Separate Hot Water Systems for Lav sinks vs. Kit / Shower / DW / Laundry?

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Speede541

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I really got sold on the idea of a switch-activated hot water loop, and it got me thinking about hot water system efficiency in general. I'll be installing an indirect hot water heater, fired off a boiler, and hopefully connected to hot water solar panels sometime down the road.

But it also got me thinking about the inefficiency of flushing a gallon or two of cold or lukewarm water back into the water heater tank, just for a simple hand washing at a bathroom lav.

So I'm laying out my water system with a planned separate on-demand heater underneath an unstairs lav. It'll serve just the lavs in the master bath, kid's bath, in-law bath, and a prep sink in the kitchen. Meanwhile, the hot water loop will service the showers, dishwasher, clothes washer, and main kitchen sink. There's a half-bath that's close to the kid's bath, but is also sits directly over the water heater, so I'm undecided as to which system to connect this to.

The longest on-demand runs are no longer than 15-20 feet through 3/8" PEX to the lavs, and 1/2" to the prep sink, so while there will still be a short wait time at the faucet, I won't be cycling the main water heater.

Anybody care to poke any holes in this arrangement? The worst case scenario I can come up with is everyone running their sinks at the same time, but I just don't see that as likely or extremely inconvenient.
 

Terry

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I like the idea of re-circing back to the water heater. Why not put back partially warmed water to the heater instead of cold ground water? Let the heater take another shot at it.
An aqua stat wired to the pump would shut off the pump when the return line was heated.
 

Jadnashua

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Using enough electricity to heat cold water to a comfortable temp will use more energy than an on demand recirculation system...the pumps are very efficient (low power), and the energy costs on the boiler to fire up the indirect are likely a lot less than electric point of use. So, I think your logic is failed. IT comes more into parity if you have a recirc system and do not have it on a timer, but even with a timer so that it's constantly doing its thing during normal waking hours in most places is less expensive to run than a point of use system.
 

hj

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The typical "on demand" water heaters use a lot of energy, but NOT enough to provide a good flow of hot water. In order to have one with the capacity to operate a shower, for example, you need the equivalent of a "whole house" tankless water heater, and they are expensive to install and operate.
 

Speede541

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I may have been unclear with the layout I'm considering:

Hot water recirc loop w/ indirect boiler-fired water tank:
> In-Law Washer > Kitchen sink > Dishwasher > In-Law Shower > In-Law Kitchen > Upstairs Washer > Kid's Shower > Master Shower

This loop would be circulated by a momentary toggle switch at the two upstairs showers, connected to a Grundfos pump & logic controls in the utility room.

On-Demand to Prep Sink, and lavatory sinks in the in-law, master, and kids baths.

The guidance I'm reading is from a Department of Energy site, stating that on-demand heating can be 25 to 35% more efficient for small for usage rates under 40 gallons a day, which would be a really high rate for 4 sinks. The DOE site backs up what you're both saying (Jim and HJ) that at higher usage rates, the efficiency of on-demand tanks.

The 60 gallon indirect water heater we're installing is something ridiculous like 99% efficient (no flue; 1° temp drop over 12 hours), that it would seem inefficient to "uncork" and have to replenish that stored heat for something like a simple hand washing.

I haven't ID'd a model of on-demand heater yet, and have seen some with a very very small amount of onboard storage, like around a gallon.

Maybe I'll have to estimate household demand and run some numbers to put a dollar figure to my options. But even with two youngsters and two elderly parents, nobody is standing at the bathroom sink with the hot water continuously running.
 

Jadnashua

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The aquastat and controller for the IWH generally requires the tank temp to drop a moderate amount before it requests heat from the boiler. As a result, with small usages, the boiler may still never come on. And, that sensor is not at the immediate bottom of the tank, so running some cold back to there may not be a big deal. They do this to allow the boiler a decent run-time rather than short cycling.
 

Speede541

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(sorry, thinking out loud in this reply....)

Then I guess the way to proceed might be a hybrid hot water loop / home run layout, with all fixtures supplied by the loop except for the 5 low demand sinks I mentioned.

But with three of those sinks across the house -- 30 to 40' runs -- I'm now eyeing my mini-tank options (7 gallon Bosch / Ariston or Eemax unit) instead of a tankless on-demand. That would give me nearly instant hot water to three of those sinks through 3/8" PEX if I make up a mini manifold distribution under the master bath cabinet, without any of the minimum flow requirements of the on-demand systems. The fourth sink is roughly equidistant from both, and as I mentioned the 5th is basically on top of the main tank.

Because even removed from the loop, I'd be pushing roughly a quart (based on the storage of 3.8" PEX so... 4 gallons) of cold water down the drain based on 20 daily usages, and have to heat up an extra 4 gallons of cold water in addition to what is actually used at the tap. The remote mini-tank would cut that 4 gallons by more than half, probably more like 3/4. The downside is it takes nearly an hour to reheat the small tank, but I still think that would rarely be a factor.

Alternatively, leaving the sinks on the hot water loop could cut wasted water to virtually zero, but would end up flushing around 10 to 15 gallons of cold or lukewarm water back into the main water heater to be reheated.

The dollar costs I came up with are $4.50 / month to operate the mini tank (assuming 8 gallons per day -- it's about $1.90 per 100 gallons at 15¢ per kWh). Using solely the main 60-gallon indirect heater with a hybrid manifold to the sinks is 60¢ per 100 gallons @ $1.10 per therm, but I'd be heating 28 gallons per day instead of 8, so that monthly cost comes out to $5.04 per month. More or less a wash on cost, but the mini tank gives faster hot water to the faucets on three of those sinks farthest from the utility room, without the inconvenience of flipping a switch. Again, this is for the lav and prep sinks only, not kitchen/dishwasher/clothes washer / showers.

The one factor I can't account for are the anticipated solar water & electric panels on the roof, but that's down the road a ways, and can only help the situation. There's no reason I couldn't divert some of that solar heated water to the mini-tank, AND power it with solar electricity.
 
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