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Joe.G Member Posts: 213


  • Holycack
    Holycack Member Posts: 12

    Could someone tell me what condensing a boiler means? Sorry for the dumb question but I am trying to get a handle on this heating stuff. BTW I am heating a distant building to 6*C with my in house boiler via insulated 1" tubing and have a return temp around 15*C will this harm my boiler? Boiler runs for around 40 minutes to heat the building. I do have a bypass return line between supply and return piping of the boiler to avoid shocking the boiler. Just wondering if I'm doing harm.
  • Holycack
    Holycack Member Posts: 12

    Thanks for all the help I've been receiving from this board (NONE)
  • brucewo1b
    brucewo1b Member Posts: 638
    Feel free to look around

    There has been more than one thread on this subject recently. The short story is that you are bringing your water return temp back so cold that you cool of the flue gases to the point that you create water in the flue gas. Some gas boilers love it but most cast iron boilers live a short life because of it.
  • Dave_12
    Dave_12 Member Posts: 77
    Condensing your boiler

    With most boilers such as cast iron, copper fin, or steel, you will have flue gas condensation is water return temperature is below 140F. Such condensation is like rain within the boiler and stack, which will ruin the boiler and stack.

    A relatively new and special class of boiler, called condensing boilers, are made of special materials and special stacks. These boilers are designed to condense without harm to the boiler and stack material.

    Unless you have a boiler made to condense, it is imperative for longivity of the boiler and stack to make sure that return temperature is 140 F or above.

    This can be accomplished in several ways. The easiest is to operate the boiler hot enough so that the water after giving up heat to the system is still at 140 F. Other ways include a properly adjusted manual by-pass, a thermic valve preset at 140 F, a pumped by-pass, an 4-way mixing valve with actuator and controller, and other similar devices.

    Take care of the basics of good boiler design and operating and the boiler will last a long, long time.

    Good luck.
  • Barbarossa
    Barbarossa Member Posts: 89

    Also the flue gas temp is low enough to recover the latent heat of vapor thus raise the eff.
  • Joe.G
    Joe.G Member Posts: 213

    How does the return hot water effect the flue gas? ( you are taklking about teh stuff that goes out teh chimney right?) also how can I check the return temp? (that is the water that is coming back in from the rads right? thanks
  • Constantin
    Constantin Member Posts: 3,796
    Some minor info...

    The temperatures at which flue gases start to form depends in part on the fuel. Natural gas has 14% latent heat and its flue gases start to condense at around 137°F. Propane has 13% latent heat and its flue gas condensation temp is a bit lower. Oil only has 9% latent heat and its flue gases condense around 117°F, IIRC.

    The percentage of flue gases that actually condense out depends in large part on how much lower than the condensing temperature you can go with the flue gases. Thus, heating systems that have very low return temperatures do very well in this regard (radiant floor systems, for example).

    Naturally, if condensation is desired, the boiler and the flue system have to be up to snuff. Depending on the boiler brand that means plastic or high-grade stainless flue liners. Most condensing boilers use stainless heat exchangers, though Aluminum and plastic have also been in use (the latter only as a secondary HX in the EU).

    However, not every non-condensing boiler fears cold return temperatures. Some units like the Buderus G215, Burnham MPO, or Viessmann Vitola can deal with return temperatures much lower than 140°F without any issues. My Vitola supplies water at up to 110°F to the system and the return temperatures are thus usually in the 90's or less. IIRC, the Vitola and G215 are limited to 50°F and up, while the MPO needs somewhat warmer temps at 110°F and up w/o additional protection (pump logic, motorized mixing valves, injection systems, etc.).

    The original poster alluded to a bypass and thus may have afforded his boiler all the protection it needs to maintain the proper temperatures inside. As you correctly point out, with conventional non-condensing boilers, using piping arrangements like primary-secondary piping, etc. affords boiler protection no matter what the return water temperature is.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Hot Water Return

    Joe, your 'take' on the terminology is correct, flue gas is the term for products of combustion and return water is coming back from the radiation of whatever type(s).

    How this affects condensing:
    Most if not all condensing boilers operate on a "counterflow" principle.

    This means that the leaving water -the hottest water- exits the boiler at the initial combustion chamber, where the hottest flame resides and the flue gasses are at maximum potential. This makes for a good send off to the hot water on it's mission to the radiators.

    At the same time, the coldest (return) water enters the boiler near where the leaving flue gasses are exiting the boiler. It pre-heats the return water and "collapses" (condenses) the exiting flue gasses into sensible moisture.

    You can see how this amplifies and supports heat transfer, using the maximum temperature differentials. If reversed, in theory, you could add heat to the exhaust gasses and that would just be so WRONG.... :)

  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Here's a very good article on condensing technology

    15°C (59°F) is an extremely low return temp almost certain to cause problems with traditional boilers not designed for condensing operation.

    What's the return temp on the boiler side of the bypass?

    What type of emission system (e.g. tube-in-slab, fin baseboard, etc.)?

    A thermostatic bypass valve (like Danfoss ESBE type TV) would be a VERY GOOD addition to this system as it seems quite likely that you are resigning a conventional boiler to a premature death. A very rapid death if this is a low-mass (like copper tube) boiler driving a high mass (like tube-in-slab) system!

  • Holy-Crack

    The men here responded to your question of what a condensing boiler is. Was this not enough? Perhaps we were not of enough help to help to you but, you get what you give.

    Wallace Radiant Design

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Brad White_24
    Brad White_24 Member Posts: 28
    I think

    the "no help" comment was posted 3 hours after the first query. Check the date, it was the second comment.

    The initiator probably did not understand what constitutes a fast reply, but knows what thorough is at this point!

    Ah, to be impetuous again....
  • Joe.G
    Joe.G Member Posts: 213

    How can I check to make sure my return temp is high enough? thanks
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    Do you mean cold enough?

    Sorry, Joe, did not see your last posting. You can measure the temperature with a thermometer. To condense the flue gasses the returning water temperature should be about ten degrees F below the dewpoint of the flue gas. Generally the dewpoint of natural gas exhaust is about 128 to 138 degrees depending on composition. So your return water temperature should be at least below those temperatures and the cooler the better. All this time your boiler supply water is leaving at the lowest temperature necessary to heat the house at that time.
  • Weezbo
    Weezbo Member Posts: 6,232
    did you not like the idea of a station by pass?

    condensing a boiler and a condensing boiler are not necessarily the same thing nor are they mutually exclusive:)

    what you are describing sounds like a boiler that is Condensing,not a condensing boiler :)

    basically a "slug" of cold water comes back at the boiler and is not mixed back up fast enough which brings on a condition where it is being Swamped by cold and is unable to keep up or hold against the cold. to do that causes cold return water temps that can cause the boiler to Break for keepers.we call that Cracking the section. the by pass at the boiler needs to step the fluid back up to keep that slug of water from hitting the boiler ,the stradgey is to maintain the boiler temp by use of some reactive valve or pump or electrical sensor that drops out the system pump.

    a by pass at the station ,could limit the water temp comming back as well as going into the slab. using an injection pump ,modulating mixer ,buffer tank or heat exchanger would allow you to return water to the boiler at a signifigantly higher temp than shooting it out to the slab and hammering the boiler with whatever temp fluid could circulate back out of it ,on any given day.

    condensing causes some minor technicalities to ensue within and around the combustion processes and in and of itself can totally ruin a boiler or worse unless something is done with the byproducts of the combustion and condensed fluids. in other words when the boiler is designed to condense it is basically taking these "features" into account and the boiler has materials that can withstand the process built into it as well as the spent exhaust fumes *~/:)

    now if you have one of those boilers designed for condensing made from high quality stainless steel ,aluminium whatever then you need to check with the manufacturer as to freeze protection and flow rates...i think most condensing boilers have a programmed chip that makes the pump turn on at 37 F or so ,i am aware that this chip may be programmed to other temps upon request.

    making sence of the minor technicalities is a full time occupation in and of itself. we are all on this rock together. Feeling helped?


  • Joe.G
    Joe.G Member Posts: 213

    What is ils temp? and you say the warmest nesseary to heat the house, is that becasue the cooler the water the more efficent the system? thanks
  • Brad White_9
    Brad White_9 Member Posts: 2,440

    The cooler the water the less work the boiler has to do. Also the cooler the water the more condensing will occur. If you can supply AND return water below the dewpoint of exhaust gasses you are doing the best you can do.

    But a condensing boiler can take it whereas a conventional cast iron boiler will not (for long).
  • Dave M
    Dave M Member Posts: 36
    Small Peerless and cold return

    I have a similar situation as in this thread. Small Peerless Pro water boiler in an older house with a fairly large amount of water in the radiation system. It's a mix of fin tube and cast iron radiators, three zones. The boiler is low mass, I think less than 4 gallons of water. If the boiler is in standby and near full temp and the downstairs zone calls for heat the boiler temp will drop for 5 or 10 minutes until it starts to rise again. The cold water returning pushes it down for several minutes. It can take 10 or 15 minutes for the water temp to get back above 160. High limit is 180. I watch the digital temp readout on the L7224U aquastat. 

    There is a bypass pump (Taco 007) direct from the boiler supply to the return. A sensor in the supply turns this bypass pump on when the return temp is below 130. It doesn't turn off the pumps to the zones though. Even with this bypass pump it takes quite a while for the boiler temp to recover.  The bypass pump piping has a ball valve that is part way off. Is it OK to run a 007 pump full speed from supply to return with only a 5 or 6 feet of pipe? If so I would open that valve fully.

    I'm just concerned about flue gas condensation when the return temps cool that long. This boiler was sold as a cold start. It does have the L7224U with a no-call standby low limit set for  140. The boiler does heat the house OK. I am wondering if fully opening that valve in the bypass line would help (and is OK) and if shutting of the zone circulators when the output temp is low is necessary and would help. Thoughts?

This discussion has been closed.