Two hot water heaters - best configuration

ensmingeraa

Longtime reader, first time poster.

Through a series of remodels/circumstances I ended up with extra hot water heaters.

#1 Right now domestic hot water in my home is actually supplied through a 50gal gas unit that I am soon getting rid of (its old/used/free, and I'm removing the vertical vent pipe it uses). So this is leaving soon.

#2 I've got a condensing boiler (natural gas) that supplies our radiant/baseboard heat, and it has capability to DWH, and the supply is piped in anticipation of future use, but actual domestic hot water output goes nowhere for now.

#3 I also have a basically brand new 40gal electric hw tank that is not hooked up. I have a circuit for this already, so that is not an issue.

So, the question is - when I remove the current gas unit #1 - should I attempt some configuration of the #2 and #3 that maintains efficiency of the "tankless" dhw from the boiler, but minimizes the downsides? My first idea is boiler->electric->house in a series configuration, but ultimately im not able to explain why that would be a good idea, and not be a energy waste. I'm guessing the electric unit uses more energy heating cold water than keeping its tank warm, so feeding it hot water would mitigate some energy use?

The compounding reason to not simply use the boiler dhw only is: there may be a summer where I decide to re-route the exhaust for that thing, and then I lose my hot water too, so I like the idea of having this electric one around, plus I already own it.

For reference/context, we are a family of 3, likely to be 4 one day. We live in the foothills of CO, and the house is like 2300sf or something, so not huge but not a tiny cabin, and we are going to be remodeling for (another) 5 years or so at least. So there are lots of configurations/reconfigurations of the house that are on the table - flexibility is a part of the equation here.

Any advice on the best approach/combo of #2 and #3 would be great. I'm just trying to put my basement water heater collection to good use!

Thanks,
aae

ps: https://forum.heatinghelp.com/discussion/comment/1191266
I read this ^^ post but thought my situation was different enough to warrant its own discussion

JUGHNE

If you want to use the 40 gal electric, you will need a heat exchanger for the boiler water (needs a pump) to pass thru and then your domestic HW would come out of the exchanger. (also need a brass or SS pump for the DHW...price one first).
This is something with 4 pipe connections to it.

An indirect tank (4 piping connections) has the exchanger inside for the boiler water to heat your DHW which is in the tank surrounding the exchanger. (only one pump...could be iron)

IIWM, I would go for a 45-50 gallon indirect (4 people) and leave provisions to cut the electric in for temp use while remodeling.

Jon_blaney

Do you have a tankless coil in the boiler or is DHW to be provided by a zone off the boiler to an indirect tank?

hot_rod

If you could use the 40 electric, heated by the boiler, then you would have electric element back up when the boiler is off. Slow recovery with electric tanks, know that.

You could use the combi feature of the boiler with a stainless recirc pump to heat the electric tank, maybe. No heat exchanger required.

I think a setpoint control (aquastat) could call on the recirc pump which would fire the boiler on DHW call. Just like opening a faucet really. That would be the least expensive way to use what you have.
Cost you a circulator and temperature controller.

JUGHNE

So this is a combi boiler? I guess I failed to read that out of the statement.
I was thinking of a ModCon with indirect priority control.

ensmingeraa

Thanks for the responses/questions! This is my first rodeo (as you can probably tell) - I'll clarify what I can, and post some pictures to see if that helps.

The boiler IS a combi. It is a Weil-McLain WMB-120C. No indirect tank or anything.

The system I'm imagining is something like hot_rod is saying, where the combi coil feeds the electric tank which then feeds the house. I didn't anticipate the need for a recirc pump, and I'm not sure I understand what thats needed for. Right now, the only output of the combi DHW is a utility tap that I put on there just to test functionality. If I turn this on, I hear the combi turn on and I get hot water pretty quickly. Tee'd off of this is the real deal future output, that currently just has a shutoff valve on it. This is what I'm imagining would feed into the "cold" input of the 40 gal electric. Then, whenever a tap in the house calls for DHW, it starts to pull from the electric tank, which, instead of being refilled with cold water, will be pulling water through the combi, so it gets heated on its way in.

Here is a photo (40 gal electric not pictured), please don't judge me too hard, but also don't be afraid to ask questions about the rest of the system if something looks whacky. It was getting into the chilly part of October a few years ago when I was designing/installing this, and this was my first experience in almost all of it - although I did have this book by John Siegenthaler as my general instructional guidebook.

ensmingeraa

As a followup, and at the risk of confusing the issue more with chicken-scratch - here is a rough diagram of what I'm imagining. And to clarify, I'm mostly trying to figure out if there is some advantage or disadvantage to doing it this way. Given the equipment I have, what would you do? Or perhaps I am making busy work for myself for no reason?

I hope this diagram clarifies the boiler DHW pictured/noted above, connected to a 40 gallon electric tank, and then to the rest of the house.

Thanks all,

hot_rod

Unless you keep the electric tank plugged in the temperature will drop when not used for a period of time. So you would need to flow out enough water for the combi to refill it with hot enough water.

So by adding a small circulator between the combi and the tank, it keeps the tank at the desired temperature.
Many suggest maintaining tanks at 140 for protection against bacteria growth like legionella. Then a mixing valve reduces it to 120 for use in the home.

You could also tie the combi and tank together with some 3 way valves, manual or automatic to select which source you want to use.

If this is a temporary use maybe just plugging in the tank is adequate, as you have drawn, the element just maintains temperature the combi does most of the heating. Basically costing you some electricity with the gas, depending how long it sits idle. Newer tanks are fairly well insulated so standby loss may not be a concern.

ensmingeraa

I think I see what you are saying. Putting a little circulator pump on means I don't have to even hook up the electric at all if I don't want to, which might actually be the best configuration. The piping is obviously a touch more complex, but thats fine.

I hadn't thought of using the electric only as a tank, unhooked. I like the idea of being able to use that 220 circuit for something else then. So, that's probably the configuration I will go with once I feel like the combi boiler is permanent in its usage. In the meantime, I think I will hook up it up in series, with electric, and hope that the standby loss isn't that bad. Maybe I'll even get one of those water heater jackets to help with that.

Thanks for the advice all, cheers!

hot_rod

ensmingeraa said:

I think I see what you are saying. Putting a little circulator pump on means I don't have to even hook up the electric at all if I don't want to, which might actually be the best configuration. The piping is obviously a touch more complex, but thats fine.

I hadn't thought of using the electric only as a tank, unhooked. I like the idea of being able to use that 220 circuit for something else then. So, that's probably the configuration I will go with once I feel like the combi boiler is permanent in its usage. In the meantime, I think I will hook up it up in series, with electric, and hope that the standby loss isn't that bad. Maybe I'll even get one of those water heater jackets to help with that.

Thanks for the advice all, cheers!

There are some advantages to having some tank capacity. I see Rinnai no has tankless WH strapped omnto tanks. It give you some dump load, eliminates the cold sandwich that tankless and combis experience.

For you, it would come down to how quickly the tank loses temperature, your frequency of use.