Electric Tankless Water Heater advise needed.

16 years ago

Have been posting on different forums here as we are completely renovating the house we recently purchased.

We are switching from oil to electric heating so we will no longer have an oil tank in the house. We are located in Montreal where it is quite cold in the winter.

I am thinking of installing a tankless water heater (electric) for part of the house, while also keeping an electric water tank for the rest. We are a family of 7 with 3 full washrooms upstairs and one in the basement. My initial idea is to have the tankless unit hooked up for the master bathroom and one of the smaller children bathrooms(for the 2 youngest ones)only. The master will have a full steam shower with side nozzles and the works, I expect to use it fully on a regular basis.

Given it will be an electric unit, how dependable are these tankless water units? Can I expect it to meet my requirements?

Any feedback is much appreciated. We are starting the plumbing work in about 2 weeks. I have heard that the tankless unit can also be automatically bypassed to the tank, not sure how this works.

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Comments (4)

vhehn
16 years ago
i went through this analysis when i built my new house. i think they are dependable but my electric utility discourages them. by the time you run all the extra wire and possibly upreade your service to meet the requirements it isnt cost effective.
tankless only advantage over a tank is standby losses and they are not that much with modern tank heaters.
http://www.blackhillspower.com/tankless.htm
jakethewonderdog
16 years ago
I completely agree with vhehn.

Just about the only time electric tankless makes sense over electric tank is for a low usage point-of-use. That could have been the case for the master bath until you started talking about the multi-head showers. Those take a lot of gallons per minute and you don't get that from electric tankless.

It might be appropriate in the children's bathrooms (assuming no multi-head showers) if they are far from the main water heater.

Unlike gas tankless, the only savings for electric tankless is the standby loss through the insulation. That is only about 3% of the total energy use. That's not enough to justify the massive electrical upgrades needed to heat enough water for a multi-head shower, for example.
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Electric tankless will require an upgrade as well ... on the electric circuit. A typical electric tank takes a 25 to 30 amp circuit. Electric tankless can be high as 120 amps depending on the needed capacity. Capacity in tankless terms means the heating power required to raise the desired rate of water flow (gallons per minute) from the starting point (lowest tap-water temp experienced during the winter season) to the desired output temperature (typically at least 120ÃÂ°F for a kitchen with a dishwasher). If your house has a large-enough existing electric drop from the pole, then you'd only have to run a larger circuit from the breaker box. Otherwise you'd also have to upgrade the service drop from the pole.
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I searched GW for this topic, read through pages and pages of threads and I'm still confused. I don't understand the technical talk on the debates for each side and need to decide ASAP what's best for our house. Just when I think I have a decision made, I read something that changes it. From what I'm gathering, many people like tanks because they're simple, easy to repair if needed, and less expensive upfront. People dislike them because they take up a lot of space and can run out of hot water when people are using hot water simultaneously. People seem to like tankless water heaters because the hot water is unlimited, they take up less space, and are the "future" of water heaters. They seem to dislike them because they're more expensive, are complicated to fix, and don't always work out the way people expected them to or how they're used to using a WH. We live in E. Tennessee where the climate is moderate but we tend to have ice storms that cause power outages in winter. We are a family of 4-two adults, a teenager, and a baby. We have relatives come visit often and may soon have a family member or 2 staying with us for an extended period. We have 3 bathrooms with plumbing roughed in for 1 more in the basement. Basement is unfinished but has a large designated mechanical room so space is not an issue. We have a propane tank for the cooktop which is large enough to also use for a WH. Our GC suggested a good ole large capacity tanked electric WH. I trust him but I also realize that he's been doing this for 40 years and he goes with what he knows and trusts. He said if we went tankless we should go propane instead of electric because of how much power it would use. He mentioned putting something under the sink of the 2nd floor bathroom to help the hot water heater distribute water faster/more efficiently from the basement upstairs. He didn't use a name for it but I'm thinking maybe he was talking about a circulator? Anyway, any input would be greatly appreciated!! Need to buy soon so we can move in.
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If you're really sure you want tankless (and you kinda seem to be), I'm not going to argue with you. I'll just say a couple of things and then I'll bow out. Disclaimer: I'm not an expert on these matters. They just interest me and I hope to be more energy independent in the future, so I read up on them. I always welcome corrections from knowledgeable folk. First point: IMO electric resistive heating of ANY kind basically just doesn't play nice with PV, off grid or on. The only time it really makes sense is when you're off grid and your system produces more energy during the day than you can store in your battery. This can also apply to some extent if you're intertied and your utility pays you wholesale or less when you sell your power, essentially penalizing you for conserving. Either way, dumping your excess PV energy into water heaters is one way to store it for later use. However, it should be obvious that to store that energy, you have to use traditional storage water heaters, not tankless. Second point: Peak demand matters. It matters to your electric utility, and it matters to your bill. You probably know more about CA rates than I do, but it appears to me that right now California is more into time of use (TOU) metering than peak demand metering. However, more and more utilities are adopting peak demand metering for residential customers (they've had it for business for ages). So even if you're not dealing with it now, it may well be in your future. Demand based metering watches your power (not just energy) usage. The higher it is, the more you pay for all your electricity. In one demand scheme, you pay a surcharge based on your peak power usage (in kilowatts), In another, the price they charge you for every kilowatt hour you use depends on your peak usage -- the higher your peak, even if it's only once in the month, the more you pay per kWh that month. Let me give you an example of the second scheme. Let's say your house's big energy hog right now is an electric range.. Most of the time, you just use one or two surface units at a time. So your peak demand is relatively low, and you pay (say) 14 cents per kWh most months. But when you cook Thanksgiving dinner, you have all the surface units going, plus the oven, and a microwave. On that one day, you have a much higher peak demand. Your utility takes note. And that month, you pay 20 cents per kWh for EVERY kWh you use all month. So even though that month's usage is only a little higher than usual, the final bill for the month is MUCH higher. Now imagine what could happen to your bill if you add an appliance that increases your peak demand every day of every month. Guess what: when it comes to peak demand, tankless electric water heaters are MONSTERS. Think about it. With its mild climate, California is a great place for heat pump storage water heaters. That's what I'd get if I were you. Not tankless, and definitely not electric tankless. And that's all I have to say about that.
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charlesa
Original Author
16 years ago
Thank you to both of you. My concern was more the fear of running out of hot water. Going from oil to electric means tanks that take up to five times longer to heat new water.

We are already increasing the electrical from 200 to 400 amps. The main water supply is in the basement, and the 2 bathrooms in question are next to each other on the second floor.

However, based on the link provided I would guess my only alternative is to put at least 2 electric tanks together (or maybe 3). It's not that we use so much hot water, it's that large amounts are needed at certain times (morning showers).
jakethewonderdog
16 years ago
The problem is that tankless heaters can heat water for an indefinite period of time (you never run out) but they can only heat so much water at one time (gallons per minute). Think about how long it takes to heat a gallon of water in a pot on the stove. It takes a huge amount of energy to heat, say five gallons, in a minute.

A standard shower is about 2.5 - 3 gpm. An electric heater would draw about 120 amps to heat water 70 degrees (from 35 to 105) at that flow rate. You are looking at twice that amount for two of the children's showers at the same time -- and that's about it. The rest of the 400 amp capacity of your electric service would have to go to everything else in your house. In fact, depending on your electrical demands, it might not leave enough.

Gas tankless (or condensing tank unit) can do a better job of delivering the required amount of heat at one time -- but I'm assuming that gas isn't available.

Your multi-head showers are probably that rate for each shower head. It's really easy to get to 10 gpm or more. At that rate, you can empty a 70 gal water heater very quickly.

I can't really help you out much more, you have pretty serious hot water demand and you need someone who can go design your system for you-- including the implications for electrical service and hot water storage.

As an aside, I think people get enamored with the multi-head showers without a full understanding of what that requires in terms of plumbing and utilities to run.

You might also consider a waste water heat recovery system. They reduce the system demands by pre-heating the incoming cold water using the warm water that is going down the drain. These don't normally have a payback except in situations such as yours.

Electric Tankless Water Heater advise needed.

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vhehn

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Electric Tankless Water Heater advise needed.

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vhehn

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charlesa
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