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Old 04-14-2013, 07:08 PM  
gottodo1
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Hello,
So I just moved somewhere cold and it snowing today (has nothing to do with post just complaining). I've been updating my house (built 1989) and today I went down to start thinking about the wiring/plumbing for a basement bathroom I'm putting in (just about finished the drain piping). When I replaced the water heater we couldn't put a bigger one due to space constraints and the power company charges a sur-charge for a whole home electric tankless and I would lose existing grandfathered discounts SO that idea went out windows. Our 50 gal can barely supply enough hot water for 2 serial showers let alone 2 in parallel.

So this new bathroom is going to put a major strain on it UNLESS I put a Point Of Use electric in. I've looked all over the i-net and because of where I live (again complaining) For the 2nd bathroom I'm looking at getting the ECO 27, it may be overkill but I like warm showers. I may or may not run the sink off it as well. As I measured our inlet water at 36F when it was -40 outside I think this is probably the right unit. Unfortunately this unit requires a 200A panel. I could go to the ECO 21 that states a 150A main but I'm not sure that's needed.

Another option I thought up would be I could run the water heater hot outlet to a POU heater and feed both showers off that (one shower is directly below the other). I'm not sure if that's a good idea or not, as I've read something about maximum inlet temperature so I'm still looking into that but that could solve both showers problems and would be swell (though I'd have to re-run larger pipe for the higher flow but no problem there).

Another thought I had was putting this at the outlet of the 50gal water heater... Yet questions for a different group, back to the electrical stuff .

That being said I went downstairs to look at my panel as I'd been a bit confused by it in the past. There's 3 boxes, an off peak circuit breaker box, an off peak control box and a usual electrical panel with a 150 main in it. The main panel has the range and dryer so, DP 40, DP30, 20A dishwasher, 10 other 20As & 4 15As. Some of these are horribly mixed and I think the guy who wired this house should be shot, none the less. There's 2 meters so I believe all the off peak water heat & electric heat is all separate from the main panel. SO after all that, my question is, since all of the heating, cooling & water heating are done off the 2nd box and not the 150A main box, would the 113A draw of the ECO 27 really have to be done off a 200A box? In doing my load estimating and something else I can't find is if the POU heaters use PWM so the 113A is their max current under 100% PWM, or do they just run it at 100% all the time and combine cold water with the heated water at the output to provide the correct temperature (that way seems awful unlikely to me).

So, any thoughts? I'm completely open for suggestions. I'll probably end up calling eco or the other brands to find out about the PWM question as I imagine I can't determine which configuration I need until I know the answer to that question.

P.S. Don't mind the messy cable & router wiring I still haven't gotten down there to tidy that up yet. Or take down the 2 dishes on the roof... Ya so much left to do...



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Old 04-15-2013, 08:44 AM  
nealtw
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I am having trouble beleiving a fifty gallon tank can't keep up.



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Old 04-15-2013, 09:33 AM  
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Neal,
I'll explain it so you can understand. I have a brand new water heater and both elements are set at 140 Deg C. I take a 15 min shower at 110C there goes about 25gal of hot water, my wife then takes a 20 min shower, there goes about 30gal of hot water, right there is the water heater, by my first child the water is luke warm as you're now getting the new water that started coming in 40 minutes ago, by my 4th child the water is cold. If I'm the 3rd person the water doesn't get as hot as I want as I like VERY hot showers, if I'm 4th the water is unacceptably cold without waiting 30 or so minutes before I take mine. My inlet temperature is only 37 Degrees, that's a 73 degree rise I expect for my shower (3rd) in 45ish minutes. that's 189 270 cm3, or 795kJ per Degree C so to get 73 degrees F of rise, the energy to heat that much water that far is 4.9kWh of energy to heat the water. BUT it's in 40 minutes so that's really 7.35kWh needed to keep up with our water use SO the tank water heater could almost do it, but doesn't because of inefficiencies. This means it can't keep up with water use over time of one shower. SO I would need 2x that or 15kW tanked water heater to maintain hot water to 2 showers. Not sure I can pick one of those up at lowes.

My question wasn't about the water though, I'm trying to determine how I could run an instant hot water heater electrically so I can run 2 showers at the same time once I get the 2nd bathroom done. As adding this 2nd bathroom will more than double my current problem as if we use 50gal of hot water in 20 minutes the 3rd & 4th showering people will be very cold.

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Old 04-15-2013, 11:14 AM  
bud16415
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I don’t know about the specifics of your power requirement problem but have some suggestions.

First if your incoming water is only 37 degrees maybe a passive tempering tank would be in order. That is very cold for incoming water. I live in the Snow Belt and with a well my water comes in at 50 plus even when it’s below zero outside. A passive tempering in the summer months will give the cold up to your house, could be a good thing. I guess you did say you have a space issue also so maybe a tempering tank wouldn’t work.

We have used hot water tanks for heating systems and had the returning water be just a few degrees below the exit water and had no issues doing that with gas. As to can you preheat a POU heater I think you would have to ask the company but seems like you could.

Will you be showering off peak or at demand hours? A circuit could be designed to switch your tank off any time the POU heater was called for. The object being the normal flow would be from the cheap hot water from the tank and the times you run the tank down to some preset low temp the POU heater would take over and lock the other load out. Maybe even using that somewhat warmed water to supercharge the POU. I have no idea what code would say about that but you should be able to make it work with a much smaller POU unit as the delta will have changed.

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Old 04-15-2013, 03:55 PM  
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Bud,
Ya, ND is pretty cold the thermal maps show our inlet temp at 37 and I measured it this winter at about that with a thermocouple, shoot we just got 10 inches of snow at my house last night . I have 0 inches of room in this house otherwise i would look into that passive tempering.

I really like the idea of preheating a POU. Eco just said I could have it coming in at up to 140 and it would work fine . They said this is actually quite common but their recommended approach is putting it before the hot water heater, that way the water for the WHOLE house is hot if someone uses the showers and say the sinks or something. From my POV & situation though it's more expensive to do this and I'm not sure I have room for it before the water heater but, Ah so happy

So final plan, 13-15kW unit into standard 150A service. Install before showers and not need to upgrade to 200A service. Currently my tanks is set for 140 so I can save a bit and knock that down to 110 or 115, then because the POU is heating it to 130 even worst case 4 showers in 40 minutes it should be able to last to the end before the POU isn't able to heat quite enough to make the shower warm enough for my tastes.

The current Water heater circuit is sufficient for this so I might look into using a 3 way switch to go between off peak power and standard power to further reduce cost, as I'm an EE designing it and building it are pretty simply. Though I'ma bit stumped; My initial thought was use 12V controlled relays and a pic to monitor the 2 input voltage input lines to switch which is being powered and when. I think Ideally a flow detection for the pipe to the POU heater would be best with the Water heater otherwise always powered. I don't think the logic is separated from the heating power on the POU so... that may not work at all or I would continually be resetting the POU logic (bad ). not sure how to get around that one without opening the board finding the rectifier and modifying it's input power (easy enough but I'm sure it voids the warranty).

Any thoughts on other control schemes that could be beneficial? I still have my old house's atmega32 that controlled my solar water heater, also a few FPGA & dev boards lying around.

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Old 04-16-2013, 05:23 AM  
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If the POU is in line and after the tank and the tank is set to say 140, and the POU’s job is to make water of a temp of 140 also it should have nothing to do during the beginning of the usage cycle. Maybe turn your tank down as you mentioned if there really is a need for lower input temp but I would say keep the tank as high as possible. As the tank can’t keep up and the temp slowly goes down the POU will ramp up what it’s doing to maintain the 140. I would apply the system to both bathrooms and there would be no need to balance who’s using what water. The only need for logic control would be if you wanted to split it between the two panels to use the cheaper power off peak somehow.

I think most people that have time of day usage have timers and heat water during off peak hours and store it in the tank that is then shut off for on peak hour use. I think you will be surprised when you run the numbers on the POU heater when the water is preheated and you are just using it to fine tune the output and not do the main part of the heating.

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Old 04-16-2013, 08:23 AM  
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A timer does sound like a good idea. I just did a bit more math and I'm a little surprised that even though it's more efficient (say net 15% tankless and lower tank temperature (10deg reduce cost about 3% right?) because of the loss of the off peak cost (so 0.09 instead of 0.06) I'll end up breaking even or lose a little bit of money to install the tankless heater... but a little cost is worth it to me so I have hot water.

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Old 04-16-2013, 11:02 AM  
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I think you are correct. There is a fine line between saving money and giving up comfort. Doing laundry on the off peak hours isn’t a big deal but when you need a shower you need a shower regardless of the time of day. The POU heater would be a good compromise. Along with having the tank maybe on a timer and storing hot water for peak hours. Water heater tanks are insulated quite well these days and the one tank will heat store and also be a form of a tempering tank in that case.

In the winter months the tank is 100% efficient as any lost heat goes to the house anyway. Hard to factor it all in.

When you get it done report back and let us know how it went.



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