering.

Effing tree huggers and engineers......

First off, gotta love your last sentence. I could understand cartridge flow restrictors in high end shower valves, but have a problem with built in flow restrictors at the typical sink. I guess this theory could be tested by swapping the hot and cold supply lines under the sink, then checking pressure?

pat72 · 05-01-2010, 10:53 AM

The faucets are recent but both shower valves have those separate hot and cold stems (and a diverter in the shower where the tub is). Do these old stems have restrictors in them? If that's the case, it only on the hot side side because the cold side sure doesn't have one (8 gal/min !!!)

**phishfood** · 05-01-2010, 01:46 PM

I had a reply to this thread typed up last night, but managed to crash my computer before I could post it.

Anyway, I routinely install smaller distribution piping on the hot side than on the cold side. Not so much because of the little bit of money that it saves my company, but because of the money and inconvenience it saves the end user. A larger pipe takes longer to flush all of the cold water out of it before hot water gets to the point of use, and also takes more hot water to fill it up, which then cools off. This ends up costing the homeowner more in both water bills and energy costs to heat the water.

So, does the lower flow affect the use of the fixtures in a significant way? IE, really weak hot showers, dishwasher taking forever to fill?

pat72 · 05-02-2010, 01:59 PM

I think phishfood and havasu nailed it. Probably something to do with the distribution pipes size. Now that I think of it, there is probably a main cold water pipe which branches out to the different units of the building and it is probably much bigger than the 1/2" supplying the hot water to each fixture. To answer your question, phishfood, it is not a bid deal but if one of my future tenant wanted to take a bath they would have to open fully the hot water valve and barely the cold valve in order to get warm water. Didn't understand why but I guess I'll go with the different pipe size theory. Is 3gal/min (unrestricted) more or less the norm for a hot water flow from, let say, a bathtub downspout or should it be higher?