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Bypassing hot water storage tank after flat plate heat exchanger.

chapchap70 Member Posts: 139
When heating water with a flat plate heat exchanger, boiler water goes through channels in the flat plate and domestic water travels through the alternate channels and into the storage tank.  It is my understanding that the domestic water is hottest after traveling through the heat exchanger.  After the water is dumped into the storage tank, it gets mixed with water that is a bit cooler.  This mixed water travels out of the top of the tank when a hot water faucet is opened.  While there is a call for hot water, a bronze circulator takes water from the bottom of the tank and mixes it with the cold water going to the domestic side of the flat plate.

My question is would there be increased hot water available for a large demand (say a 800 gallon baptistry) if the storage tank were bypassed and the water went directly from the flat plate h/e to the domestic hot water line?  Would one be able to fill a baptistry say 20% quicker and at the same temperature?

I have some thoughts on this but I am curious what you all have to say.


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,968
    In a word......

    Nope. When you say 'hot water storage tank', are you talking about an indirect water heater? Can you post the make and model? You need the volume, otherwise the boiler would never be able to keep up, temperature-wise, with the hot water demand. There are other ideas you could try, but I would need some more information--boiler type/buts, indirect tank model, and how is everything piped/controlled. Pics always help.
  • chapchap70
    chapchap70 Member Posts: 139
    edited February 2012
    not an indirect

    40 gallon storage tank as described in my post.  #18 flat plate heat exchanger by EK.  I am getting 4 or 5 gpm so I believe it is working as it should.  I was thinking instead of dumping into the storage tank after the flat plate, would it make a difference if instead I went directly into the hot water line.  Just curious as what others would have to say.  Boiler is plenty big enough;  it is set up for 2.25 gph (oil) so output is 257,000 btus.  The heat exchanger is rated at 200,000 btus.
  • billtwocase
    billtwocase Member Posts: 2,385
    I can't see why

    you would want to do that. How would you temper the water temps?
  • chapchap70
    chapchap70 Member Posts: 139
    Just curious

    It would just be to fill the baptistry for a few hours.  The temperature would be regulated at the faucet.  This is more my curiosity of how things work more than anything. I could give you reasons why I think it would lessen the fill time at the same temperature and some why it wouldn't. 

    Couple of tees, an elbow, 3 feet of pipe, a ball valve, and sweat equity to bypass the tank would do it.
  • icesailor
    icesailor Member Posts: 7,265
    Heating Water

    Maybe this will help you with the concept of what you are trying to do and the difficulties.

    A Marine Biologist came to me with a problem. How could he raise a large amount of water rise in temperature so he could make bay Scallops spat. He wanted to do shellfish propagation. How much water are you trying to raise? 20,000 gallons. What is the temperature you are starting within what is the finish temperature and how fast do you need to do it. If you need to do it quickly, you need one mother of a boiler. But, if you can do it over a longer period of time, you can do it with smaller equipment, He needed four days to raise the water. I told him to do the math. How much dies 20,000 gallons weigh in pounds. How many degrees do you want to raise it, and how long. He used a small boiler. I didn't do the install. That usually happens. I'm too free with information. It was for a good cause though.

    You would be surprised at how much water may be in that pool. And how much energy it takes to heat the water. 
  • billtwocase
    billtwocase Member Posts: 2,385

    another thing to consider is how you are going to activate the boiler. System 2000 does not maintain temp, and the manager will need to be energized somehow. A storage tank has a thermostat that will do that, also I think they don't recommend direct flow thru the plate heat exchanger. Was told that back in the 80's, thats why they use a cold water make up design with a check valve
  • chapchap70
    chapchap70 Member Posts: 139
    edited February 2012
    Clearing it up

    This unit has already been installed and when the baptistry was filled for a recent baptism, I was told that it took 3 hours to fill which was an hour quicker than the old boiler setup.  The water was warmer as well.  It probably could have been done quicker since he filled it for about 1:45 the night before and 1:15 in the morning with no cover so plenty of heat escaped.

    This is just an intellectual exercise.  I was just wondering if the storage tank caused less hot water output than if there was none. 

    The tank aquastat would still call the manager.  After a few minutes, the storage tank would be bypassed (ball valve) until the baptistry was full.  The bronze circulator would have to be shut off or else it would dead head.  The tank aquastat would call continuously so as soon as it was full, the hot water tank would have to be put back to normal or else the hot water call would never end.

    With charts I have looked at, I should be able to get a 77 degree rise at a flow rate of 7.6 gpm.  At this rate, I could fill the baptistry in about 100 minutes.  Since the hot water storage tank is upstairs from the boiler and I have lots of elbows, I do not think I am getting the 10 gpm boiler flow rate required.  I believe I need a flow rate of 5 gpm on the domestic side (I would have to look at the chart again, I don't have it here.)  but the 006 bronze circulator does not give me that.

    I don't know about the problems of cold water directly through the heat exchanger but it seems to me that there would be better heat transfer at a higher delta T.  With the domestic flow rate not being slowed down, I am thinking that the better heat transfer may allow the baptistry to be filled quicker and at the same temperature bypassing the storage tank although it probably would not be worth the hassle.

    I didn't realize I posted this thread in oil heating; I meant to post it at the main wall.

  • Ironman
    Ironman Member Posts: 7,066

    What's the difference? The water gets mixed down somewhat in the storage tank, but you also mix it down at the fixture. If it didn't get mixed in the tank, then you would have to mix it down more at the fixture. Same end result.

    A btu is a btu is a btu. You need x number of btu's to fill the baptistry at say 92*. The phx is producing say 160* water. It has to be mixed down to 92*. What's the difference if it's done in one step or two? Answer: none.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • chapchap70
    chapchap70 Member Posts: 139
    Having the storage tank increases flow rate?

    If the flow rate with the storage tank at cooler temperature is 5 gpm out of the hot faucet, this would mean that bypassing the storage tank would slow down the flow.  If there is no difference in temperature or flow rate in the end, I would get say 3 gpm at the hot faucet and I would have to mix it with cooler water at the faucet at 2 gpm to get the same flow and temperature.

    What makes the flow rate faster because it goes through the storage tank?  I'm not trying to debate; just trying to learn.  Thanks.
  • Ron Jr._3
    Ron Jr._3 Member Posts: 603
    Another problem

    might be the boiler won't keep up with the cold water flow through the HX . If the hot water goes directly to the faucets from the HX .

     I know boiler temp drops pretty fast with an internal coil . And that's with a boiler holding 10 - 15 gallons of water . An EK1 holds 2.5 I believe ?  Someone asked about this scenerio in an EK class I was in . I'm sorry but I forgot the answer .......... 
  • billtwocase
    billtwocase Member Posts: 2,385
    As Ron said

    You may not achieve success running directly thru the H/X. They are miraculous, but not a total miracle worker. They have always specified in there piping diagrams that only the tank circulated thru the H/X. One thing that comes to mind is the unfiltered water containing sand, etc that can plug them up. I live on an island, so sand comes to mind here. Generally the tank will store all the impurities, so transferring between the two will help keep that out of the H/X. We set them all up to back flush, as we have has some plug up do to debris. 
  • chapchap70
    chapchap70 Member Posts: 139
    edited February 2012
    Boiler flow rate

    If I have 10 gpm boiler flow rate, I have better output.  More boiler side flow rate equals better hot water output from the h/e.  I either do not have 10 gpm flow rate (which I am guessing is more likely because the h/e and tank are one floor above the boiler with lots of elbows in the 3/4" piping) or the h/e is producing maximum output.

    I piped the boiler with circulators.  The hot water zone kit came with a 007 but I put it on an air handler zone.   I instead put a Grundfos 1558 in the hot water zone.  I set it to run on high speed which I believe gets me better flow than the 007 would have.

    I tried to see how much hot water recovery I could get.  I could only get about 5 gpm before the water out of the faucet got colder but the burner still shut off on high limit.  This led me to believe that I did not achieve the maximum flow rate available.

    I suspect had I put a 0010 in the hot water zone, the burner would not be shutting off on high limit on a hot water call at 5 gpm.  I think the 1558 on high speed is equivalent to about a 008. 

    According to the chart in the EK manual, the #18 PHE has a Rated Hot Water Output of 200,000 BTUs (100 degree temperature rise) with a Hot Water Output of 7.6 GPM and a boiler flow of 10 GPM.  I think if I had 10 GPM boiler side flow, I would be getting the 7.6 GPM and the burner would still be shutting off on high limit since the EK3 has an output of 272,000 BTUs per hour at the firing rate it is set at. 

    I think I have less than 10 GPM boiler side flow which gives me a lower than 7.6 GPM Hot Water Output and a more frequent high limit burner shutoff.

    Since I am (maybe too) curious about exactly how things work, I was trying to figure if bypassing the tank (thinking the water out of the h/e would be hotter) would have the same flow rate as it would have going through the storage tank first.  If that was true, it would seem that I could have a higher gpm domestic hot water flow which would be beneficial for very long hot water calls.

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