Makes sense to me although a trunk system is more useful with a recirculation loop (pumped or natural convection) to get quick hot water.

I've never seen an analysis of why Legionnaires disease isn't a greater issue with home-run hot water systems.

Why would Legionnaires be a greater issue with a home-run system? I'm not understanding where that question would come from.

You have more sections of pipe that get used only occasionally (ie, are stagnant).

WIkipedia: Legionella... reproduce at the greatest rate in stagnant water at temperatures of 35 °C to 46 °C (95 °F to 115 °F).

The water in the pipes would not be sitting at 95 to 115 F. That would require constant heating.

Is an association with Legionnaire's something you have actually read about regarding home-run systems, or is this just a theory on your part?

Neither, my understanding is that the association isn't there.

I also wondered about your hybrid appraoch. Seems logical to me. Would also reduce materials on the manifold.

What I'm reading is that a home run system is better for pressure distribution across multiple fixtures being used at once, but that trunk and branch systems have slightly better pressure at the fixture (as long as nobody flushes the toilet WHILE you're taking a shower!). So an infinitesimal decrease in isolated fixture pressure with the home run system is very probably worth it in exchange for the pressure equalization.

Another option to weigh - someone suggested a hot water recirculation loop instead of the home run. I guess this system guarantees quick hot water. But is there an energy cost associated with such a loop which is greater than the energy savings you would get with the home run system?

IMO, incremental energy costs are difficult to accurately measure or characterize in a typical home hot water usage situation. Even tankless vs tank can be argued either way depending on usage habits, length of runs, pipe insulation, etc. Convenience of quick hot water, to me, is the primary consideration. My Manabloc installation manual says do not include the manifold inside a continuous hot water recirculating loop. If the manifold is some distance from the hot water tank then a recirc line from the tank to the manifold is OK, but not through the manifold. That's what I did for the manifold in the utility room since it's about 40' from the tank.

Then there's seasonal considerations. If the hot water line is running through space into which you are adding heat, or is naturally hot, most to the year then the heat loss from the pipe isn't really lost. But if runs through space that you are cooling, or stays cold, then it is lost.

So I say do what will make your system comfortable for you to live with and don't look back!! I could be wrong, but I think this is another nickel and dime aspect of an overall system where you're going a long way already to save dollars.

I agree. If you let 10 gallons of warm water go down the drain waiting for hot water, that is wasted energy (and time). But so is keeping the pipes hot (when not heating).  PEX loses less heat than copper and one could insulate the pipes.  IMO, thermostatically controlled shower valves are nice to eliminate changes in shower temperature caused by pressure or tank temp changes.

They are very good, but not perfect! The same can be said for a PEX manifold home run arrangement. I have Moen ExactTemp controls in my showers and home run configuration with the manifold in the utility room. When I'm in the shower and the wife flushes the toilet in the guest bath the temp in the shower goes up a little bit. Not scalding or uncomfortable by any means, but noticeable. I don't know if it's a reflection of slow response of the shower valve or the cold water pressure drop from the flushing. Probably a combination of the two. The toilet is a Toto Drake which has a very fast refill rate so that may contribute to a high pressure drop.

Pressure drop. We replaced clogged metal plumbing lines with PEX using a home run setup in a small (625 sf) house. We expected to have constant pressure with two fixtures operating at the same time, yet we get a pressure drop. We had the city check pressure at the meter which was fine. SO, we figure the service line from the meter to the house shutoff valve inside the house is the old clogged metal pipe which is restricting flow.

DMac, I know you built from scratch and should have a new service line. Do you have the expected pressure at the meter?

I don't know what the pressure drop may be out at the meter. I've got a pressure gauge connected to the PEX manifold in the garage. Static pressure there right now is about 42 psi. With utility sink faucet in the garage opened, both hot & cold, the pressure drops to about 38 psi. With the master shower turned on full force with both hot & cold flowing, and both lavatory faucets in the master bath turned on full force with half hot & half cold, the pressure drops to about 35 psi. The master bath is on the utility room manifold which is about 40' from the garage manifold. The service line from the meter to the garage manifold is 1", about 40' of poly from the meter to the garage and about 45' of PEX inside the garage. The lines from the garage to the utility room are 3/4" PEX. With the garage utility sink faucets on, the shower on, and the two lavatories on, the pressure drops to about 30 psi. I have an outside hydrant connected to the line at the juncture of the poly and PEX lines where the poly comes into the garage. I just turned it on full open and the pressure at the manifold dropped to 35 psi. That tells me the pressure drop for water flow is occurring mostly somewhere between the house and street and not in the house PEX system.

I think a good bit of that pressure drop is probably occurring in the main line, or at or through the meter. Then again, the way the excavator, who dug the trench for the service line, did things he may have buckled or pinched the service line during backfill. Maybe I shoud get the city guys to connect up a pressure gauge at the meter. I could then measure the flow rate and pressure drop and see if it's reasonable for that length of service line. The feeder from the street main to the meter is 1", I believe, and serves two homes.

You can ask a plumber about the correct pressure, but I would not want anything less than 35 psi at each fixture.  The only time I would go below 35 psi is for a drip irrigation line and they usually have their own pressure reducing valve.

We installed a PEX homerun water distribution system in our new home.  All lines are 1/2" Uponor (Wirsbo) AquaPEX.  No fixture is more than 60' from the hot or cold water manifold.  The city water pressure at the manifold is 55 psi.  Flow rates are fine, but I should have used 3/4" lines to the tubs for faster filling.  I've installed all copper systems, but for low cost and ease of installation, you can't beat PEX.

Hot water arrives at the fixtures faster with a homerun system than with a trunk-and-branch layout, but not always fast enough.  The powder room lav is typically used to wash ones hands after using the toilet, so waiting even 15 seconds for the hot water to arrive is too long.  Once the hot water arrives, the hand washing takes but a few seconds.  At each use, most of the hot water goes toward filling the pipe, so each time the lav is used, heat energy is wasted in addition to the cold water lost down the drain.

Our solution was to install a small point-of-use (POU) instantaneous water heater under the powder room lav.  It supplies hot water by electrically heating water from the cold water supply line to about 110F.  We get immediate hot water, heat only the water used, and reduce our overall water usage.

We're thinking about putting a POU heater on the kitchen sink, as well, but connected to the hot water line.   In that case, we would use a thermostatically controlled heater which shuts down once the hot water from the tank arrives at the sink.  The cost for a thermostatically controlled heater is about $200+, making it harder to justify, but the convenience and comfort of instant hot water is attractive.

Clark, even the smallest point of use heaters that I've seen are at least 200 dollars, but those are electric tankless. Are you talking about the tiny storage tank version? I reckon I should probably install a point of use for the lav sink in the farthest guest bathroom. But more than one or two starts to get pricey, plus there's the amperage to consider.

I have a friend who has a passive circulation system for hot water - ie, it has a loop with a line returning from the faucet back to the water heater but no pump. He says it works well.

If the POU system is electric and you have nat gas or geothermal hot water heat, I would expect a recirculation system to be more cost effective.

Yes, you're right, tankless POU heaters can get expensive, but shop around. I bought a Bosch AE-9.2 at Menards on sale for $148.95 last month (not a closeout or damaged goods price either.) It requires a 40 amp double-pole breaker and a length of #8 3-conductor cable (10' in my situation.) I think the smaller (30 Amp) Bosch AE-7.2 would have worked fine for a lav, but Menards wanted $179.95 for it (not on sale.) I'll check their price this week as prices seem to bounch around a lot these days.

Here is what I do in all of my client homes (I'm not saying that it is better than another way, it just makes sense to me.)

We run a modified home run system, I will not allow my plumber to install a trunk system in my houses. Hidden connections and couplings in wall and floor spaces can (and often times will) come back to haunt you, even if the pressure test was fine I don't like the idea of having random connections off the trunk line all over the house. We run a 3/4" cold line to a header that is mounted in the bathroom (in an accessible location, behind an access panel if possible), from the header we run lines directly to the fixture (again no connections between the header and the fixture, just a solid run of pex). We also run a 3/4" hot line to a reote header in the bathroom, but we pull a recirculation line off of the header to insure instant hot water all the time, you are only draining the line between the header and the fixture (6'-12' max usually). Our houses typically end up with 2 or 3 (ocassionally more, if the house is quite large) remote header locations, centralized around a bathroom or group of bathrooms. If there is a random individual fixture (like a single bar sink) I run a complete home run for that fixture. I also run kitchen sinks as a home run system so that the kitchen water can easily be isolated from the water softner. Hose bibs are also run as a home run so that they can be isolated from the water softner. For the hot water recirculation, I use a pump on a timer, so that we are not cycling hot water 24 hours a day. Since switching to this system, I have had happy home owners and I feel better knowing that there are not any hidden plumbing connections in the house. The only possible spot for a leak is right at the header, even if it did leak, it is easy to find and easy to repair.