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Heat dump zone for OWB

warnowarno Member Posts: 227
This summer I'm going to be replumbing my heat dump zone for my outdoor wood boiler. I know typically a dump zone is a gravity fed system with a N/O valve that allows flow in a power outage. I'm not worried so much about the power outage situation as I am an over fire situation.

Right now my dump zone is tied into it's own loop with a water/air heat exchanger and fan. I'm wanting to tie it into my current garage heating loop and do away with the other WAHX setup. The dump zone will only be used in the case if over load the firebox and my hot water storage tanks won't take anymore or at least won't take the BTUs fast enough to cool the boiler. I'm trying to avoid my boiler going into idle.

My boiler will be heating storage tanks which I draw from for my heat loads. It heats the tanks via a plate heat exchanger. The storage tanks are in my garage as the boiler is in a shed just outside. The FPHX is also in the boiler shed. So from plate HX to storage tank it runs underground.

I've drawn up a plan I'd like feedback on, good or bad, to see if you guys think this will work. Anything i should change let me know. My thinking is if the dump zone needs to come on it will open the diverter valve to the dump side. This will send water up into my current garage heating loop. When the dump is activated it will also switch on my current garage loop circulator to help with the added head of the garage loop. Thought being the pumps would be in series so it should be ok?

Let me know what you guys think.


  • Solid_Fuel_ManSolid_Fuel_Man Member Posts: 851
    I would think you'd still want to circulate through storage while in dump mode. Could you add a circulator instead of a diverter valve in that location. Basically have everything all piped into the supply header and the return header.
    Master electrician specialising in boiler and burner controls, multiple fuel systems, radiant system controls, building controls, and universal refrigeration tech.
  • warnowarno Member Posts: 227
    @Solid_Fuel_Man thanks for the reply. my thinking was the over heated water would circulate directly through the garage loop. If the dump is needed it's because the storage isn't taking any more heat from the boiler or at least not fast enough. The boiler is only plumbed to a FPHX which has the storage plumbed to the other side. So the storage tank gets heated before anything else.

    What if I take out the diverter and just put a tee there with a N/C zone valve on the vertical line that opens if the dump is needed? Then some flow would also go to the storage.

    Or would i basically achieve the same thing by just using the temp controller to switch on the garage heating loop. With where the header connections are on both the supply and return headers it would draw in most of the FPHX water and return it to where the circ pulling for the FPHX is drawing from.

    The 2 pictures are of my storage tank headers.

    The first picture is the supply header. The connecting nearest the wall wall and on the bottom is the FPHX return. And the center connection is the garage loop supply.

    The second picture is my return header. The connection nearest the wall and vertical is my FPHX supply. And the last connection that's horizontally is my garage loop return.
  • GroundUpGroundUp Member Posts: 26
    My first question would be why the need for a dump? Is this a batch burn type scenario with no damping on the boiler? I'm having trouble understanding your expectations from the system. The tank pictured does not appear to be of much volume, a batch burn does not work well unless there is a HUGE amount of stored energy- like several thousand gallons for a 24 hour batch with an average home. I can understand the want for avoiding idle time, but a damper to send the boiler to idle is a much safer bet than a wild fire and wasted heat via the dump IMO.
  • warnowarno Member Posts: 227
    edited March 13
    I run batch burns in my boiler. My storage tank is actually 3) 250 gallon tanks stacked. Typically I judge the wood load good enough the fire dies before anything over heats. But if the wood load is too much the boiler goes into idle. I'm wanting to start dumping heat at 190°. If the storage tanks are getting close to boiler temps and there is still a fire going, albeit small fire either way at this point, I would rather dump the heat to a building then send it up the smoke pipe.

    With my new plumbing and FPHX I'm putting in I don't think the heat dump zone will be used but I want the insurance if it is needed. One of those rather have it and not need it than need it and not have it things. Thats all.

    Attached is a pic of the tank stack
  • GroundUpGroundUp Member Posts: 26
    Understood. After some thought, I'm thinking you could benefit from swapping your drawn diverter to a tee, and moving said diverter up to the tee location between the check and garage circ so you are not drawing from storage while in dump mode. Also, I see that you have drawn bidirectional flow in your supply and return headers. In this fashion, during any call for heat you will likely be drawing through your underground/FPHX and bypassing the storage tank altogether. To make storage work in this fashion, I believe we need 4 ports to the tank itself. Counterflow in a header is bad juju
  • warnowarno Member Posts: 227
    edited March 13
    The design of the diverter tee only allows flow in the center bottom port and out one side or the other. I think to locate it on the garage circ line I would need a zone valve on the vertical dump line and on the garage supply line. The One on the garage would be open and other closed on the dump liner during normal operation then switched during heat dump. which if this did the trick I'm ok with that.

    My thinking in the header design using 3" headers with 1" connections was, the larger header would slow the velocity of the incoming and outgoing water and a slight mix of both storage water and FPHX water would be picked up as heat load supply. But this would only happen during boiler firing. I've been running the headers with supply and return this way for 2 years and it seemed to work fine. I've never seen any parasitic flow through the FPHX when boiler wasn't running. The lines go "cold" until the next fire. Would it work better if the FPHX pickup and return were on their own line instead of load headers?

    What bad juju do you say is happening in my headers?
  • GroundUpGroundUp Member Posts: 26
    The way I'm looking at the diverter (or 3 way ZV) is if you were to pipe the garage loop through the run of a tee (instead of a bullhead where you have the diverter drawn in, turn a tee vertical so the tee is straight through to the garage and the branch of the tee looks to the storage tank) and put a 1 way ZV above this new tee to send all flow to storage while not in dump mode, through the branch of the tee. Water will take the path of least resistance during a dump when said ZV opens and head straight through the tee right? I had my wires crossed thinking about the diverter only being able to be fed from the bottom.

    As for the headers, if you say they are working I guess that's that. My way of thinking is that incoming FPHX water is able to short cycle in the header and go right out to a load on a call for heat, which you say it does only during a burn? If it does it during a burn it's likely to do it during a no-fire draw as well, but seeing as you've run it this way and noticed no ghost flow perhaps there is enough hydraulic separation in the header to keep this ghost flow from happening. The bad juju mentioned is mostly just from a flow standpoint. Turbulence is a flow killer and counter flow in a pipe makes turbulence, similar to bullheading a tee, which is also a no-no. Notice almost any factory made buffer/storage tank has 4 ports, usually opposite each other, 2 for heat and 2 for load to keep things separated and allow full uninterrupted flow. If it works for you no sense in changing it because of what some clown on the internet said, but those are my thoughts anyway.

    Side note though, how often are you burning and what is your heat load? 750 gallons would only make 312,375 BTU at a 50 degree delta T or 187,425 BTU at a 30 degree delta; 8k-13k BTU/H is pretty impressive for most homes with a garage if you burn once a day! Then again I'm using my Minnesota calculations where it may be 50 degrees colder than it is where you are lol
  • warnowarno Member Posts: 227
    edited March 13
    The way it's plumbed now there is a short cycling effect in the headers while there is a fire burning. But while no fire is burning if there is any ghost flow through the lines to the FPHX it's so small the copper lines never get warm.

    To answer the side note. I burn twice a day on the coldest days. Once in the morning before work and once at night before bed. During the more mild days I burn once a day at night. And shoulder days I burn every other day. I live in central Illinois so it winters probably aren't as crazy as yours.
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