Operation cost of electric vs. natural gas tankless water heater for radiant floor heat
We’re thinking of replacing our Rinnai natural gas tankless water heater with an electric tankless water heater such as a Stiebel Eltron. I’m having trouble figuring out the likely difference in the annual cost of operation. We use the tankless water heater both for on-demand hot water for domestic use as well as for heating a tank that heats the coils for our home’s radiant floors. We use about 683 therms a year for the radiant floor heat, and in our area of Montana, that costs around $700 a year. A conversion chart shows that 683 therms is equivalent to around 20,000 kWh of electricity, which would cost around for $2,500 a year at our utility rate of $0.125 per kWh. That seems very expensive for the cost of switching our gas for electric and I’m wondering if I’m taking into consideration all the relevant factors and calculating this correctly. Any thoughts on this are very welcome! BTW, we’re motivated by two reasons for considering this switch: First, our Rinnai shuts off with an error code whenever the outside temperature gets down to around -20F, which happens several times a winter and could be a disaster if we weren’t home. Second, we would like to have a greener home.
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Replies
WDS,
Not sure how reliable the electricity is in your area, so maybe the error code issue is better to pursue than a new form of heater. The wiring for what I am guessing would need to be a very large tankless electric heater could incur a big charge just to install. Operating it, as you note, would be 3.5 time more expensive. If you have demand pricing once going over a certain kwH usage, then maybe much more expensive. Check on that first.
Depending on the source of your electricity, the environmental difference could actually weigh in favor of the gas.
A tankless electric heater is a poor fit. An air to water heat pump would be the green, electric option. Should be about a third as expensive as tankless electric. You also won’t need 683 therms worth of electricity as some of those therms are wasted.
Agreed, there's almost no scenario where running resistance heat is greener than burning gas.
That said, a heat pump water heater won't work, he'll need a hydronic air-to-water heat pump. Which is niche technology. I think the only one that might work in his climate is the Arctic, https://www.arcticheatpumps.com/specifications.html
Keeping a gas boiler around for -20f is an easy compromise
The rather confusing chart for the Arctic heat pump seems to suggest that at -4F the output water is only 113F. The COP seems to be down below 2 as well.
I completely forgot about the Sanco heat pump water heaters. I believe they can extract useful heat down to -20F and the output temp is higher. They seem to have newer systems available now that may allow for useful residential heating capacity in cold climates. The one risk factor often mentioned is the water goes out to the pump and back. This necessitates a secure heating cable on the line to prevent freezing. Not sure what might happen if the power goes off and the whole unit freezes. The units are considered a bit pricey, but worth exploring.
The Sanco unit is only good for 8000BTU of space heat. You need a very efficient home to be able to make it work. It also doesn't fare well when used in space heat, unless the return water temperature is very cold the COP of the unit drops. In cold climate it will be bellow 2, maybe closer to 1.
Their technical documentation says that for a heating application:
* Maximum heating capacity must be less than 8,000 BTU/h.
* Minimum design ambient temperature must be above 27°F.
With a heat pump the COP depends on the difference between outdoor temperature and water temperature, so you have to specify both when you're talking about performance. As the chart shows, the bigger the difference the lower the COP. To figure out what kind of performance you're going to see you have to know the distribution of outdoor temperatures you're going to see. Basically you need a COP of 2+ for electric to be competitive with gas.
Arctic also offers a gas backup mode where it switches to gas when the outdoor temperature gets below a certain point. So you could keep your existing gas heater.
What temperature are you running now? 113F is actually kind of hot for radiant floors.
The lowest cost option is to install an air to air heat pump. Could be as simple as a hyper heat wall mount in the main living space or even better a ducted unit in the beasement. Since this will provide the bulk of your heat, it will be much cheaper to run than a resistance only unit.
The bit of heat load left over can then be handled by a resistance boiler and the existing hydronic system.
The rest of the options that are cheaper to run than your gas are a very expensive install.
Sorry to mislead. I thought I had seen a new large size Sanco that addressed the formerly low output. Maybe that was the one for apartment buildings. I was under the impression that the whole point of CO2 compressor tech was the ability to extract heat to much lower temps. I guess I better go find their literature again.
If WDS is already experiencing -20 frequently then ASHPs are going to be operating in a less favorable regime as well. The existing floor plan may not lend itself to easy substitution. The idea of a tankless water heater as a backup might be less onerous on the total panel demand.
Since the gas one is kicking out when its -20F, it might do to look for frosting effects that cause it to throw the code. I know the outside gas meter I had in Chicago would frost out a small screened port that caused a lockout of the gas until cleared. Perhaps mediating the intake air temp would put it back to rights.
Burning natural gas directly is almost always going to be cheaper than using electric resistance for heating, so your calculations are probably correct. Going to electric RESISTANCE over natural gas for heating is also usually NOT "greener", since the majority of electric power generation in most areas of the country is sourced from a mix of natural gas and coal, and after you factor in the system inefficiencies, you end up burning more fuel for the same amount of heat with an electric resistance heater. All you do is move the emissions somewhere else.
A heat pump is greener because it MOVES heat instead of MAKING heat directly, which is a big difference -- and it's how you can get more BTUs out per watt in compared to using those watts to heat directly with an electric resistance heater.
Your simplest option is probably to contact the manufacturer and see if there is some way around that error code and associated lockout.
Bill
All I can say is we have 14 year old electric boiler(don’t recall brand/model) for a 23 year old 3 story 1700 sq ft house with in- floor heat only in basement floor but all 3 floors are open to each other. Live in western Wisconsin.(basically mpls/st Paul)…
I’m going to gues for the coldest 6 months of heating season, electric for boiler runs about $500 a month.
I would say your calculations are probably correct…it is horribly expensive!…we supplement with 3-4 full cords of wood in a rais wood burning stove in living room…
Recently re-sheathed exterior with zip r sheathing and sealed seams, rain screen and then corten steel siding…guessing we are saving $25 bucks a month because of it.
One might say the $80,000 spent re-sheathing and re-siding may not have been cost effective…???