I’ve been researching hot water, trying to devise the perfect system for us. There are some things about our present system that will require upgrading before I can paint me happy. When I did the research to figure out which tankless heater we should get, I think I did a good job. I got one that will heat water for our whole house (when it’s built) and will accept pre-heated water so I can add a solar conditioner to the system when I’m ready. Pretty smart, but . . .
Tankless heaters have a rather serious drawback. If the water flow through the heater isn’t sufficient, it stops heating the water. Say it’s summer and you want a cool but not cold shower. The amount of hot water you need to mix with the 55° well water to bring the shower water to 75 or 80° isn’t a fast enough flow to kick the heater on. The kick-on flow rate for most tankless heaters is a set measure, and it’s higher than the hot water flow needed for a tepid shower. Oops. You can have a cold shower or a hot shower, but not a tepid shower. To get a tepid shower you need to run hot in the sink while you take a shower. What a waste! I am SO not going there.
So we need to increase the flow through the hot water heater without wasting either the water or the cost of heating it. I have two ways I can do this. I can purchase another tankless heater and plumb it in front of the tankless we’ve got. It doesn’t need to be a whole house biggy like we’ve got, it can be a little 1 gpm baby that can produce a trickle of hot water. When the water’s running slow it will heat the water, and when the water’s running fast enough to turn the big heater on, it acts as a preconditioner for the big heater. That’d work. But I have an idea I like better. I think it will work better and cost less . . .I’m looking for a programmable flow sensor switch which I can plumb into a loop 
through our tankless heater to turn on and off a water circulating pump. If you look at the diagram, you’ll see the flow sensor switch is connected to the pump. The arrows show water flow direction.
The programmable flow sensor switch needs to switch the pump on when the water flow is > .25 gpm but less than 3 gpm. Switching on a pump to increase the flow through the heater will ensure we get hot water for a tepid shower and it will do it without wasting water or the energy to heat it. The pump switches off when it’s not need (> 3 gpm), which saves energy. (If we had colder ground water I’d be tempted to ignore the upper shutoff to ensure the water made it to temp.) Because our heater is a model which has both a large capacity and the ability to bring the water to temperature regardless of the temperature of the incoming water, cycling hot water through the heater will save energy because the heater will only heat the incoming water the amount necessary to bring it to temperature. To me, that seems like a pretty simple solution. Let’s see if I can find a programmable flow sensor switch that will do the job.
There’s another energy/water waster associated with hot water heaters. One of the downsides of any hot water heater is the amount of water you waste waiting for hot water to get where you need it. Over the course of a year, it’s thousands of gallons of perfectly good water poured down the drain! Ugh! If you’re on city water, that’s a pretty big waste of a family’s resources! If you don’t pay for water, you’re still wasting the cost of pumping the water and adding thousands of gallons of water to your septic system unnecessarily. If you have a limited amount of water like we do, pouring water down the drain is something that really must be avoided.
Let me walk you through what I’ve learned.
There are two types of water recirculating systems designed to keep/get hot water to the faucet without wasting water. In reviewing both systems, it appears one’s just a bit of a brighter idea, at least for us.
The idea is to conserve the unheated or below optimum temperature water in the hot water line. This first system uses a time or temperature controlled pump attached to the hot water line where it exits the hot water heater. This can be any type of hot water system, gas, electric, tankless or solar. When the temperature in the line drops below a set temperature, freshly heated water is circulated through the line. The flushed water is returned to the hot water tank or system by a dedicated return line which runs from the farthest faucet back to the hot water heater.
This system certainly does the job, AND it’s more energy efficient than a hot water system with no recirculating pump, but there are two downsides to this system. It’s expensive to install, not only in the cost of the pump, valves and switches, but it’s also expensive in the amount of plumbing that must be added. Plus, it isn’t the most efficient system. The heat loss from hot water sitting in the pipe is wasted, though this can be reduced by making sure the hot water and dedicated return lines are well insulated. This is a good system, and it’s definately an improvement over wasting water, but it’s still an energy waster. Ths system requires more plumbing, so costs more initially, and because it keeps hot water in the lines either continuously or on a timed cycle, it’s wasting the energy used to heat the hot water that isn’t used for anything but keeping the line charged. There are a lot of sites that sell this type system, so there’s plenty out there to read if you’re interested.
The cold water return system can be a better system in some cases, but it too has its drawbacks. This system uses the existing cold water line as the return line, which obviates the need for a dedicated return line. The pump is mounted at the point of use (faucet) and is engaged manually prior to using the hot water. Run the pump, and when it quits you can turn on the faucet confident you’ve got hot water. You spend the same amount of time waiting for hot water, you just don’t waste water in the process. But the system is reportedly noisy, and you end up waiting for cold water instead of hot water as the cold water line is the return for the water that isn’t up to temperature, but isn’t necessarily “cold”.
The best system may be a combination of the two. If you’ve got a faucet that’s far away from all others you can save yourself the extra plumbing for the return line for that faucet only by plumbing it with the second system, using the first system for all the clustered/close faucets.
UPDATE: We moved the location of the cold water connection and replumbed the hot and cold lines between the Airstream and our laundry room (location of our on-demand hot water heater), shortening the run by almost half and reducing the size of the hot water line from 1″ to 1/2″. I could have made the line even smaller, but I used what I had instead of buying new. This is line I will be able to use as main feed lines in the new house, so it isn’t wasted. When I plumbed it, I used the new shark bite connectors which will allow me to disassemble and reassemble the system at will. I won’t have to buy new connectors to replace the ones in this system. And we now get hot water in the bathroom AND the kitchen quite quickly, so this is a good revamp.
We also insulated the hot water lines starting at the heater instead of where it leaves the laundry. This helps retain the heat in the water a bit better.
Winter ’08 produced temps hovering around 10°F, which is really low for us. The changes meant we didn’t have to run water in the sink through the night during the coldest temps to keep the pipes from freezing. Between hot water faster and no frozen pipes, we’re pretty pleased with the changes. Hopefully by next summer I’ll have the solar conditioner on the roof of the laundry and plumbed in to preheat the incoming water. Now that we’ve moved the cold water inlet, connecting this system will be much easier.