An interesting concept. I wonder if it is worth what must be a high initial cost. Upkeep would be another matter with the Water Furnace pump plus 2 hot water tanks to go bad.
I find the estimates of water heating expense very high compared to my wife and I use with just 2 of us in the house.
The delta cost was negligible, since I was installing the HVAC system anyway. The gas heater has the same risks it’s always had (it’s still original equipment). The electric tank is just a tank; remember that it’s not connected to anything. As for the pump, I’m satisfied with the MTBF data.
Have you considered on-demand heaters or solar heating?
It is common in commercial buildings and high end homes to have a small circulator that runs a parallel loop so the hot water is always at the tap.
Of course that’s done mostly for convenience; the convenience of not having to wait for the cool static water that’s in line to run through until it’s hot.
You’ll waste less water that way, but the circulator costs some small amount of electric to operate, so I doubt its a net gain, water savings vs electric cost.
Interesting.
Do you get “free” hot water? We install these units and our experience is that the gain for the domestic hot water means that the hot water is “supplemental.” IOW, we’re pre-heating the tank, but IME we’ve never been able to get to the point where the main tank doesn’t fire. (whether its gas or electric)
Its still a great deal and idea, but we’ve never gotten to free hot water. Have you been able to figure just how much you’ve saved specifically with your hot water needs?
Plus you have to re-heat the water that’s cooled in the lines.
as a preheat for the supply into the water heater it is a great savings with a short payback. most of the energy is used is to heat the water from its source temperature. if the water heater (the final storage tank) is insulated well then the maintenance heat required is low.
with thermal solar as the preheat source you can put water into the water heater that is above safe water temperature for use. the water heater needs to have a thermostatic mix valve on the outlet to add cold water to keep the water at a safe temperature. totally free hot water (aside from electric for circulator pumps) will depend on your preheat storage and water heater capacities, your water usage in conjunction with the solar collection (that is you would need to use more than a tank of water on a really sunny day).
See slightly above. There are periods when I don’t. When I was living in Mexico and made trip homes, there was plenty of hot water for my needs, just from the HVAC. The gas tank was left in permanent vacation mode. Now the water wasn’t piping hot, and I wasn’t doing laundry, but I had hot enough water. (For those interested in why the water wouldn’t mix in the gas tank and get cold, etc., consider dip tubes.) I think I was home full-time for more than a week before I realized that I needed to turn the thermostat on the gas tank back up (it was a day my wife wanted to wash literally everything in the house after being empty for so long).
Unfortunately, no. Because the previous HVAC system was gas, I only know the overall energy savings, not just that attributable to hot water. I still have (insist!) on a gas stove and gas laundry dryer, and my gas usage is so inconsequential that I don’t know what it is (I still have a huge credit with the gas company for overpaying after changing to geo, over four years ago!).
I spent considerable time investigating tankless heaters. I concluded the advantages were a bunch of marketing hype based on invalid assumptions.
I see. And solar?
My neighbor had a solar system in the early 90’s. She said there was never any hot water first thing int he morning, when she really needed it. There was lukewarm, but seldom hot.
I’m guessing the technology has improved substantially since then though. . .
if solar was the only heat source for the hot water then in the morning you would only have the stored hot (lukewarm) water from the day before. Taking that water and bringing it up to temperature with some other heat source is still a large heat savings.
Having a storage tank of solar heated water fill the water heater tank gives a savings on energy used by the water heater.
That’s not how you measure efficiency. Efficiency is useful output divided by input, in this case (power transferred to water)/(power consumed).
You can’t say “those remaining 3 watts heat the water more than 15 watts of resistance heating” - if 3W is being transferred to the water in one case, and 15W transferred to the water in the other case, then you **are ** going to get 5 times as much heating in the second. If you’re not, then the power isn’t being transferred to the water.
I currently have solar as a pre-heater. It probably saves me 50-75% on my water heating electric bill. I used to have a roof-top storage system that allowed me to completely turn off the electric water heater in the summer. Even in the morning, the water was plenty hot. It would get so hot on summer days that I had to be careful of scalding - I once heard the OTP valve trip on a particularly hot and sunny day!
with that type of system it is good to have a thermostatic mixing valve (mixing cold water with the outlet of your water heater) to prevent it from being distributed at an unsafe temperature.
What I have seen of it looks like at my age, I might die before it paid for itself. Even at today’s interest rates, I would be better off with the money in the bank.
Not for Solar DHW. In most areas of the US, it’s a clear win, financially.
I think this circulation scheme wastes much more heat. Without the circulation, the water sitting in the lines would usually cool off before you use it. The heat departs the place you’re trying to keep it at this time. But, once it’s gone, that room temperature water in the lines is no longer wasting heat. In the circulation system, it’s wasting heat at the maximum rate all the time. To take an extreme example, if you use hot water at the tap once every year, you waste as much heat as it takes to heat maybe a gallon of water with the old method, but with the circulation pump going, you waste as much heat on this plumbing run as if you used water constantly. The power to run the pump would just be a little more waste on top of that.