General question on HW pipe sizing

[Giantsean]

Hi All,

My house is 1951 and has a 1" supply into a shutoff after which it reduces to 3/4". The house has been remodeled/changed up several times over the years and it is not a straight run of 3/4 to the water heater, rather it branches off in places to other 3/4 and 1/2 cold lines first. After the HWH (tankless Navien) it reduces to 1/2" and tees to all fixtures, except for the upstairs baths (2) increases to 3/4" to go upstairs and then splits to 1/2" again to each bath (1/2" shared by sink and shower)

I have been told by some pros that resizing the hot water piping to 3/4" and branching would improve pressure losses which we currently face when showering while a dishwasher or washer is filling, which I was keen to do until I read too many posts about fluid dynamics and such. Even the experts couldn't agree.

So in general, if you have a 3/4" supply can you also have a 3/4" hot, or is it a must to reduce all hot and cold by one nominal size (ie all 1/2") and branch 1/2" to all fixtures?

I'd really like to upgrade but at least clean up the mess that I have. However if I'm already the best I'm gonna get, no reason to change too much. Would love to see if I can get a consensus here.

Thx as always!

 

[Houptee]

New houses are using PEX manifolds with 3/4 in and 1/2 out in home run layout. You can shut off each fixture from the manifold.

 

[Sponsor]

 

[wwhitney]

the HWH (tankless Navien)
. . .
pressure losses which we currently face when showering while a dishwasher or washer is filling

I think the tankless itself is likely the biggest source of the pressure losses. If the demand flow exceeds heating capacity, I believe they intentionally throttle flow to maintain temperature. And even if not, at maximum design flow I think the pressure loss is significant.

So I doubt that repiping will help much or at all. It's just one of the tradeoffs with a tankless.

Cheers, Wayne

 

[Giantsean]

I suspect you are right. That said, the DHW supply is good for 5.6 gpm for my model (CH240) out of the box. The manual interestingly says to use 1" piping for DHW - which is weird because the outlet is only 3/4 - but the CH manual is also notorious for some oddball statements.

All that said, is it fair to say that moving to a 3/4" hot trunk and teeing off to 1/2" would not hurt anything? Even if I redid it with 3/4" PEX the ID is not that different from 1/2" copper, which it is now. What I'm trying to do is eliminate the runs of 1/2" PEX out of the Navien, which I think might be choking things up some.

 

[wwhitney]

For cold water, the only downside for large pipes is the one-time cost of the pipes.

For hot water, the larger the pipes, the more water they store that cools off between uses. So the more water you have to drain each use, unless you have a recirculating system. So for hot water it's best to "right size" the pipes, just large enough to provide the required flow without excessive pressure loss.

Given all that, in a trunk system, I would think you'd want 3/4" for any pipe that serves, say, 4 or more fixtures. The way the upstairs baths was done makes sense--3/4" for the 4 fixtures in both baths, then when the supply splits, 1/2" to each bath.

Cheers, Wayne

 

[Sylvan]

If you have a supply line of 3/4 " then you should supply the tank full size (3/4) and run the 3/4" line and reduce at the fixture

For example you have a 3/4" line hot and cold gong to a bathroom on the cold I would install a 3/4 x 3/4 x 1/2" line going to the toilet and 3/4 x 3/4 x 1/2 x 1/2 for the basin and shower

On the hot 3/4 x 1/2 x 1/2 for the basin and shower

 

[Giantsean]

Thanks fellers. I am coming up w/ my planning diagram now and hence needed some feedback to lay it out first. With this, I think I am still planning on using a 3/4 trunk for my hot when it's time. In most cases (the only exception being aforementioned upstairs baths - the only two in the house are upstairs) they will tee off to 1/2" to individual fixtures. Due to the existing piping/geography, the trunk itself is only about 12-15 feet long from the HWH, and I will wrap it nice. I wrapped all the hot lines when we renovated before we closed up so I feel pretty good about the plan. I just got concerned a bit hearing so many people talk about the 3/4 supply splitting to 1/2 hot/cold as soon as the water heater and didn't want to create problems. Maybe that was the rule of thumb way back in the day and old timers have problems letting it go.

 

[Sylvan]

The "Old timers" were rightly concerned about volume drop

Someone in a shower and another person flushes a toilet a half 1/2" line has limited volume and there is a drop also in pressure on the CW line for just an instant BUT because of the flow restrictions in a shower head the water spikes to a dangerous level and causes scalding

 

[Jeff H Young]

Undersized piping certainly not helping. no a 1/2 inch hot line is not proper plumbing for 2 or 3 bathrooms a kitchen D/W and a washing machine. not good work , sorry to say, but changing that might not "Fix" it either point is though that isnt right and would not pass a real inspection or meet any code and for good reason its way undersize!