Tankless Water Heater: Electric vs. Liquid Propane
Pinoli
| Posted in General Questions on
If you have decided to replace an old tank water heater, currently using liquid propane, with a tankless water heater, would you continue to use liquid propane or would you switch to electric. Assume your existing panel could accommodate the electric. What are the advantages and disadvantages? Keep in mind I am a homeowner and NOT a professional builder or have other expertise. Thanks in advance for your insights.
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If the total cost (wiring, operation, etc) is similar, go with "getting cleaner" electrification. But a heat pump tank water heater would be greener.
The only real knock against electric is that they can get expensive to operate if you need a lot of water.
I have limited space, which was the driver behind the tankless. Also, the house is at 8500 feet and we get cold in Colorado, so it must be inside the house. I like the heat pump idea, so suggestions are welcome for space considerations.
At 8500' in CO the wintertime incoming water temps are cold enough that even the largest electric tankless systems don't quite cut it. Even if the electricity is free and you don't mind spending the money for wires fat enough to tow the F-150 out of the snowbank with the limitations on max flow are severe.
Heat pump water heaters are pretty bulky- slightly bigger than a typical electric water heater of equal volume, and 2/3 or more of the heat is being supplied indirectly by the heating system. If the heating system runs off LP, it's not substantially more efficient than a condensing LP tankless.
Even the 2nd largest Stiebel Eltron Tempra 29 will heat 2.7 GPM from 32F to 105F. That's about triple the GPM I shower at. They recommend a not-unusual 200A service.
Heat pump water heaters don't work tankless, the BTU/hr just aren't there.
A conventional electric tankless water heater uses a lot of electricity, make sure your service can handle it.
If you're definitely going tankless, propane is usually the best bet.
Thanks for the comment about propane. Can you tell me your thinking why propane is better? Not challenging you, just don’t know anything.
Does your house have solar? If so, then electric may be the way to go.
Let me just throw out some numbers for the sake of example. Let's assume you like your shower water at 105F, your showerhead produces 2.5 gallons per minute, and that in Colorado your incoming water in the winter is at 40F. Heaters are rated in BTU/hour, where a BTU is the amount of heat it takes to raise one pound of water by one degree. A gallon of water is 8.3 pounds. So to heat that 2.5 gallons per minute by 65 degrees takes 2.5*65*8.3=1348 BTU/minute. Multiply that by 60 to get BTU/hour= 80,925. So that's the size of a water heater you need. (If you want to support multiple simultaneous showers or other high-usage you'd need a bigger one.)
That's a pretty small propane heater, it will burn about a gallon of propane an hour, your existing tank and plumbing will be fine.
Electric heaters are rated in watts. The same BTU/hour is 23,500 watts. You need almost 100 amps of electricity to produce that much heat. Many older houses have only 100 amps total capacity for the whole house. Even if you have 200 amp service you probably don't have 100 amps of excess capacity. You'd probably need to install a new electrical service to get the capacity.
Generally demand for hot water is sporadic -- high demand in morning and evening, low demand the rest of the day. Tankless water heaters have to provide enough heat for the highest possible flow. Tank water heaters provide surge capacity and can be sized to the average demand. A 40 gallon water heater might be rated at 4500 watts, or less than 20 amps. Heat pump water heaters are significantly more efficient and might provide the same performance with less than 10 amps.
On your existing water heater it will say how many BTU/hour it's rated for. Where your electricity enters the house it will say how many amps of capacity you have. Those two numbers will help us help you.
No solar, as it is a very tree filled lot. The local solar guy, who knows his stuff, assessed it and said it was a no go. Good idea.
You're unlikely to be able to fit enough solar in to run a typical tankless water heater.
Downsides to electric water heaters are that they are a MASSIVE electric load -- the utility industry uses 1kw as an average load for a house (this is averaged over thousands of homes), so a 24 kw tankless heater uses about as much power as 24 homes while it's running. The transformer feeding you may not be large enough (typical transformers will be 10, 25 or 50 KVA in a residential area). The capacity of transformers is usually stenciled on the side, so if you see a transformer on a pole with a big "25" on it, that means it's a 25 KVA transformer. For a resistive load like a tankless water heater, KVA is the same as KW -- kilowatts. If the utility needs to put a bigger transformer in for just your tankless heater, there will be larger no-load losses in that transformer for all the rest of the time when that extra capacity is not needed.
This isn't really an issue for the utility to supply from a generation standpoint, but it's a BIG cyclic load on the local grid running your home, so you're likely to see lights dim when it's on. Many homes need upgraded electric service too, which is going to be expensive with today's crazy copper prices!
I would absolutely go with the propane heater if you already have propane available. Tankless hot water heaters are not nice electric loads, and aren't a good fit if you want to go all electric. If all electric is your goal, heat pump hot water heaters are a far better option.
Bill
In summary:
Tankless electric WH is not a good idea.
Tankless propane would be OK.
If you have the space for a tank type, a heat pump water heater could be the best option. It will probably have a higher initial cost. It needs some ventilation to the garage or house. It will have lower CO2 emissions and probably have the lowest operating cost. You have some flexibility to use offpeak energy which won't be an option with the tankless. I think Time-Of-Use rates are going to be more common in the future.
Time of day rates have been the standard for large commercial and industrial electric service for decades, they even used to have mechanical electric meters that could do it. Variable rates are somewhat new for residential customers, but many residential customers can actually save money with these rates -- you're usually at home during the off-peak hours after all :-)
I'm on a time of day rate myself (by choice), and I typically save around $20-25/month compared to the regular residential rate. I also have my car setup to charge during the cheaper off peak times, which is easy to do since electric vehicles all (as far as I know) have the option to set when these times start and end.
The purpose of these rates is to encourage load leveling on the grid and reduce the difference between on-peak and off-peak load levels. This is a Good Thing for all involved.
Bill
Thanks everyone for the explanations and thoughts on my choices. I always learn a lot and that makes the decisions easier.
Lydia,
Circling back a bit - what is your motivation to go tankless?
The reason to go tankless is space in the existing 3/4 bathroom. It will allow for a new 30 inch shower stall instead of the existing 24, which is a tight personal fit. The best option would be a heat pump electric tanked but no location to place it. Does that clarify? Today leaning to using LP as the fuel source, yesterday I was on to electric. It is a balance of doing the right thing and doing what I have for electrical panel and existing venting, wiring etc.
In many ways LP is more efficient than electric for tankless water heaters. In much of the country, a large part of the electric supply is from natural gas fired generation. After system losses and energy conversion losses are factored in, you end up burning less fuel to locally heat your water with a direct flame than you would if you went with an electric heater.
The reason heat pumps are different is that they MOVE heat, they don't MAKE heat, so they have a "coefficient of performance" -- you basically get more hot water per unit of input energy than you would if you used that same amount of energy to heat the water directly. That is why heat pump water heaters let you go electric and reduce energy use, it's not because electricity is always a better option in terms of an energy source. When your goal is to reduce your overall energy use, you always have to consider the entire system and not any one small part of it.
Bill
Incredibly helpful explanation, I did not know that. Thanks.
Lydia,
If you do go with propane or nat gas be sure to have the installer certify in writing that the unit is set up correctly for your altitude. The gas port size needs to be changed to run properly. I am at 8000' and during my early research on gas heating and cooking I discovered that virtually no one had a clue about what I was asking for. Even a fancy appliance purveyor located in a fancy unnamed town at 8000+ feet had no clue. Ended up going all electric since nat gas was not available and propane fluctuates too much.
We simply don't have as much air up here and the pressure is different. Sea level is nominally 14.7 psi and under 10.9 psi at 8500'. The ratio of oxygen/nitrogen doesn't change much (if at all), it is just thinner up here. This is equally important when sizing fan units for heat or cooling. Think of the difference between a cubic foot of cotton candy and smoke to be a little extreme in my example. There are simply fewer molecules of oxygen available per cubic foot as you go higher. The risk of unburned fuel may only be wasted fuel, but I suspect poor combustion will also generate bad side products.