Point of use water heater as whole home booster?
I currently have two tank water heaters in series. One is a 50 gallon heat pump unit that I bought used which only ever worked in pure electric mode, the other is an 80 gallon electric. The reason for two water heaters is that the house was designed with in floor hydronic heating, sharing with the DHW. I’ve pretty much abandoned the in floor heating in favour of air source heat pumps. I’m looking to install a heat pump water heater, and because of the size of it I’ll probably have to remove both the current tank water heaters. I’d kind of like to maintain some amount of extra capacity, and I just came up with the idea of putting a 7.2kW tankless heater in series with the heat pump water heater. The tankless would be connected to the output of the heat pump water heater. The idea is that if I set the temp of the tankless unit below the temp of the tank unit, so that it would only come on in the rare case of a large hot water draw that exceeds the tank’s draw down capacity. Since the tank will still be supplying pretty warm water, the tankless should be able to provide 49degC water for quite a while. The bonus is I already have the wiring there for it, it takes up less space than an extra tank, and it will have no standby losses. Is there a problem with this plan that I cannot see? Will the tankless heater not like receiving hot water at its input?
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I'm bored waiting in an emergency room, so I went and read the specification sheet for an electric tankless water heater.
They list a minimum / maximum pressure, but not a minimum or maximum input temperature.
They did list a maximum output temperature, but it's probably high enough that it wouldn't impact the application you suggest.
Rheem actually makes a product like this...
https://www.rheem.com/innovations/innovation_residential/water-heater-booster/
Not an expert, but I don't see a problem. It's equivalent to having a desuperheater tank from a ground source heat pump. And that is done all the time.
I would check with the manufacturer first. Tank-type water heaters are used after tankless heaters all the time, but the tank-type water heaters operate very differently, with heat exchanger systems designed to operate with a big tank of water that is usually mostly hot. Tankless water heaters have a heat exchanger that has to transfer heat from the heat source to the water at a very rapid rate, and if the incoming water is too hot, you MIGHT have issues with heat rise of the heater itself. This might just cause cycling, but there may be other issues.
It would be worth a call to the manufacturer to be sure what you want to do will be safe and not cause any issues with the tankless unit or it’s warranty.
Note also that electric resistance tankless water heaters are generally frowned upon as a green way to heat water. They don’t play nice with the electric grid.
Bill
Bill,
Given the existence of the above mentioned for-purpose booster, I suspect there may indeed be an issue of the heater trying to heat already heated water. All that would be required to solve this is a sensor that doesn't allow the heater to turn on above a certain incoming water temperature. They appear to charge a lot for this added feature which probably costs about 10 cents, because they can. I'm also now having a hard time finding a unit for sale that I can hook up to my extant 10AWG wiring. Most are 30A or higher. The electrical code here in Canada is more conservative than the US, and a 30A device can't be run on 30A wiring or breakers. That limits the heater size I can use to 6kW. As for playing nice with the grid, that concern is valid for whole house heaters, drawing anywhere from 15-45kW. This booster heater doesn't draw much more than a standard electric tank heater, and less than a cooking range with multiple elements running.
Steve,
Thanks. That Rheem unit would work nicely, except that it requires 8AWG and a 40A breaker. Weirdly, the install instructions don't directly specify the wiring requirements, but it does say they have to comply with local codes, which contradicts another part of the manual that says to use existing wiring for the water heater. It says to hook up the existing wiring to it, then daisy chain that to the tank water heater. This is bananas. This thing draws 30A. A standard tank water heater draws about 20-25A, and will almost certainly be hooked up with (at most) 10AWG wiring. There's nothing in this setup that will prevent both of the units from running at the same time, which means 50-55A on a 30A (at most) circuit.
I think you will find code will require 2 circuits one for each heater
To my eye the Rheem tankless electric water heaters look to be rebadged Eemax units.
http://www.eemax.com/
Walta
Yes, those are definitely the exact same unit, and even the same user manual. I think the way they are able to hook the two devices up to the same circuit is that the unit actually shuts off power to the slave tank water heater when it is actively heating; they mention a relay in the troubleshooting section relating to if the tank water heater is not heating. This is fine for a "dumb" style tank, but it's obviously less than ideal for a computer controlled device, and especially one with a compressor that you don't want repeatedly having its power cut in the middle of a cycle.
These folks spec their heater with warmer inlet temps:
https://www.ecosmartus.com/support/manuals
They also modulate to keep output temp at set point, so it should work. The issue is the smallest unit needs a feed sized for a stove...
Their lowest output booster is 8kW, which I can't hook up to my 10AWG pre existing wiring. The 6kW model is "non-thermostatic", which I think implies that it just goes balls to the wall as long as water is flowing through it, which obviously isn't suitable for on-demand boosting.
Edit: I see they have a line that appears to be thermostatically controlled, ending in the letter T. Now my only problem is finding a place to buy one, and seeing if it is economically feasible.
Might be easier to get a small tank water heater with a big element.
There are 4.5kw 20 gallon units, would hook up to your existing wiring and about the only extra running cost is a bit more standby loss.
If you are looking at the smaller units be sure to check the max flow rate. A lot of the fitting look like 3/8-1/2. If you feed all the hot water for the house thru a 3/8 lines you could see some pressure problems.
Again code requires each appliance be on its own circuit. I try to remember the code is written in blood each line is there because someone died and not to inconvenient me.
You currently have 2 heaters presumably on 2 circuits and when you are done you will have 2 heaters on 2 circuits, as long as you do not exceed the capacity of the present wires and breakers. If that is true I do not understand this quote “It says to hook up the existing wiring to it, then daisy chain that to the tank water heater. This is bananas. This thing draws 30A. A standard tank water heater draws about 20-25A, and will almost certainly be hooked up with (at most) 10AWG wiring. There's nothing in this setup that will prevent both of the units from running at the same time, which means 50-55A on a 30A (at most) circuit.”
I am also very concerned about a “pretty much abandoned” combined Hydronic and DHW system. Many locations prohibit combined system for this very reason. The water in the abandoned part of the system is stagnant over time bacteria can grow in the stagnant water. When there is a disruption in the system the bacteria can enter the domestic part of the system and make people sick. I think you should disconnect and abandon the heating loop or use it. Pretty much abandoned is a dangerous game.
Walta
Two clarifications:
Yes, I do have two 30A circuits running two water heaters. I quoted exactly what was in the install manual mainly for interest's sake, not because I had any intention of following it.
You've said a couple of times that code requires each appliance to have its own circuit. I don't think this is absolutely true. I can only speak for the Canadian code, which as far as I can tell is actually more conservative than the American one. But here the default way of installing a separate cook top and wall oven is to feed them both from the same circuit. This is spelled out specifically in the code. I'd be surprised if it's different in the US. Rheem is a well established leader in water heating equipment. If they have a product manual that explicitly tells you to hook up the water heater through the device, I suspect that it is code compliant and I suspect this is because it is designed to disable one when the other is operating (in effect making the booster and tank heater operate as a single device).
By pretty much abandoned, I mean that it doesn't get used for heating the house anymore, and is not likely to do so again. The design of the system means that all water that goes into the water heater goes through the floor loop first; this is equivalent to the well just being further away, and so there is no more chance of the water stagnating as the water in any plumbing system.
You are generally allowed to tap multiple devices off of a single circuit as long as that circuit is sized sufficiently to be safe, and each device can handle running on whatever size circuit you use. Sometimes in the first case you are required to provide separate disconnects for each device (which is always nice for serviceability anyway), and a disconnect is not necessarily the same thing as a circuit breaker or fuse. In the second case, you get into issues with “tap conductors” such as is the case with 30A circuits not being permitted to feed 20A and smaller receptacles. 18 gauge wire is considered too small for a tap off of a 30A circuit, and smaller receptacles would allow connection of little appliances that could burn up with tripping a 30A breaker in case of a fault.
Trevor, your Canadian code probably doesn’t want an appliance connected to a circuit that could run continuously at the maximum power for that circuit. We have that too in US code, it’s commonly known as the “80 percent rule”. You can’t load up a normal circuit to more than 80% of rated capacity in continuous service. The result is a need to run a “30A” device on a 40A circuit where it would only be 75% of that circuits rated capacity.
What I’ve always found interesting in Canadian code is that T90 wire is almost like our NM-B, but with a slightly beefier jacket. I like the red and blue jackets you guys have for 240v-only and AFCI circuit identification too. Our jacket color code (white, yellow, orange) is only for wire gauge (14, 12, 10), basically to make the inspectors job easier.
Bill
“By pretty much abandoned, I mean that it doesn't get used for heating the house anymore, and is not likely to do so again. The design of the system means that all water that goes into the water heater goes through the floor loop first; this is equivalent to the well just being further away, and so there is no more chance of the water stagnating as the water in any plumbing system.”
You must have a very clever plumber if the system is laid out in such a way that every pipe has water flowing thru it even if the pump never runs. My guess is if you draw out your system on a sheet of paper you will find the pipes going to and from the pump have no flow unless the pump runs and opens the check valve.
If you want to keep the system as an option, put a timer on the pump and run it the 5 minutes a day or week.
It's actually pretty simple. You literally cannot get water into the tank unless it goes through the whole radiant heating circuit. Check out the diagram at the link.
https://www.radiantcompany.com/system/opensystem/