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Inline tankless water heater for master bath?
Old 12-13-2013, 07:47 PM   #1
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Inline tankless water heater for master bath?

T-Al's threads have made me wonder...Our master shower takes a few minutes to get hot water from the water heater (gas, conventional tank) which is on the other side of the house, probably 70 feet of pipe travel. We live in Houston so winter is not that long, but it is still annoying to wait 3 minutes for the shower/sink to heat up when it's cold. The shower has easily accessible attic space above it.

Do they make an inline (e.g. "hot" in, hot out) tankless electric water heater that I could install above the master bath? It would draw cold water from the "hot" water line until hot water reached the bathroom, at which time it would just be a pass-through.

We are replacing the pipes now anyway so this would be a good time to do it.
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Old 12-13-2013, 08:25 PM   #2
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Just saying - having investigated the cost of tankless and the problems they have (we're in hard water area, which doesn't help) - put a cheap easy pump back by your water heater and a circulating valve by your most distant sink. simple and less expensive is good. over-complicating and spending is easy.
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Old 12-13-2013, 09:05 PM   #3
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I don't know of a water heater like that, but it sounds like something that somebody probably makes. Like any tankless design, it has more things to go wrong than a regular WH with a tank.
Questions:
- Is your regular WH electric, too? If so, then there's little/no advantage to running the (not yet) "hot" water line into this remote heater, just run a cold line into it.
- Where are your pipes (in a slab, basement, or attic)? A recirculation system probably is easier, cheaper, and more reliable if you can gain access to the lines and insulate them. Better yet, as Travelover suggested in an earlier post, if your WH is lower than the distant bathroom (e.g. water heater in the basement) you could make this whole thing run without any pumps atr all using just passive thermosyphon recirculation.
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Old 12-13-2013, 09:42 PM   #4
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Our main water heater is in the attic, very close to the master bathroom, which is great for showers in that bathroom. However, it's pretty far from the kitchen sink, and also the guest bath. I'm not too worried about the guest bath, because it's not going to get used so much. However, since hot water use occurs pretty much everyday in the kitchen, since T Al's post, I'm thinking about how many gallons of water we're wasting while waiting for it to become hot. I'm just beginning to research the under-sink water heaters. My water bill for the first month of living in this house (last month) was $124. That includes water, sewer, garbage pick-up, and some "fee/tax" for fire department. Anyhow, I hate to think what it will be like in the summer when we have to water the grass to keep it from dying. I need to find ways to cut back.
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Old 12-13-2013, 10:02 PM   #5
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Our main water heater is in the attic, very close to the master bathroom, which is great for showers in that bathroom.
Oooh, water pipes in the attic. Uggh. Well, make sure the pan under the WH is in good shape and that the drain is clear. Definitely buy a water alarm for that pan: you want to know as soon as there's a problem, not after the water is really gushing out and overwhelming the drain.

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However, it's pretty far from the kitchen sink, and also the guest bath. I'm thinking about how many gallons of water we're wasting while waiting for it to become hot.
- I'm guessing that if you crunch the numbers you'll find the water you use waiting for it to warm up in the kitchen isn't very much compared to the costs of installing a new WH (with its own standby losses, etc).
- Will the city let you run a separate meter for your outside watering? That allows them to not charge you sewer fees for the irrigation water. Outside water use can be a lot, and the sewer costs can be as much/more than the charge for the water. And, is water high enough that it's worth drilling a small well for irrigation? This is common in some areas.
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Old 12-14-2013, 07:14 AM   #6
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I just do not see the benefit of putting a hot water heater inline with another hot water heater... especially a tankless....


Just put your tankless in as the main heater for that location and be done with it... It should be able to provide you with all the hot water you need... I would think that it would also help out on the temp changes due to flushing etc. that happens...
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Old 12-14-2013, 07:32 AM   #7
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Originally Posted by Texas Proud View Post
I just do not see the benefit of putting a hot water heater inline with another hot water heater... especially a tankless....


Just put your tankless in as the main heater for that location and be done with it... It should be able to provide you with all the hot water you need... I would think that it would also help out on the temp changes due to flushing etc. that happens...
Regarding tankless heaters, keep in mind that they are notorious for having a limited heat gain. If you look at the charts for each specific heater, take note of the temperature delta for the specific model you're looking at. If a heater only provides a 55 degree rise, that means in the winter when your incoming water is 45 degrees, your 'hot' water will max out at 100 degrees.

Also keep in mind the flow rates - an instant heater will only be able to provide that X degree rise at a MAX flow (usually 2.5gpm for the smaller units). So that means if you want to fill up a bathtub, you can't just open up the tub valve all the way, because a tub valve flows MUCH more than 2.5 gpm. So the water flow is far greater than the heater can heat up, and your tub will be full of water that's maybe 65 or 70 degrees in the winter (with cold incoming water).

On the opposite end, also note the minimum flow rate to turn on the instantaneous heater - either 0.5gpm or 0.7gpm. If your hot water demand is less than that, the heater doesn't kick on. Not a big deal if you want a hot shower, but if you have a low flow faucet or want just a small flow of warm water, it might not be enough hot water demand to kick on the heater.

Finally, remember that electric instantaneous heaters take HUGE amounts of power (relatively speaking, for an average residential home), and that if you somehow only have 208V available to bring to the heater, 208V will cut your heater's output by 25% compared to the output listed at 240V (so take that temperature rise listed for 240V and reduce the temperature rise by 25%).
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Old 12-14-2013, 08:09 AM   #8
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OP: I would put a small tank water heater up there above your master bath. Cheaper, simpler technology.
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Old 12-14-2013, 08:31 AM   #9
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Originally Posted by calmloki View Post
Just saying - having investigated the cost of tankless and the problems they have (we're in hard water area, which doesn't help) - put a cheap easy pump back by your water heater and a circulating valve by your most distant sink. simple and less expensive is good. over-complicating and spending is easy.
+1, adding a recirculating pump should solve your problem, but it will slighly impact your electricity usage.
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Old 12-14-2013, 08:34 AM   #10
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+1, adding a recirculating pump should solve your problem, but it will slightly impact your electricity usage.
These can be put on a timer, too, if you are on a regular schedule.
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Old 12-14-2013, 09:00 AM   #11
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We put one of the recirculating pumps in our master bathroom. It is on a timer and we calculated that it is cheaper than the cost of all the cold water we were wasting. It worked well for us. The only hassle was installing an electrical outlet under the sink but luckily there was a line nearby.
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Old 12-14-2013, 09:14 AM   #12
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Another approach to getting hot water to a distant point with less wait and less water down the drain: Run a smaller direct line from the WH to the distant point. Most older homes are set up with 3/4" copper (or CPVC) "trunk" lines and then smaller branch lines. When you turn on the hot water at a distant shower you have to wait for all the water in the 3/4" and 1/2" lines to run through that smaller shower valve opening before the hot water finally arrives. A direct 1/2" line will normally get the hot water to the distant fixture a lot faster (a foot of 1/2" pipe contains only 45% as much water as a foot of 3/4" pipe). These "home run" lines directly back to the main WH can be easy to run using flexible PEX pipe.
It's not perfect (you still have to run the water and wait, but for less time), but it's an improvement, costs very little, has no additional energy use or maintenance. As a bonus, if it turns out you later want instant hot water at the distant location, the PEX pipe you already put in can be used as the return line for a recirculation system.
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Old 12-14-2013, 09:15 AM   #13
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On This Old House, they used a motion sensor/timer on one install. Walk into the bathroom, and the pump runs for X minutes.

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Old 12-14-2013, 11:31 AM   #14
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We just keep a bucket in the bathtub and fill it up until the water gets warm. We use the water to flush the toilet (some times!), water the plants, for household cleaning, etc. It's also the cats favorite watering hole. Cheaper than installing another heater!
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Old 12-14-2013, 01:16 PM   #15
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Here in La Quinta Saturday morning yardsales are a staple. Today I noticed a cardboard box labeled TACO 006-BC4-PNP. In the box was a TACO 006 -BC4-PNP, complete with analog timer on the pump bottom. The top of the pump was off, the sweat on connections showed no sign of solder.. My guess is that it is a warranty replacement pump - someone kept the sweated on pump top and replaced the rest. For the princely sum of $1 ol' Diamond Jim here took a flyer on it. Amazon has a new one for >choke< $280.

Edit: unit only hums. motors are available, worth the buck just to take it apart and see how it (doesn't) work. Maybe I'll repair and install it up north.
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Old 12-14-2013, 02:16 PM   #16
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Originally Posted by Texas Proud View Post
I just do not see the benefit of putting a hot water heater inline with another hot water heater... especially a tankless....

Just put your tankless in as the main heater for that location and be done with it... It should be able to provide you with all the hot water you need... I would think that it would also help out on the temp changes due to flushing etc. that happens...
Ditto. A properly sized instant tankless water heater should be piped with cold in hot out. It doesn't need any preheating so why couple it in series when it's not necessary. Check Eemax or Chronomite brands.

They work perfectly when properly sized by someone who knows what they're doing and I have chosen to install them in buildings requiring as little as .5 GPm with only 30 degree temp rise to 40 GPm units at a 100 degree rise.
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Old 12-14-2013, 02:33 PM   #17
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The D'mand system is a user activated pump that works like a plumbed-in recirc system. it only runs when the user wants it to. They may now have a motion sensor version, I haven't looked in a few years.

Taco-Hvac: D'MANDŽ System

There are a couple others, iirc.
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Old 12-15-2013, 07:59 PM   #18
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Thanks all. I may run a direct line to the shower first and see if that cuts down on the warmup time before getting into the more complicated solutions.
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Old 12-15-2013, 09:00 PM   #19
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Regarding tankless heaters, keep in mind that they are notorious for having a limited heat gain. If you look at the charts for each specific heater, take note of the temperature delta for the specific model you're looking at. If a heater only provides a 55 degree rise, that means in the winter when your incoming water is 45 degrees, your 'hot' water will max out at 100 degrees.

Also keep in mind the flow rates - an instant heater will only be able to provide that X degree rise at a MAX flow (usually 2.5gpm for the smaller units). So that means if you want to fill up a bathtub, you can't just open up the tub valve all the way, because a tub valve flows MUCH more than 2.5 gpm. So the water flow is far greater than the heater can heat up, and your tub will be full of water that's maybe 65 or 70 degrees in the winter (with cold incoming water).

On the opposite end, also note the minimum flow rate to turn on the instantaneous heater - either 0.5gpm or 0.7gpm. If your hot water demand is less than that, the heater doesn't kick on. Not a big deal if you want a hot shower, but if you have a low flow faucet or want just a small flow of warm water, it might not be enough hot water demand to kick on the heater.

Finally, remember that electric instantaneous heaters take HUGE amounts of power (relatively speaking, for an average residential home), and that if you somehow only have 208V available to bring to the heater, 208V will cut your heater's output by 25% compared to the output listed at 240V (so take that temperature rise listed for 240V and reduce the temperature rise by 25%).
Ok (excuse the metric units but things are easier that way). Assume 5 liters per minute, starting at 20c and going to 50 c for a 30 c rise. That is then 5000 grams per minute at 4.84 joules per deg c per gram so we get
12,000 joules per second, or 12,000 watts now at 240 volts that is about 50 amps. But if you have a dishwasher there is likley no restriction to 1.5 gpm as it may be up to 3 gallons per min or 11 liters per minute, making the energy added 28,800 jps or about 28.8k watts which works out to a circuit of 120 amps +. At this level you may well be talking a new service entrance and a new box at a minimum. An alternative is to put a 5 gal storage water heater for the kitchen direct from the cold.
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Old 12-15-2013, 09:08 PM   #20
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But if you have a dishwasher there is likley no restriction to 1.5 gpm as it may be up to 3 gallons per min or 11 liters per minute, making the energy added 28,800 jps or about 28.8 watts which works out to a circuit of 120 amps +. At this level you may well be talking a new service entrance and a new box at a minimum. An alternative is to put a 5 gal storage water heater for the kitchen direct from the cold.
Or put a low restictor in the line so the DW doesn't flow more than 1.5 GPM? I'd sure rather wait an extra 120 seconds for the dishwasher to fill than spend $3000 for additional electric service.

I run the hot water at the sink so the DW gets hot water from the start. It's not much water, really, and if someone is conserving there's always a good use for a bucket of tap water.
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