Can I use an tankless electric DHW heater like this?

[mpilihp]

So we heat our DHW year round with our wood boiler using two indirect DHW tanks. This has worked well with our older small wood boiler. I replaced it with a THermoControl 2000 unit last fall and while it works great in the winter but during the summer I'm finding it takes too much wood to bring it up to temp again and because its physically larger it puts off a lot more heat.

So what I want to do is put a tankless electric DHW heater in line after my indirect DHW tank. Reading about one model the EeMax 240013 it sounds like it can take in pre-heated water and only heat to the selected temp, IE say 120 deg. This way its automatic and I do not need to select either the indirect or the tankless source of hot water.

Does anyone have experience with using a tankless unit like this?

Thank You

~ Phil

 

[maple1]

You should be able to do that. Tankless haters take big expensive wire & breakers though. And big panel space.

I would consider just putting in an ordinary electric hot water heater and letting the fire go out all together. I could heat DHW all summer only burning once a week, but I don't. Just not worth it to me unless I have junk wood to get rid of. Electric only runs me around $25/mo.

 

[mpilihp]

Hi thanks ill try to verify ot with the manufacturer also. A electric tank is expensive and takes up a lot of space which i dont have much of. The tankless needs a 60 amp breaker which i have an open one in my panel already and ill just need to buy some #6 wire.

Thanks

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[Ashful]

Why not just replace the most downstream of your current two indirect tanks with an electric water heater, and run it as you had proposed for the tankless?

 

[maple1]

Ordinary electric water heaters are cheap. How much are the on-demands?

 

[mpilihp]

Well having two indirects let us fire every other day during shoulder season and have plenty of water. Also i dont want to pay for running the electric unit when i am going to do a firing.. i like the idea of the tankless unit being able to top off the temp only when we need it.


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[mpilihp]

Ordinary electric water heaters are cheap. How much are the on-demands?

The one im looking at is $300, a 40 gal electric one is 350 and up and it takes up a lot more space.



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[nhtreehouse]

A couple of comments - Although the on-demand electric heaters seem like a great idea, the other item to think about is the lifespan of the on-demand units. I looked at them about 6 months ago and the warranties I saw were in the 1 year range. After some reading on the internet I learned that scaling is a serious issue with these units - so you ought to think pretty hard on what kind of water you have and if that is an issue. $300 seems awfully cheap for a on-demand which has a 14kW rating (60*240).

FWIW, I'm heating DHW with a 23 year old 40 gallon propane tank unit. I seriously doubt that you will get that kind of lifespan out of a on-demand unit. The physics are totally different than a tank unit.

I'm working on installing a wood boiler and from the research I have done, electric tank units like maple is referring to seem like the way to go. Once I get the time or when this one dies, I'll probably go with a small electric tank unit, possibly preheated in the winter with an indirect if I can figure out how to plumb it up.

 

[mpilihp]

Well the tankless model I am looking at has a 5 yr warranty, at Lowes the Whirlpool 40 gal electric tank has a 6 year warranty.

Also I dont think my water is too prone to scaling as our old oil boiler which has a coil for DHW in is about 26 years old and when I switched over to wood and put in the indirect tank (just 1 to start) it was probably 18 years old and the DHW coil in it was still working fine. The coil in that is very small diameter pipe and it was still flowing so probably not a concern for me.

The same thing can happen to indirect tanks and these have been working fine. Also for the majority of the time the water will be heated in the tanks and probably sit there for some time so I would think the creation of scale would be happening in the tanks mainly.

~ Phil

 

[maple1]

If I was doing over, I would likely consider an indirect that also had elements in it. I don't think they are that common, but might be in the solar world. That would eliminate my heat exchanger and one circulator and some controls.

I was also thinking that scaling goes up with the dT at the heating source. Which is a lot bigger at an electric element than an indirect or tankless coil. And likely even more at an on-demand element using 60 amps. But I don't really know.

 

[Chris Hoskin]

Well the tankless model I am looking at has a 5 yr warranty, at Lowes the Whirlpool 40 gal electric tank has a 6 year warranty.

Also I dont think my water is too prone to scaling as our old oil boiler which has a coil for DHW in is about 26 years old and when I switched over to wood and put in the indirect tank (just 1 to start) it was probably 18 years old and the DHW coil in it was still working fine. The coil in that is very small diameter pipe and it was still flowing so probably not a concern for me.

The same thing can happen to indirect tanks and these have been working fine. Also for the majority of the time the water will be heated in the tanks and probably sit there for some time so I would think the creation of scale would be happening in the tanks mainly.

~ Phil

I agree, Phil, it sounds like your water is not particularly prone to scaling. Also, scaling tends to happen more when the same water is heated and re-heated (like in a tank) and when the water temp is heated to higher temps which drives the minerals out of the water. Shower temp is about 105F so tankless water heaters tend to operate at much lower temps than tank-type heaters. Also, no need for high temps to eliminate legionella (sp?) because the water heater is always being flushed with fresh water. As long as the Eemax recognizes incoming water temperature and you have the space in your electrical panel, I think this is a great idea. My only hesitation is that 14kW is a bit small if you have no pre-heating at all. If I remember correctly, 19kW will give you about 2gpm at 65F degree rise (50F to 105F, for example). Good luck with the project and let us know what you end up doing and how it works out.

 

[NateB]

What about a heat pump water heater for your indirect tanks? It would keep the tanks hot and dehumidify the space they are in.

 

[Chris Hoskin]

What about a heat pump water heater for your indirect tanks? It would keep the tanks hot and dehumidify the space they are in.

Is anyone aware of a heat pump water heater that also has an input coil in it? That would be a fantastic solution, I think.

 

[maple1]

What is an input coil?

A Geyser is what I was picturing re. Nateb's post.

 

[NateB]

Is anyone aware of a heat pump water heater that also has an input coil in it? That would be a fantastic solution, I think.

The heat pump I got from Tom had a 1/2 inch input and output. I am going to use it to heat my 820 heat bank, so I have dhw in the summer. Oh and a dehumidified basement.

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[mpilihp]

Im only looking for a solution for when we run out of hot water, ie kids visiting, or its too hot to fure the wood boiler. Ive figured out i can change a few breakers and dit a 60 amp in and still have some open slots. U have to travel for work next week but plan on ordering one soon.

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[Chris Hoskin]

What is an input coil?

A Geyser is what I was picturing re. Nateb's post.

An indirect water heater has a coil in it through which hot water from a boiler system circulates to heat the water in the indirect water heater. This instead of electric heating elements or a burner of some kind. A "solar indirect water heater" has two coils - one for heat coming from the boiler and a second for heat coming from a solar collector. This is what I meant by "input coil". I was thinking that a heat pump water heater that also had this kind of coil would allow you to heat the water heater with the wood fired boiler/thermal storage and the heat pump would take over automatically if the wood system was not running or able to keep up. Of course you could do this with a regular electric water heater or oil/gas -fired water heater, but the heat pump is much more efficient and could more easily be run off of solar PV.

 

[Ashful]

The heat pump I got from Tom had a 1/2 inch input and output. I am going to use it to heat my 820 heat bank, so I have dhw in the summer. Oh and a dehumidified basement.

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This is my issue with the indirect HPWHs. I haven't found one with 3/4" or larger in/out, or a means of connecting into my boilermate without going coaxial and restricting flow. These things are aimed at tiny houses.

 

[duramaxman05]

If you are wanting to use your owb to heat the water, we use a plate exchanger as a tankless water heater. In the winter time we turn oue electric hot water heater off and turn the valves off to it. The plate exchanger puts out plenty of hot water for a long shower. I would recommend a mixing valve because the water is hot. We have been using this set up for 3yrs no and no problems.