Is there a general guideline for how much more expensive it is to use an electric boiler versus a gas boiler for hydronic radiant floors?

In case it's important: We're building an ICF home. 3000 sq.ft on 3.25 floors (including basement). 3/4" Wood floors on upper floors.

Ideally we'd like to use no gas in the house at all.

This part is possibly the wrong forum but: we're looking at an inline hot water heater and it's been suggested that we might want to get a 300 gallon water tank so we have an ample supply of "room temperature" water. Does anyone have experience with doing this?

It looks like getting 400A service to our house will be costly but I'm not sure if 200A is enough. The maximum draw of the hot water heater alone is 120A. I'm trying to imagine if I'll be able to have a shower in winter when the furnace is on, and someone is running the oven.

Thanks,
Colin.

For electric vs gas the rates vary by quite a bit across the country. If you know the rates we could tell you what it might be right now, but who knows which will cost more down the road. If you know the numbers you can try filling in this online form and see what it comes up with.

That would cheap natural gas, we are paying just over $1.10 per therm. Roughly 100 cu ft is a therm, but here is a more detailed description.

But yes in your situation it looks like electricity will be three times the cost of natural gas.

Doing a electric boiler set up for a passive solar home in Boulder Colorado. We are doing electric boiler because we will have 6kW PV system, passive solar and active solar heating. So electric boiler will be backup to active hydronic solar which is backup to passive solar. The heat loss / demand will be too low to justify paying $15/month just for gas meter. $180 per year in meter charges (without any gas) is about what we project as our heating costs! Lower construction costs (no gasline, meter or venting), lower insurance for all electric and better air quality. Induction cooktop solves biggest problem with all electric homes.

300 gallons of "room temperature" water is an interesting idea. We have 45-50 deg water supply that needs to be heated to 120 degrees. If it could be PRE-heated to 70 degrees first, the recovery and efficiency of any hot water heater would be improved. My thought is you would only need 20 -30 gallons (or one long shower). Instead of a tank it could be a 200' coil of 1' copper pipe in the utility room. Would have more surface area exposed to air temperture and would not have mixing problem.

It would be very helpful when asking for an option on whether a system will work, if you stated WHERE the project is located.
Your project could be in Florida or Alaska, and you will get different answers in those two extremes.
The single most important piece of information when discussing a HVAC system, a solar system or an insulation system is location, location, location.
Depending on location, solar hot water could be the solution!

Hmmm... more interesting numbers:

"2005 Department of Energy report that rated residential energy sources by the average dollar cost per million BTUs of heat they produce. Natural gas was found to be $15.13 while electricity was $27.26."

Since electrical heating equipment lasts longer, costs less to buy and less to install (no venting) I wouldn't mind if the actual premium were 80%.

I wonder why our calculations aren't even close to that?

There are 35.3 cu.ft. in a cu.m. so we're paying $1.19/therm for gas.

Re: Location

Sorry, I forgot to add that bit: We're in Toronto. We measured the tap water temperature (where we're renting during construction, but close enough) at 38'F! Yikes! It's too cold/not-enough-light to make solar hot water heating a practical option. We've done the calculations and it never pays back.


Re: Lots of line instead of tank

I'm still working out the details on the tank option, but I *think* because the water leaves a line it doesn't re-pressurize when its leaving the tank and then requires a pump on the other side. So keeping it in one long pipe would be a better option. (Part of me thinks it shouldn't matter because it's a closed system, but I seem to recall, that one can't just expand and contract piping at will -- it loses its pressure every time the pipe grows.)

The incoming line is 3/4". And I think that gets split into two 1/2" lines (hot/cold) -- we only want to bring the hot line up to room temperature.

It takes about 5.1" of 1/2" line to make 1 cu.inch. That makes 61' of line for 1 cu.ft. or 7.5 gallons. So I guess 200' is actually still about right, but 1/2" line instead of 1". If we could put that into a large thermal mass (concrete) that could be our source of 'room temperatures' to be drawn on as the water comes in. When water isn't flowing, the block could gradually come back to room temperature. Hmm...

Have you done a manual-J type heat loss calc on the house? That & that alone should tell you how much boiler you need.

The 300gallon tank may have been recommended not for it's mass, but for the rating of the elements, to be able to supply then necessary heat.

$0.13CDN/kwh would be CHEAP in my neighborhood, as would $1.19CDN/therm gas, but there are plenty of places in the US with rates that low.

One therm=29.4kwh, but whereas electricity is used at near 100% efficiency, natural gas needs derating by the average efficiency. AFUE is a funky test designed to give a sense of the average annual operating efficiency, but it has several flaws. In order to have any semblance to reality the size of the burner needs to be somewhere around what a manual-J heat loss calculation would give you, and the implementation within the heating system (like the required operating temperature you need to deliver the heat to the zones) makes a huge difference too. That said, with radiant heating most houses can run at low temperatures, and a properly size modulating & condensing boiler can deliver in-system performance of ~90% efficiency. (I wouldn't assume more than that unless the radiant tubing is embedded in concrete everywhere.)

So, for every therm in, you get 0.9 therms, or 26.5kwh-worth delivered to the radiant floor. In electricity that'd cost you

$0.13 x 26.5= $3.45

whereas in natural gas it costs you $1.19

So let's say you cheap out, go with an 80% AFUE properly sized boiler. One therm in delivers 0.8 therms, or 23.5kwh-worth to the floor. In electricity that would be costing you:

$0.13 x 23.5- $3.06

vs.

$1.19, if delivered as natural gas.

The back of my napkin sez that's a 157% fuel premium you'd be paying for doing it with electricity vs. gas with a barely-legal efficiency gas burner or a 190% premium over going with a high-efficiency burner.

It's your money, your decision. But whatever you do, calculate the heat load carefully, and don't buy any bigger equipment than absolutely necessary. Cycling any heating equipment more often than necessary reduces it's service life, and the operational efficiency of gas burners drops significantly with oversizing (somewhat less so with condensing gas appliances but still some. Don't oversize it by more than 50%.)

200A @ 120V= 24 kilowatts , or ~82KBTU/hour, which would be more than enough strictly for heating for most ICF houses that size in my neighborhood, not sure about yours. If the elements on your electric boiler/tank are rated 120A, that's 14.4kw or ~49kbtu/h, which still might do it, depending on the amount of glazed area you have and the quality of those windows. (My ~2000sf house in central MA has a heat load about half that, and I'm sure it's not as tight or well insulated as new ICF construction, but our mid-winter weather isn't as cold either.)

Maybe I've been misconstruing you, perhaps by "inline water heater" you mean "on-demand" electric hot water, and the 300gallon tank is a tempering-tank for the incoming water (for storage, allowing the water to heat up slowly from room heat, not actively heating the water)? If that's the case, letting it sit at room temperature may take the peak load off the on-demand heater, but doesn't improve the efficiency. At 300 gallons you might as well run IT with lower-wattage elements, since unless you're filling multiple hot-tubs you're not going to use anything like 300gallons/day, and the standby losses of electric tank heaters are extremely low compared to their gas-fired-tank brethren. Leaving volumes of potable water at tepid-temps for long periods of time may in fact be dangerous, since pathogens (eg. legionella) breed more quickly at warmer-than-cool temps, and aren't necessarily killed instantly in your flash heater (but would be in a 120F+ tank.) Smaller tempering tanks are done all the time as pre-heat to point-of-use mini-tanks or flash water heaters. Keeping the size small keeps the stagnation/breeding time low enough to avoid the issue.

>Have you done a manual-J type heat loss calc on the house?

I'll have to check with the mechanical eng. I'm sure they had to do some sort of heat-loss calc for us to get the permit.


>don't buy any bigger equipment than absolutely necessary

I believe we were looking at a triangle-tube system that scales itself up and down based on the most efficient operation.

> perhaps by "inline water heater" you mean "on-demand"

Sorry; yes I did. The large tank was for storage of room temperature water. The quantity was so that as cold water came in, there would be too much to room temperature water to be significantly changed.

It seems to me even if we have to go with a gas system, it would still benefit from the idea of having the water travel through 200' of pipe before it gets to the domestic hot water heater.

Thanks for all the feedback! Perhaps we'll revisit the issue when PV hits the big time and resulting electricity costs are much lower.

If all goes well, our footings are getting poured tomorrow! Woo!

If you embed that 200' of pipe in the slab-edge of the radiant basement slab it should be plenty of pre-heat tempering with minimal condensation potential. If it's suspended in air it'll "sweat" considerably during periods of high humidity.

It's still possible to oversize a modulating-condensing boiler like the Triangle-Tube (which have an excellent reputation, BTW). It doesn't hurt to ask where they got the numbers, but if they did their homework the mechanical contractors probably recommended the right one. Mod-cons are pretty flexible though, and it's easy to get lazy based on that, and some (incorrectly) assume that the most-efficient modulation is at lowest fire, and intentionally oversize 'em. (At lowest modulation there is typically less turbulence on the fire-side of the heat exchangers, reducing the heat transfer efficiency. They could modulate lower, but the efficiency drops off a cliff if they do. The sweet spot is usually somewhere in the middle-third, but it varies by heat exchanger design, and absolute return-water temp, etc.)

A separate question for the professional gallery here:

In low-loss high mass high thermal-lag envelopes like ICF buildings, do any manufacturers have different algorithms on outdoor-reset control? Seems to me you could squeak a few percent better efficiency out of a mod-con using less modulation and longer averaged temps based on high lag of the thermal mass using tweakable PID algorithms, etc.