pre-heating with electric tankless

IZZY

Sometimes I hear an idea an immediately shut it down as ridiculous, but after a little thought realize that maybe not so much...My mother's house has an old, but totally functional, natural gas boiler for a single pump, 3 zone hot water heating system (baseboard)- standard builder's special for where she lives. She recently put in a solar electric system which produces way more electricity than she needs, especially in the winter when the central air is not used.
So here's the question, could an electric tankless water heater be cut into the 1" return before the water returns to the boiler to use less gas for heating the return water? Obviously this would be beneficial only on start up, since once the water hits 190 and the delta T is around 20 degrees, it would basically be passing through the tankless without gaining much, and the electricity is basically free anyway.
The one issue I can think of is potential condensation from short cycling of the burner.
Any thoughts would be greatly appreciated.

Jamie Hall

Well, the most obvious questions are first, does she really have a surplus of solar electgricity? Some programs use net metering and average over a year, so that a surplus in, say, the winter, is used to offset an excess demand in, say, the summer. That's worth checking.

Second, how much power does the array generate in full sun (obviously it doesn't generate any on cloudy days or at night) and how is that accounted for in the electricity use/payment scheme? Again, net metering?

Related to the second, what would the power demand of your tankless be vs. the output of the array? Nist tankless heaters use a lot of power, Even a fairly small one may use upwards of 30 KW. How does that compare with her array power?

IZZY

ooh, that's exactly the kind of direction I was hoping for! Thank you. I'll have to find out about that. Outside the box ideas are great, but it's always the things you don't know that trip you up.

EBEBRATT-Ed

Not a good idea.

Hot_water_fan

Unconventional, but if it’s truly being otherwise unused, do it! You could also just use an electric baseboard. 

@Jamie Hall as a supplementary system, it can be small. 30kw is not a helpful size here. 

IZZY

thanks guys for the quick responses, much appreciated 👍🏻 

bburd

A tank type electric water heater would make more sense than a tankless. It could store the solar energy in the form of hot water slowly, even when water is not being drawn.

IZZY

that's true, but I don't have much floor space, so the tankless seemed like a more practical option. thank you. 

GroundUp

A tankless WH has a ridiculously high pressure drop/resistance to flow, so piping it in series would destroy the flow in the rest of the system. Do not do that. Perhaps something like an electric boiler may be of assistance, with minimal pressure drop and only heating until setpoint is met or something- depending on how much excess power is available.

IZZY

that's a good point, I hadn't thought about that. I'm not looking to change the boiler out, maybe when this one craps out though. thank you.

Hot_water_fan

that's a good point, I hadn't thought about that. I'm not looking to change the boiler out, maybe when this one craps out though. thank you.

An electric boiler is what you want to use. They're cheap. They're small. Don't use a tankless water heater (like for a shower sold at Home Depot). You want want designed for central heating - they have close to 0 head loss.

IZZY

thanks. seems like that's the way to go, but definitely not doing something radical to the heating system that doesn't need to be done smack in the middle of heating season, that would just be asking for trouble.

jumper

Hot_water_fan said:

Unconventional, but if it’s truly being otherwise unused, do it! You could also just use an electric baseboard. 

;

Simple to do. Often we overlook the obvious. Plug in oil filled is even easier.

jumper

bburd said:

A tank type electric water heater would make more sense than a tankless. It could store the solar energy in the form of hot water slowly, even when water is not being drawn.

Thermal storage is most economical. Change controls for more capacity by means of higher temperature.

Zman

Prices will vary from region to region, but in general, natural gas is much cheaper on a "per BTU" basis. Unless she is producing way more electricity than she uses on an annual basis, she is probably better off heating with gas and paying little or nothing for electricity.

hot_rod

Resol makes a PV control that can determine when there is excess PV generation. Instead of sending it back to the grid at low KWh prices it turns on a thermal tank

so a large  electric water heater tied just to a PV array would store excessive PV as thermal for heating

Tie this tank into the heating system as a parallel load. It runs down to the lowest usable temperature, then the gas take over

A simple delta t controller would make that decision. Once the tank gains temperature again  boiler goes off line, PV electric/ thermal takes over again

IZZY

that's awesome. she really doesn't have the room for a separate storage tank, and I'm not particularly savvy with controls wiring, but that is definitely something I'll explore for after the heating season when I'm not under the gun and have time to "play around", thank you!