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Point of Use to supplement combi boiler.

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SteamtoHotWater
SteamtoHotWater Member Posts: 122
edited April 6 in Domestic Hot Water
I recently replaced my boiler and tank DHW heater with a combi boiler. So far, I'm pretty happy. However, the combi boiler is on the other side of the house while the 2nd floor bathroom and first floor kitchen are on the other. It takes a full 90 seconds before hot water comes out of the tap. For the shower, that's not too big of deal - though it sucks wasting water. But 90 seconds is a long time to wait when you just want to wash your hands.
I was thinking of installing a 2.5 - 4 gallon electric point of use heater in the basement, right below the kitchen to feed both the kitchen and the bathroom above the it. One hot water line branches off to feed the kitchen sink, bathroom sink and the shower - I think installing it before that branch would knock off quite a bit of waiting for all three.
That leads to two questions:
1) The install guides on these small units always show a cold inlet and hot outlet. But for my usage, shouldn't I feed the unit with the hot water supply from my combi? That is, wouldn't it be a hot water inlet and not a cold water inlet?
2) Is anyone aware of one of these small POU heaters that has an integrated scheduler? Monday through Friday, I'd only need it running in the morning and evening when we're home.

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,442
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    Do you have the electrical power available at that location? And in your breaker box? They take a fair amount of power...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SteamtoHotWater
    SteamtoHotWater Member Posts: 122
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    I have both the outlet and a spare breaker. And I'm sticking with a small unit so I can do 120V and 12 amp.
  • pecmsg
    pecmsg Member Posts: 4,888
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    Water is still relatively cheap
    electric is not. 

    Will you see a pay back I doubt it
  • hot_rod
    hot_rod Member Posts: 22,260
    edited April 6
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    Small 2.5 gallon tankless electric heater work for that. 1300W is the typical element. In a warm space it would not run that often. You could add a timer to turn it off for 10 hours or so a day.

    I would guess these well insulated tanks are less that 1° an hour temperature loss, like most indirect tanks.

    I suspect an uninsulated recirc line could cost as much or more to run as a small tankless.

    It depends on what you want to spend for the connivance of instant hot.

    Put a Kilo Watt meter on it for a month or so to see what it costs to run.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • SteamtoHotWater
    SteamtoHotWater Member Posts: 122
    edited April 6
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    pecmsg said:

    Will you see a pay back I doubt it

    Though I'm not thrilled about paying for electricity, it's not the cost of water vs electricity. It's about not standing for 90 seconds when I want to wash my hands.
    hot_rod said:

    It depends on what you want to spend for the connivance of instant hot.

    Two or three hundred seems like a reasonable upfront cost. Though I do wonder about ongoing electric costs.
    And am I correct that I would plumb it inline with my current hot water line?
    Combi Boiler hot water outlet + 20' of existing pipe -> POU "cold" water inlet -> POU hot water outlet -> branch that feeds the kitchen sink, bathroom sink and shower.



  • hot_rod
    hot_rod Member Posts: 22,260
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    Yes just pipe through the tank at the point of use.

    when you run water waiting for hot its not just the water cost, but the energy you used to heat it at the tank,

    suppose it runs an hour a day total, probably a high estimate,
    1.3 kw times what your kwh cost is. At .20/ kwh that 26 cents a day, possibly less 

    Is that worth it?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • SteamtoHotWater
    SteamtoHotWater Member Posts: 122
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    hot_rod said:

    suppose it runs an hour a day total, probably a high estimate,
    1.3 kw times what your kwh cost is. At .20/ kwh that 26 cents a day, possibly less
    Is that worth it?

    Yeah, probably.
    Thanks for the input.

  • pedmec
    pedmec Member Posts: 988
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    us a laing eco circ recirculation pump. Great little pump install under the farthest sink. has built in thermostat and timer. just need a receptacle. also made under different name
  • john123
    john123 Member Posts: 83
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    if gas is cheaper than electric then
    what about using a conventional tank --say 20 gallons---heated by gas instead of electric would give lots of hot water for the shower and the basin and quicker than the combi?
  • SteamtoHotWater
    SteamtoHotWater Member Posts: 122
    edited April 7
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    pedmec said:

    us a laing eco circ recirculation pump. Great little pump install under the farthest sink. has built in thermostat and timer. just need a receptacle. also made under different name

    I definitely looked into a recirculation pump, but ultimately ruled against it. I have a pedestal sink in the bathroom, there's no place to put it other than the basement branch that feeds the kitchen and bathroom. Also, they don't seem ideal for a combi boilers - as I can't warm the house and heat water at the same time. Lastly, I don't see how keeping hot water in uninsulated pipes could possibly be as efficient as keeping a small amount of water in an insulated tank.
    john123 said:

    if gas is cheaper than electric then
    what about using a conventional tank --say 20 gallons---heated by gas instead of electric would give lots of hot water for the shower and the basin and quicker than the combi?

    Interesting, but I really don't need "lots of hot water." I just need a little bit of hot water to wash my hands. I'm hoping to get hot water in the upstairs sink in less than 20 seconds vs 90 seconds. I sealed off the exhaust that my 40 gallon tank heater used when I installed the combi. It would really suck to have install a new exhaust.
    Though I do wonder if keeping 20 gallons hot via natural gas is any more efficient than keeping 2.5 gallons hot via electricity.

  • SteamtoHotWater
    SteamtoHotWater Member Posts: 122
    edited April 7
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    Is there such thing as a small, passive insulated tank that just stores and passes hot water? I guess it would need some sort of thermostatic input and output to make it work.
  • kcopp
    kcopp Member Posts: 4,443
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    These work well and dont use much power.
    Simple 120v outlet. run the hot from the tankless into the cold and the hot to the sink. I have done it many times.
    Lots of happy customers.

    https://www.eemax.com/products/eemax-minitank/emt2_5/
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,336
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    Hi, Sinks are no problem with a point of use tank-type heater, but the shower could be. Whatever tank you use needs to have at least three times the volume of water that you need to run before the water gets hot. Even with that, I'd expect to see a dip in temp that might make the shower user slightly unhappy. Perhaps a thermostatic shower valve could help???

    Yours, Larry
  • pedmec
    pedmec Member Posts: 988
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    you need to get the pro eco circ. the laing eco is design for the on demands and combi.