Mitigating Point of use hot water wait time

scootersyme

I'm a General Contractor and have installed numerous Radiant systems in homes, both for domestic hot water and radiant floor heating. I have a tankless domestic hot water system in my own home and have come to really appreciate tankless systems both for domestic and radiant floor heating.

One of the biggest complaints I get from customers and honestly one that I also have is the time it takes for hot water to arrive at the point of use, I realize on one hand if this is our only problem.... then we are doing pretty well considering, but I've been thinking of a way that both solves this problem with minimal extra cost, and hopefully will help others create a simple solution to alleviating this first world problem.

So as a precursor I am building a new home with retirement and the thought of growing older something that needs consideration. This house is a single level with no basement, which means there will be a mechanical room off the attached garage housing the tankless heater 30' to the kitchen and almost 50' to the furthest bathroom. I can't tell you how long it will take if you want warm water to arrive at that distance considering the cycle time of the tankless heater etc.. etc.

Add a small holding tank around 10 gallons (in the attic and well insulated) directly above a bathroom or kitchen with a thermostat where the tankless heater will cycle off and on via a TACO circ. valve ect. to keep the water there being held at a preset temp. This will give you the instant hot water needed for the point of use, and with a mixing valve once the hot water from the tankless kicks in it would be the primary hot water. I have attached a PDF for a schematic and would appreciate and feedback for this project.

Thanks in advance!

Intplm.

Try this. I've done this in numerous homes. It works great.

It's a recirculation system made by the Watts company. Should work great with the situation you have described.

hot_rod

Or consider one of the tankless heaters that has a small buffer tank inside. That at least eliminates the ramp up time delay.
Also look at a Manablock distribution system Viega still offers them. Each fixture gets a dedicated line, 3/8 in many cases. So the line flushes the cold out much quicker. Compared to a 3/4 trunk line piping method.
And no fittings buried in walls floors or ceilings.

https://www.viega.us/en/homepage/systems/manabloc.html

Larry Weingarten

Hi @scootersyme , There are a few things you can do. First is the reduce the volume of water between heater and point/s of use. 3/8" tubing is code, using appendix M in the UPC. This can go a long ways in speeding hot water delivery. If you have 60 psi and no long runs, 1/4" tube can give you 1.5 gpm and hot water in seconds. I've got the engineering document to verify this along with its compatibility with code. A different approach is to use demand controlled pumping. This approach requires you push a button or activate a motion sensor to prime the line with hot water. Another way is to use a small tank at the point of use, and have it be fed by the hot line. As long as the tank holds at least three times the volume of the line, you get steady hot water. The simplest and my favorite approach is a well insulated copper manifold directly on top of a tank, (so it stays full of hot water) and 3/8" or 1/4" tube from there to the points of use. This is "instanter" hot water than a tankless heater can provide

Yours, Larry

hot_rod

I think this would work with off the shelf parts.

A small electric point of use heater in series with the tankless. With low heat loss the tank should not need to run often. Depends on how often you run hot water. This keeps everything in the mechanical room.

The entire system could be set at 120 and used directly from the boost tank.

Of if for bacteria prevention you want the tank and tankless at 140, use a thermostatic mix valve. In 3 or 4 stares mix valves are required on all water heaters.

john123

@hot rod: What problem would you see in using a larger (super insulated?), gas (or electric?) larger (20 gal) tank in series after the combi boiler as per your second diagram? Would it provide any advantages in getting a hotter "hot" flow when the incoming water is super cold and there is a larger demand for DHW than was expected when the combi boiler was first put in. perhaps like 2 showers at the same time instead of one?

If the combi boiler is just not giving enough hot water, could you put a "larger" hot water tank in series after the combi boiler without doing re-circulator piping?

It would solve the delay issue at the same time?

scootersyme

Larry Weingarten said:

Hi @scootersyme , There are a few things you can do. First is the reduce the volume of water between heater and point/s of use. 3/8" tubing is code, using appendix M in the UPC. This can go a long ways in speeding hot water delivery. If you have 60 psi and no long runs, 1/4" tube can give you 1.5 gpm and hot water in seconds. I've got the engineering document to verify this along with its compatibility with code. A different approach is to use demand controlled pumping. This approach requires you push a button or activate a motion sensor to prime the line with hot water. Another way is to use a small tank at the point of use, and have it be fed by the hot line. As long as the tank holds at least three times the volume of the line, you get steady hot water. The simplest and my favorite approach is a well insulated copper manifold directly on top of a tank, (so it stays full of hot water) and 3/8" or 1/4" tube from there to the points of use. This is "instanter" hot water than a tankless heater can provide

Yours, Larry

Hi Larry, and thanks for the input, I am pickin' up what you are puttin' down here, although I am not seeing the "benefit" of having a copper manifold on top of a tank. If in fact the tank is being preheated by the tankless system via a circ-loop and water is fed from the tank which is in close proximity to the point of use then how does a copper manifold help in the delivery of said-almost instant hot water? Also am I on point with the schematic I posted on the recirc or should there be any tweaking involved there?

Thanks much for your input!

scootersyme

hot_rod said:

I think this would work with off the shelf parts.

A small electric point of use heater in series with the tankless. With low heat loss the tank should not need to run often. Depends on how often you run hot water. This keeps everything in the mechanical room.

The entire system could be set at 120 and used directly from the boost tank.

Of if for bacteria prevention you want the tank and tankless at 140, use a thermostatic mix valve. In 3 or 4 stares mix valves are required on all water heaters.

This definitely provokes my interest. I was thinking though of having one of these "booster tanks" at each point of use, mostly for bathroom sinks, shower and kitchen sinks the clothes washer would not require, dishwasher ect. mostly just the kitchen and bathroom sinks. The cost of a small electric water heater is pretty cheap, I just wouldn't hook it up "electrically" and utilize the thermostat for a signal for the tankless to heat up the water being stored via the recirc circuit. What say you?

hot_rod

They all leak at some point, I’m not sure that you want those small tanks scattered around your home?
They need a dedicated 15 amp circuit also. One back by the tankless should do a good job even without a manabloc upgrade. It could be a 6 gallon for a bit more storage

Larry Weingarten

Hi @scootersyme , A reason for a copper manifold above a tank is that code prohibits plastic too close to the tank. It's also a great place to put individual valves for each line, so you can easily control all fixtures, even the shower. By simply using a union and copper directly above the tank, the manifold can stay full of hot water, making it so you get fast hot water delivery with minimal water and energy waste. Of course, using pipe/tubing as small as can be, will greatly speed hot water delivery also. Also, the system is simple, with no moving parts, or electrical demand. This means it can live a long, no-hassle life. It would work less well being fed directly by a tankless heater, but with your small buffer/booster tank, it could do a nice job, and still help simplify the system.

About the plumbing, it's basically either manifold or trunk and branch. If you want to recirculate through a trunk and branch system, a manifold has no place. If you have long runs, with the need for high flow, a trunk and branch system is the way to go, though I'd look into demand pumping instead of timer based, so you're not keeping it hot when it isn't being used. For more on these concepts, have a look at Gary Klein's site: http://www.garykleinassociates.com/

Yours, Larry

Ps. Unfortunately I was just confused by the schematic. Maybe adding some arrows for intended direction of flow, and any valves, like check or ball valves would help clarify. I didn't understand the hot bypass around the tank or lack of cold feed to the mixing valve... but it's late in my day and my brain might have left already