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Boiler return temp

northernsoulnorthernsoul Posts: 112Member
edited October 19 in Gas Heating
Hi

I have a slant/fin gas boiler, probably close to 20 years old, which is connected to a single zone of old cast iron rads.

Even after running for an hour (PSI normally around 20-22 hot) the return temp gauge usually shows around 135-140. Rarely it gets to to 150 on the coldest winter days.

I don’t know if this is normal due to he type of boiler and the fact the rads and piping is prob 80 years old or if the aquastat is set low or some other issue?

Is there a way to access it to see what the low and hi temps are?

I know there can be issues when the return temp is cold and leads to condensation

Comments

  • SnowmeltSnowmelt Posts: 1,104Member
    I’m lost, you said it doesn’t get to 150, you want a high return temp that’s not to low.
  • northernsoulnorthernsoul Posts: 112Member
    > @Snowmelt said:
    > I’m lost, you said it doesn’t get to 150, you want a high return temp that’s not to low.

    I’ve read that a return temp under 140 causes condensation which can rust out the boiler and flue.

    Is there anything I can do to increase the temp to prevent this?
  • northernsoulnorthernsoul Posts: 112Member
    Also is there a way to boost operating temps or is this a product of the boiler and the old pipe and rads and so it might not operate warmer ?
  • Jamie HallJamie Hall Posts: 11,204Member
    You can... but I'm not sure you need to, if the house heats satisfactorily.

    Heat delivered is related to the temperature drop of the water and the flow rate, so to increase the return temperature you need to reduce the temperature drop -- so for the same heat delivered you'd need to increase the flow rate (probably a different pump). The heat output of each radiator, on the other hand is determined by the average temperature of the radiator and hence of the water going through the radiator -- so if you aren't warm enough, you need to increase the water temperature on the average. Again, greater flow will do that, but so will raising the initial temperature.

    Now having said all that... somewhere on that boiler's piping near the boiler there will be an aquastat. That should be adjustable to command a higher circulating water temperature.
    Jamie

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • hot_rodhot_rod Posts: 11,874Member
    does it run non stop and never get above 150 at the supply on the coldest days?

    As Jamie mentioned first check the temperature setting on the control. If it is set at 180 and the boiler never gets there?

    Or the return never gets above 150?

    If it has run 20 years with too low of a return you would see signs, rust around the burner tray, diverter hood, metal vent pipe, roof terminal.

    Ideally all conventional boilers like to be above dew point within 10 minutes of start up. Around 130- 140 at the return.

    At least a 20 minute run cycle would be desirable also.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • northernsoulnorthernsoul Posts: 112Member
    > @Jamie Hall said:
    > You can... but I'm not sure you need to, if the house heats satisfactorily.
    >
    > Heat delivered is related to the temperature drop of the water and the flow rate, so to increase the return temperature you need to reduce the temperature drop -- so for the same heat delivered you'd need to increase the flow rate (probably a different pump). The heat output of each radiator, on the other hand is determined by the average temperature of the radiator and hence of the water going through the radiator -- so if you aren't warm enough, you need to increase the water temperature on the average. Again, greater flow will do that, but so will raising the initial temperature.
    >
    > Now having said all that... somewhere on that boiler's piping near the boiler there will be an aquastat. That should be adjustable to command a higher circulating water temperature.

    Thanks - the house is normally comfortable other than the upstairs usually being a degree or two colder (old house with no wall insulation)- my main concern was damage to the boiler with low temps - id like to preserve the current boiler (mid efficiency) as long as possible

    I think the aquastat is inside the boiler housing - will take a look. I am 90% sure it would be a Honeywell
  • northernsoulnorthernsoul Posts: 112Member
    (> @hot_rod said:
    > does it run non stop and never get above 150 at the supply on the coldest days?
    >
    > As Jamie mentioned first check the temperature setting on the control. If it is set at 180 and the boiler never gets there?
    >
    > Or the return never gets above 150?
    >
    > If it has run 20 years with too low of a return you would see signs, rust around the burner tray, diverter hood, metal vent pipe, roof terminal.
    >
    > Ideally all conventional boilers like to be above dew point within 10 minutes of start up. Around 130- 140 at the return.
    >
    > At least a 20 minute run cycle would be desirable also.

    Thanks -I’ve never seen it get much past 150 even on coldest days. Normally around 130-140 most days

    It seems to take much longer than 10 min to get above 130.

    Is there a way to see what the operating temp is , other than the return temp ?

    It normally runs at least 30 min
  • ZmanZman Posts: 5,262Member
    The key is for it to reach those non condensing temps before the end of a cycle. If you are seeing rust on the vent termination or dandruff in the combustion chamber, you are probably condensing at least some of the time.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • northernsoulnorthernsoul Posts: 112Member
    We did replace some of the vent pipe that goes to the chimney flue liner that had rust.

    Not sure how old the pipe was as we’ve only been here 2 years
  • STEVEusaPASTEVEusaPA Posts: 3,352Member
    If you are seeing signs of condensing and you probably are, turning up the aquastat will not help.
    You can either pipe in a mixing valve on the return to protect the boiler, or if your well for the aquastat is in the boiler itself, switch to an aquastat that has circulator hold off, set to 130° or 140°
    steve
  • northernsoulnorthernsoul Posts: 112Member
    > @STEVEusaPA said:
    > If you are seeing signs of condensing and you probably are, turning up the aquastat will not help.
    > You can either pipe in a mixing valve on the return to protect the boiler, or if your well for the aquastat is in the boiler itself, switch to an aquastat that has circulator hold off, set to 130° or 140°

    Stupid question but is it safe to open up the aquastat?
  • ZmanZman Posts: 5,262Member
    Unless you are experience working around live wires, you should turn off the power first, but yes, open it and see what it is set to.
    From what you describe, it sounds like you have so much radiation that the boiler, as piped cannot reach it's set point.
    You would benefit from a re pipe with a smart mixing valve like the taco I-series which would give you outdoor reset and condensate protection.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • DZoroDZoro Posts: 902Member
    can you take a big pic with the boiler and all the piping?
    You can turn the power off if you feel more comfortable and open it up.
  • northernsoulnorthernsoul Posts: 112Member
    > @DZoro said:
    > can you take a big pic with the boiler and all the piping?
    > You can turn the power off if you feel more comfortable and open it up.
  • ZmanZman Posts: 5,262Member
    Unless the vent is rotting or you are seeing damage to the boiler, you might consider running with your current setup and saving your pennies for a mod/con boiler in the future. With your large radiators, you can expect savings in the 20% range with the new boiler and outdoor reset.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • DZoroDZoro Posts: 902Member
    It's hard to see but I think you do have a hot pipe bypass to the return piped in, but it doesn't have a balance valve installed. If this is the case, then that would be saving your boiler, and you probably, I dare to say have almost the perfect size boiler for your home. Does the system keep you warm during really cold spells?
    D
  • hot_rodhot_rod Posts: 11,874Member
    somebody had worked on it recently, That Caleffi fill valve isn’t 20 years old Are you sure the boiler is?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • northernsoulnorthernsoul Posts: 112Member
    Yeah I looked up the slant fin serial number- it’s made around 2000
  • northernsoulnorthernsoul Posts: 112Member
    > @hot_rod said:
    > somebody had worked on it recently, That Caleffi fill valve isn’t 20 years old Are you sure the boiler is?
    I had the fill valve iput n last year
  • northernsoulnorthernsoul Posts: 112Member
    > @DZoro said:
    > It's hard to see but I think you do have a hot pipe bypass to the return piped in, but it doesn't have a balance valve installed. If this is the case, then that would be saving your boiler, and you probably, I dare to say have almost the perfect size boiler for your home. Does the system keep you warm during really cold spells?
    > D

    How can I tell if I do have a hot pipe bypass? Is there a different angle photo I can show?

    It does keep us warm enough - the cast iron rads do hold the heat well even if there is no insulation in walls
  • DZoroDZoro Posts: 902Member
    The pipe I'm looking at has the blue valve above the pump. Then goes to a copper pipe, and then I believe to the main supply pipe from the boiler????? Or no??? I see the room is small to get a good side view for us.
  • northernsoulnorthernsoul Posts: 112Member
    That seems right . Ir does connect back to the supply line out from the boiler. What exactly is that doing?
  • DZoroDZoro Posts: 902Member
    That is bringing some hot water back to the boiler, protecting it from condensing too cold of water. So what I can't see is where the copper pipe goes to possibly the pressure tank? If that is the case then IMO it sounds like it is performing like it should, and the blue valve is the balancing valve that is needed. I personally would leave everything alone. Sounds like you have a proper sized and temperature set up system.
    IMO
    Congratulations, because that doesn't happen to often :)
    D
  • northernsoulnorthernsoul Posts: 112Member
    Pic > @DZoro said:
    > That is bringing some hot water back to the boiler, protecting it from condensing too cold of water. So what I can't see is where the copper pipe goes to possibly the pressure tank? If that is the case then IMO it sounds like it is performing like it should, and the blue valve is the balancing valve that is needed. I personally would leave everything alone. Sounds like you have a proper sized and temperature set up system.
    > IMO
    > Congratulations, because that doesn't happen to often :)
    > D

    Yes a pipe runs off it (black valve handle) to the expansion tank in the ceiling
  • DZoroDZoro Posts: 902Member
    Nice :)
  • DZoroDZoro Posts: 902Member
    Check the blue valve. It shouldn't be closed all the way. Should be opened about a turn or so. If it is closed then open it 1-2 turns, then your return temps will come up even more.
    D
  • northernsoulnorthernsoul Posts: 112Member
    > @DZoro said:
    > Check the blue valve. It shouldn't be closed all the way. Should be opened about a turn or so. If it is closed then open it 1-2 turns, then your return temps will come up even more.
    > D

    Thanks will do- I
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