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Auto fill/back flow preventer. Is it necessary?

idahohridahohr Member Posts: 13
I am putting together a radiant hydronic system in my shop that will be pressurized. There will be a number of Webstone purge t's and after the system is filled and purged is there any reason why I can't maintain system pressure via a hose and valvesand simply disconnect when I'm done?


  • STEVEusaPASTEVEusaPA Member Posts: 1,500

    Not at all. 

    1.  Just make sure you put in a low water cutoff.

    2.  Make sure you disconnect the hose when you're done.

    I assume you don't have running water to the shop, otherwise it's not that expensive to put in the auto fill/back flow.  Make sure you buy a cap for the boiler drain.  After a couple times over the years, it may leak after use.
  • idahohridahohr Member Posts: 13

    The system in question is on the pressurized side of a fphe, the heat source is an OWB on the other side so I'm assuming no low water cutoff involved.
  • heatpro02920heatpro02920 Member Posts: 991
    edited April 2014
    I have seen a few systems like this...

    a customer of mine has a hydronic heating system in a barn with no water in it, they didnt want a fwa because the sound scared the horses, so they used radiant panels and lots of them an outdoor wood boiler feeds the fphx, he has 2 large expansion tanks {sx 30s maybe 40s}, he fills it with a hose and it works well.....

    Not sure if the expansion tanks aid in anything, I didnt design it, and he has a water feed and back flow, the hose connection is before the 911s water feed...
  • SnowmeltSnowmelt Member Posts: 751

    Need more info, do you have a boiler inside. Are you going strictly on the outdoor boiler?

    What is your pressure / relief valve rated at?

    As of right now I ask why wouldn't you want the BFP why take a chance of boiler water going back into your domestic side.
  • idahohridahohr Member Posts: 13
    edited April 2014
    A little more about my system

    The outdoor boiler is non pressurized. Water loops inside through a fphe and back to the boiler. The indoor heat load side will be closed. There will be a primary loop that circulates through the fphe and two pumped zones off the primary loop via closely spaced tee's. The closed side has expansion tank, air eliminator, a number of fill and purge possibilities, pressure relief valve, and a total volume of no more than 35 gallons.

    I don't have anything against an autofill setup. Just running out of money and doing this build paying as I go. Don't see any reason why I couldn't add it later. Any thoughts?
  • SnowmeltSnowmelt Member Posts: 751

    I would put a potable expansion tank on the water heater side. If your not going to have city water pressure in your heating loop you don't need it. Still neeed a pressure relieve valve
  • ZmanZman Member Posts: 3,517

    Your heating piping has an extra circ. There is no need for the primary loop and closely spaced tees. You could instead run your 2 mixing loops right off the exchangers. You just need a beefy header size and check valves on the circs.

    The same applies to the owb and hot water heating piping. Right now the water heater is heating a cold wood boiler.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • 4Johnpipe4Johnpipe Member Posts: 476

    I am confused...Is the water heater connected to or in use as a water heater? Or is it strictly the boiler? If its just the boiler...why the need for FPHEX's?
    Considerate People, Considerate Service, Consider It Done!
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  • idahohridahohr Member Posts: 13
    Consider this

    The low temperature loop that goes down will require 5.16 gpm with 30% glycol and I'm trying for a DT of 10 deg. 4.8 psi drop for pex in slab, 1.2 psi for mixing valve, 5 psi for fphe, without any further added drop for fittings I was looking at 25.4 ft. head and 5.16 gpm. Going with a primary loop lowers the secondary zone to 13.8 ft. head which seems like a more reasonable target. So 2 high head pumps or 3 low head pumps?

    How is the water heater heating a cold wood boiler? The wood boiler is 180 degrees, the water heater is 130. If the water heater drops to 120, aquastat turns on both pumps until water is back to 130. My reason for piping the fphe for the dhw this way is so I can heat water with small clean burning fires and bypass the main heat exchanger if I want to.

    This will be he only heat, the OWB is non pressurized, the dwh is on a well water system. The domestic water cycles between 40 and 60 psi, has a pressure relief valve. Sorry I wasn't clear on this . The closed primary secondary piping will have expansion tank and pressure relief valve.

    Any thoughts would be greatly appreciated.
  • ZmanZman Member Posts: 3,517
    I see...

    I think what you have will work.

    It is to bad you can't dual purpose the DHW tank to give you some more system buffer.

    Have you considered putting a reverse indirect buffer/dhw tank on the heating side?

    Wood boilers need something to heat, they don't idle well.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • GordanGordan Member Posts: 891
    edited April 2014

    OWBs typically have some built-in buffering capacity, but a smarter control strategy (than a simple aquastat) could ensure that the DWH is used as a dump zone of first resort. A tempering valve at the DWH outlet would be necessary, but realistically it's necessary in any case.

    To the OP, I don't see how the way you have it laid out allows you to bypass the space heating FPHX?
  • idahohridahohr Member Posts: 13

    On my OWB the buffering amounts to 85 gallons of water.

    Sorry about the lack of certain details in my schematic. One valve is shown on the return side of the main fphe, there is to be another on the supply side.(doesn't show) i would do this on both the OWB side and the load side to aid in removal for maintenance and if both valves on the boiler side are closed and a crossover valve opened(valve shown in drawing) water now loops back but can supply the water heater fphe via the closely spaced tees and a Tacio 006. Two pumps keep the water heater up to set point, and no need for a thermostatic mixing valve.

    Actual physical situation is hard to show in a schematic( at least for me) The OWB will never get glycol in it and my intent is to make it simple to drain down when I want to leave, but the interior hydronic radiant will have glycol and never be drained.

    Any thoughts or suggestions are appreciated. I have other questions like how close can my circulator be to my thermostatic mixing valve . Maybe sould start another thread?
  • GordanGordan Member Posts: 891
    Thermostatic is for safety

    Arguably you need it on the DHW even more than you need it on the space heating side. On the heating side it's there to protect your emitters from damage due to overtemperature; on the DHW it's there to protect your butt from damage due to overtemperature. An aquastat will interrupt the charging of the DHW tank when the water around it is at the set temperature, but what happens to the water higher in the tank? It may approach the temperature of the boiler water unless you're purposely diminishing the efficiency of the heat exchanger somehow.
  • idahohridahohr Member Posts: 13
    Couldn't the water heater

    be maintained at 125 degrees by using the two taco 006 stainless circulators I show in my diagram? An aquastat would control both pumps at the same time. One circulates water from the non pressurized OWB loop through the fphe (via closely spaced tees as shown in diagram), the other circulates water from the water heater through the fphe. If the aquastat was set to 125 deg. why would the water get up to the temp of the boiler? Maybe a check valve or anti siphon loop would prevent this?

    Not against a mixing valve on the water heater but can't comprehend why the tank temp can't be maintained without one. I thought by pumping a fphe at the manufacturers recommended rate would maximize efficiency , not diminish it.
  • GordanGordan Member Posts: 891
    Ok, try it

    What on god's green Earth does an aquastat buried within a tank have to do with the output water temperature from the heat exchanger? That will depend on the flow on either side of the exchanger, the input DHW temperature (which varies), the input boiler temperature (which varies) and the efficiency of the heat exchanger. Think about it some more.
  • idahohridahohr Member Posts: 13
    The aquastat

    On God's green earth, said aquastat measures water temperature in the water heater. The flow on each side of the heat exchanger is controlled by said aquastat. The aquastat doesn't really care what the temperature of the heat exchanger is but if it could talk would say : "It's cooling down in here, I think I will exchange some BTU's with the hot OWB by turning on a couple of pumps and when I'm satisfied (125 deg.) I'll just turn the pumps off"

    A more common way to heat domestic water with a wood boiler is to direct all flow from the OWB through either a fphe or a sidearm using the circulator at the OWB. Water heater intake passes through fphe when in use. No control on temperature so there would be a need for a mixing valve.

    I'm trying to keep pressure drop in the OWB loop down. Stainless steel pumps get expensive and high head pumps are higher wattage.
  • GordanGordan Member Posts: 891
    Good luck.

  • idahohridahohr Member Posts: 13
    Thanks Gordan

    All water heater manufacturers should read that. Apparently they send out all domestic water heaters set at 120 now to avoid scalding but must not know they give you a legionella farm.
  • CanuckerCanucker Member Posts: 321

    Manufacturers aren't responsible for the install, they make the equipment that has the ability to satisfy the code and the installer takes it from there. Most instructions mention that the unit should be installed to applicable codes. They won't take the blame if you don't turn up the temp of the tank to mitigate the risk.
    You can have it good, fast or cheap. Pick two
  • MattMatt Member Posts: 163
    you should listen to Gordon

    I agree with Gordon. If you tee in the return from the heat exchanger to the hot water supply as drawn you have a scald hazard. Hypothetically - 6 yr old grandchild starts washing hands in sink. Water heater decides to turn on pumps. Wood boiler water is at 200 degrees for some reason or flow through domestic side is restricted. 120 degree water enters heat exchanger and leaves at 150. 150 degree water enters tee and proceeds to faucet and grandchild gets scalded. Tempering valve prevents this.
  • idahohridahohr Member Posts: 13
    I see your point.

    Good point Matt. On the domestic side, would it be better to circulate water from the top of w/h to the fphe, and return it to the bottom?
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