Tankless Water Heaters

[BrettS]

My wife and I recently adopted three kids and her grandmother has moved in with us for the winter, so our family has quickly grown from 2 to 6 and our 50 gallon water heater isn't quite able to keep up with us. We regularly run out of hot water for showers... Especially on the weekends when everyone showers at close to the same time.

In any case I'm considering getting a tankless water heater and wondering if you guys had any suggestions.

As far as sizing goes, though, I'm having a little trouble. Everything I read says to figure out what the maximum flow rate is and what temp rise you need and go from there, but from my calculations it looks like I'm way high.

We have three showers and it's not uncommon for all three to be in use at once, so that's 2.5 gal/minute times 3, or 7.5gpm total, plus a little more in case someone has a faucet or the dishwasher or something on puts me at like 9gpm.

Luckily I live in Florida and according to the maps our groundwater is just above 70 degrees in this area (seems to range from about 72-75 depending on which map you look at).

Unfortunately, we have no gas service, so I'm looking at electric only. Looking at Home Depot their largest unit is a 36kW model, but it will only do 5.46gpm at a 45 degree rise (again, according to the sizing charts I saw, one should aim for 120 degrees). I considered doing two units in series, but I'm not sure my electric service will take more the 36kW... That's 150amps when it's on.

Am I making a mistake in my calculations or is tankless just not going to work for me?

Any other suggestions on brands or anything?

Thanks


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[randy]

We have three showers and it's not uncommon for all three to be in use at once, so that's 2.5 gal/minute times 3, or 7.5gpm total, plus a little more in case someone has a faucet or the dishwasher or something on puts me at like 9gpm.

I'm afraid I can't be a lot of help but I can offer one possibility where your calculations may be high. If the shower flow rate is 2.5GPM, it is a blend of hot water and cold, so the actual hot water flow is probably more like 1-1.5GPM in the showers.

[S-F]

The best units available are made by Steibel Eltron. Hands down. That being said, you might also want to look into a huge hot water tank. It will use less electricity and provide hot water quicker. In your cooling dominated climate you should consider a heat pump water heater. Again, Steibel makes the best solution. The only time that an on demand heater makes financial sense is when there is either an extremely large or an extremely small domestic hot water demand. You might fit the bill for an extremely large demand.

And you should probably stop looking at Home Depot for a unit. You're not going to get anything worth the money there. You need to find a GOOD HVAC tech. Look for someone with years of experience with alternative energy equipment. Look for people with a decade or more experience with ground source heat pumps and solar thermal. For the record I don't recommend either of these approaches in this day and age, but a decade or more ago this was where it was at so such a person should be knowledgeable.

[Richel]

I live in NE Florida and have been using electric tankless for years. I replaced 2 standard water heaters, one on each side of the house. Presently, I have one Stiebel-Eltron (http://www.stiebel-eltron-usa.com/tempra.html) and one Ecosmart (http://www.ecosmartus.com/), which is a Florida company. Both have worked well for me.

Your calculations are probably correct. Consider low-flow shower heads to reduce by half the flow rate. Also, 120 degrees is a bit high in my experience.

Elliott

[S-F]

Also, 120 degrees is a bit high in my experience.

The national code won't let you go too low due to the possibility of legionnaire's disease. Never mind that it can't form in a tankless heater because the water doesn't have the chance to incubate.

[Richel]

The national code won't let you go too low due to the possibility of legionnaire's disease. Never mind that it can't form in a tankless heater because the water doesn't have the chance to incubate.

Interesting. Thanks.

[BrettS]

Here's another question as I do a bit more research. It looks like some places are recommending two units instead of a single unit. With my 200amp service I think I'm probably pushing it to get a 36kW unit as that can draw 150amps by itself, so I'm not going to increase capacity at all by getting two units as I can't draw any more power anyway.

But I am wondering if it makes sense to get two units just for reliability. If one unit fails then I'll still be able to produce some hot water while the bad one is getting replaced. It looks like it may be a bit more expensive to go this route and obviously somewhat more work to install everything, so maybe it's not worth it.

What do you guys think?

If I do go with two units, then series or parallel? I've seen recommendations for both options.


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[Uncle Michael]

. . .you might also want to look into a huge hot water tank. It will use less electricity and provide hot water quicker. In your cooling dominated climate you should consider a heat pump water heater. . . . You need to find a GOOD HVAC tech.

+1
I wonder if there is a way to use the heat pumped from cooling the house to help heat water?

PS. Congratulations on your expanded family!

[Steve Q]

My neighbor just evaluated adding a tankless water heater but decided against it because the cost of adding the wiring needed to power it was more than the water heater! Don't forget about the wiring cost.

Steve Q


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[S-F]

+1
I wonder if there is a way to use the heat pumped from cooling the house to help heat water?

Basically, this isn't possible. At least it isn't commercially possible. You could of course cobble something together but I don't think there are any products on the market that will do this. You'd need valves, pressure regulators and controls to switch from the exterior condenser to the DHW tank condenser. A friend of mine recently installed the refrigeration gear for his son's farm's multi 1,000 Sq. Ft. fridge. It uses ground source for the cooling in the summer and regular condensers in the winter. It puts out so much waste heat that they're melting snow on driveways and walkways with it. So he patched together three different condensers for one set of heat pumps. I don't know how it's managed though. Probably manually. BUT! A heat pump hot water heater will certainly cool and dehumidify whatever environment it is in.

[Uncle Michael]

A heat pump hot water heater will certainly cool and dehumidify whatever environment it is in.

That's part way there. I didn't realize they could be set up so as to reduce the load on the AC. I assumed the exchanger would be outside conditioned space.

[S-F]

A heat pump water heater is basically an electric water heater with an AC compressor on the top. The condenser is inside the tank. Recently some models with an outdoor condenser have hit the market but I've yet to see any in action and I'm not really familiar with their performance. If they perform as well as the newer mini splits they might be a viable option in a cooling dominated climate, like where I live.

All that being said, as far as I can tell the Steibel units seem to be 7,000 BTU/Hr. So that's more than 1/2 ton of air conditioning capacity. They also seem to roughly have a COP (coefficient of performance) somewhere around 2.5. So while it is true that electric resistance water heaters (all normal electric tank and on demand water heaters) are 100% efficient, these units are closer to 250% efficient. For every watt of electricity you put into one of these units you get 2.5 watts for making hot water. Add the rebates you get from these things and it's usually a no brainer. It's cheaper than making hot water with oil, propane or anything else. It's even cheaper than solar thermal under most circumstances. That is unless you have line gas. Then you should get a 96% efficient condensing DHW heater.

As for the whole series / parallel question, I'm not even sure if it's legal to run water heaters in series. At any rate, the first one in series would be doing most of the heavy lifting. I'm not sure it's good to over load a resistance heater too frequently.

[ITguyHS]

If I do go with two units, then series or parallel? I've seen recommendations for both options.

How about neither? If you split the load, [maybe running two baths off one unit and one bath and kitchen off the other], wouldn't that allow you to use two smaller units?

[ewkearns]

[S-F]

Depending on propane and electricity rates that could be a LOT more expensive.Here propane is the most expensive fuel. I shudder at the thought of how much propane would be sucked down by the kind of heaters we're talking about here.