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Do I need a low-limit set on my non-condensing boiler if it is piped with a bypass?

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  • Do I need a low-limit set on my non-condensing boiler if it is piped with a bypass?

    Hello. Here is a question for those familiar with radiant in-floor heating set-ups.

    Recently bought a new house. It has a 52000 BTU non-condensing gas-fired cast iron boiler that provides heat for a radiant in-floor heating system. The in-floor heating has six zones, each controlled by a zone valve. Each valve is connected to a separate thermostat. The boiler is only used for the in-floor heating, as there is a separate gas-fired conventional hot-water heater tank making water for household use.

    Currently, it is becoming warmer and warmer outside and as such I was a bit surprised when I noticed that the boiler was still firing periodically for a few minutes at a time (and then going off for a couple hours). None of the zones were calling for heat. A bit of investigation showed that the boiler is maintaining a temperature of between 120 and 170 degrees (lower than 120, it fires until it hits 170, then stops). I did some reading and learned that this is probably to prevent the system from becoming too cold and causing condensation on the boiler which could rust it, etc.

    Problem is, I did more reading and realized that, in addition to the three-way valve that mixes 170-degree water with cold return water to make a temperature suitable for the plastic in-floor heating pipes, there is also actually a bypass loop piped in to the system. The bypass (I think) ensures that newly heated hot water is piped directly back into the intake of the boiler, mixed with the cold return water from the system to prevent too-cold water from entering the boiler and possibly also causing acidic flue gases from condensing.

    Ideally, I'd like to remove the low-limit configuration from the system somehow to prevent the boiler from firing up unless at least one of the zones is actually calling for heat. In the summertime, having a low-limit of 120 degrees is a huge waste of energy. Almost like leaving the kettle on all summer just because you might want a cup of tea once a month on an especially cold morning. Would removing the low-limit and letting the system cool right down to whatever it would be with just a pilot light a good idea given my non-condensing boiler set-up? I'm thinking that the bypass should work to minimize the amount of time that the boiler is dealing with cold return water and also ensure that the boiler's hot side is only slightly hotter than the colder return side, preventing any damaging temperature gradients as well.

    Thanks in advance for your help with the above.


    Bit of a separate question: If I were to remove the old-style non-condensing boiler from my house (to try to up my energy efficiency to something better than the ~80% or so that it provides along with the very warm boiler room and very high gas bills), could I replace it and my 10 year-old conventional water heater (probably will die soon anyways) with a single high-efficiency hot water heater system? I have read that hot water heaters can often provide enough hot water for in-floor heating systems while also providing domestic hot water for showers - especially if you include a priority system that turns off in-floor heating after a couple of hot showers until the system recovers. How do I know if the new system would provide enough BTUs for my needs? I live in a 2400 sqft house in Vancouver, BC, Canada - my house seems to be well-insulated, and our winters get down to maybe 10 degrees Fahrenheit (-10 degrees Celsius) at the coldest. I've heard that I need to do some heating calculations but don't know how to go about that.

    Thanks for all your help!
    Last edited by mattw63; 05-28-2013, 02:43 AM.

  • #2
    Re: Do I need a low-limit set on my non-condensing boiler if it is piped with a bypas

    The boiler has a maintaining control. That is, it maintains the maximum temperature of the boiler as frequently as possible. It's usually only needed in commercial setups. I've seen it only in residential in very old units. Preventing condensation in cast iron is important. Having water come back under 140 degrees is the main culprit. However, for radiant applications this may be too hot. Replacing the aquastat control is usually the only way to prevent the 'maintaining boiler' problem. The primary-secondary loop you have is to help prevent too cold of water from entering the boiler, mainly to prevent thermal shock.

    Since you want to replace the older inefficient system here's what I would do:

    1- have a heat loss calculation done for your house. A manual calculation is not inexpensive and requires a competent HVAC installer
    2- remove old cast iron boiler and hot water tank - replace with high efficiency boiler and indirect hot water tank

    A water heater is really not designed to provide domestic heating. Don't try it, you won't like it, and it may not pass any inspections.

    If this were my house I would do the above with:

    - Viessmann Vitodens boiler with Lambda Pro controller
    - Viessmann indirect hot water storage

    This setup heats your house based upon the outdoor temperature. The boiler adjusts the firing rate up or down as needed and also adjusts the combustion ratios for best burning efficiency. There are all heavy (heavy!!) gauge stainless steel and I've seen these boilers have return water temps of 36 degrees F and have no problems. For you, this means no more primary-secondary setup, less pumping using a low loss manifold, less electricity, and higher efficiencies without a problems of condensation. Priority is built in so the unit knows when to crank up for domestic hot water production.

    The difference between this HE boiler and others is that it will run more frequently. It nearly never shuts off (during cold weather). It maintains your house temperature without the startup and shutdown that other units have. It's hard to believe but it actually saves in comparison with other similar units.
    ~~

    ... it was plumbed by Ray Charles and his helper Stevie Wonder

    Comment


    • #3
      Re: Do I need a low-limit set on my non-condensing boiler if it is piped with a bypas

      Originally posted by Plumber Punky View Post
      The boiler has a maintaining control. That is, it maintains the maximum temperature of the boiler as frequently as possible. It's usually only needed in commercial setups. I've seen it only in residential in very old units. Preventing condensation in cast iron is important. Having water come back under 140 degrees is the main culprit. However, for radiant applications this may be too hot. Replacing the aquastat control is usually the only way to prevent the 'maintaining boiler' problem. The primary-secondary loop you have is to help prevent too cold of water from entering the boiler, mainly to prevent thermal shock.
      140 degrees would be too hot for in-floor radiant applications - which is why we have a three-way valve as part of the system. Thus, 180-degree water becomes more like 120 or so which is fine for the PEX in the floor. And yes, the water will come back cool initially, but after the system heats up, the return water will be mixed by the water from the bypass line to be higher than 140. Would that not re-evaporate any condensation that would have built up over the few minutes of start-up time?

      And if the system turns itself off due to no more calls for heat, the water stops flowing, so at that point I don't think we should have any more condensation issues (no flow means no more cold water flowing into the boiler = no condensation?).

      As far as the boiler goes, it is a Weil McLain CGA Gold series 1, model CGA-25-SPDN. It is only 52000 BTU so I would assume it is not for commercial applications... and I wouldn't class it as a very old unit (likely from 2003 or so when the house was built). Should it have a maintaining control? Maybe this is the problem - we had a guy out last year and maybe he changed the controller to an inappropriate one for the model of boiler.

      Comment


      • #4
        Re: Do I need a low-limit set on my non-condensing boiler if it is piped with a bypas

        Originally posted by mattw63 View Post
        140 degrees would be too hot for in-floor radiant applications - which is why we have a three-way valve as part of the system. Thus, 180-degree water becomes more like 120 or so which is fine for the PEX in the floor. And yes, the water will come back cool initially, but after the system heats up, the return water will be mixed by the water from the bypass line to be higher than 140. Would that not re-evaporate any condensation that would have built up over the few minutes of start-up time?
        Yes, but with a condensing boiler designed for radiant it would not heat that high, except for when domestic hot water calls for heat.

        And if the system turns itself off due to no more calls for heat, the water stops flowing, so at that point I don't think we should have any more condensation issues (no flow means no more cold water flowing into the boiler = no condensation?).
        Ideally, yes, unless there is moist air blowing down the chimney.


        As far as the boiler goes, it is a Weil McLain CGA Gold series 1, model CGA-25-SPDN. It is only 52000 BTU so I would assume it is not for commercial applications... and I wouldn't class it as a very old unit (likely from 2003 or so when the house was built). Should it have a maintaining control? Maybe this is the problem - we had a guy out last year and maybe he changed the controller to an inappropriate one for the model of boiler.
        i've never seen the gold series of boilers have a maintaining control. it should have an electronic module that controls the pump and firing. the zone valves make or break the end switch to activate a call for heat. there may be an external high limit control but I cant recall 100%. is there a zone valve sticking in the ON position? is there any other zone controller that may be causing the problem? perhaps the control itself is initiating the call (stuck relay, etc).

        this boiler is very picky when it comes to condensation. causing it to condense will rot out the iron as the condensation has no good path to the drain (there are many flat, horizontal surfaces inside).
        ~~

        ... it was plumbed by Ray Charles and his helper Stevie Wonder

        Comment


        • #5
          Re: Do I need a low-limit set on my non-condensing boiler if it is piped with a bypas

          i've never seen the gold series of boilers have a maintaining control. it should have an electronic module that controls the pump and firing. the zone valves make or break the end switch to activate a call for heat. there may be an external high limit control but I cant recall 100%. is there a zone valve sticking in the ON position? is there any other zone controller that may be causing the problem? perhaps the control itself is initiating the call (stuck relay, etc).
          It definitely does have an electronic/electrical module of some sort. I'm not at home right now and am heading out of town for a few days so I can't check on the model number until maybe Friday but when I can I will read the sticker on the side and post the model number of the control module here. I'm pretty sure that there isn't a zone valve stuck open because I've taken the covers off and looked at them all, and can usually feel the pipes to see if they are getting hot, etc. Let's just say that I'm very sure that when the boiler has recently turned on, it's doing so when it drops to 120 F and stays on until it heats up to 170 F. When it's doing this I have put my hand on the circulating pumps and they are definitely not on (they do turn on when a zone valve calls for heat).

          Comment


          • #6
            Re: Do I need a low-limit set on my non-condensing boiler if it is piped with a bypas

            should be a black box with a few LEDs for power, prepurge, igniter, flame, etc. i have never seen one that cycles the boiler with no pump action at all. does the internal pump above the boiler turn on?

            definitely post the control number so we might be able to research this a bit more.
            ~~

            ... it was plumbed by Ray Charles and his helper Stevie Wonder

            Comment


            • #7
              Re: Do I need a low-limit set on my non-condensing boiler if it is piped with a bypas

              Got home tonight.... the controller is a Honeywell R8239D. It's not black - more like silver metal-covered. It doesn't have any LEDs at all. I think it might be PLC controller rather than an electronic one. Also, numerous wires inside the wiring area are disconnected. Most notably (for me at least) is that the diode-looking thing that sits way down near the burner isn't connected to anything. It has yellow wires running from its leads all the way up, but one of the ends is just sitting in the air. What is that thing that is down by the burner called? If I can figure that out I might be able to find it on the schematic.

              Comment


              • #8
                Re: Do I need a low-limit set on my non-condensing boiler if it is piped with a bypas

                Did a bit more looking around - seems that what I thought was a controller is probably just a transformer... The main controlling feature of the system is the aquastat. It has only one dial on it - which is set to 170. That seems to be the high limit of the system... so I bet that the aquastat is maintaining a closed circuit when the temperature is lower than 170, but higher than 120. If the temp drops below 120 again, the aquastat trips and the boiler heats up again. There doesn't seem to be a control that sets the low limit - maybe the aquastat just infers that information from the high limit?

                I'm thinking that we just need to wire the zone valves into the loop somehow - such that the aquastat can't turn on the flames unless at least one zone valve is calling for heat. How to do this without buying a bunch more relays?

                Comment


                • #9
                  Re: Do I need a low-limit set on my non-condensing boiler if it is piped with a bypas

                  ok, you have an older Gold unit, before the newer Gold units with the electronic control. some of what i said above pertains to the newer units. here is a manual for your unit:

                  http://www.weil-mclain.com/en/assets...1-009_0107.pdf

                  the transformer box thing is a fan center relay usually used for pump control. I cant remember if it is also used for main burner power too. The schematic is on the back of the front panel door. The aquastat is the main temperature controller while the high limit is a dual safety and only shuts off when the set temperature is reached. It is usually set at 220 degrees.

                  all the zone valves have a TT or end switch circuit. each terminal of one switch gets wired to each other identical terminal on the others. then the last valve in line runs down to the TT terminals of the aquastat. thus, when any one of the valves calls for heat, the end switch closes. power from the aquastat then completes a circuit and turns on the aquastat. there may currently be a jumper between the tt which is causing it to call continuously, or maybe the aquastat is bad.

                  you need a different company to come out. the diode thing near the (right hand side? ceramic looking thing?) burner is a roll out safety switch that has been disconnected.
                  ~~

                  ... it was plumbed by Ray Charles and his helper Stevie Wonder

                  Comment


                  • #10
                    Re: Do I need a low-limit set on my non-condensing boiler if it is piped with a bypas

                    Originally posted by mattw63 View Post
                    Got home tonight.... the controller is a Honeywell R8239D. It's not black - more like silver metal-covered. It doesn't have any LEDs at all. I think it might be PLC controller rather than an electronic one. Also, numerous wires inside the wiring area are disconnected. Most notably (for me at least) is that the diode-looking thing that sits way down near the burner isn't connected to anything. It has yellow wires running from its leads all the way up, but one of the ends is just sitting in the air. What is that thing that is down by the burner called? If I can figure that out I might be able to find it on the schematic.
                    The flame rollout safety switch has been bypassed and that's a big NO-NO! You need to have the control wiring checked and corrected ASAP. Sounds like someone has recontrolled the burner by using the limit switch as the thermostat. He has adjusted the temperature differential on the limit all the way out. (i.e. comes on at 120, shuts off at 170) Typically, you would want it closer to a 10-20 degree difference, depending on the system.

                    Just so you know, your current configuration has the potential for catastrophic damage and loss of life. You're lucky it's only a 52,000 BTU boiler. That's only big enough to take down your house but probably not your neighbors. No, I'm not exaggerating....

                    Comment


                    • #11
                      Re: Do I need a low-limit set on my non-condensing boiler if it is piped with a bypas

                      Thanks for the advice, Punky. I feel like I'm understanding what is going on in there now and will at least be able to determine when something is being done incorrectly in there.

                      Originally posted by plumberscrack View Post
                      The flame rollout safety switch has been bypassed and that's a big NO-NO! You need to have the control wiring checked and corrected ASAP. Sounds like someone has recontrolled the burner by using the limit switch as the thermostat. He has adjusted the temperature differential on the limit all the way out. (i.e. comes on at 120, shuts off at 170) Typically, you would want it closer to a 10-20 degree difference, depending on the system.

                      Just so you know, your current configuration has the potential for catastrophic damage and loss of life. You're lucky it's only a 52,000 BTU boiler. That's only big enough to take down your house but probably not your neighbors. No, I'm not exaggerating....
                      Plumberscrack, thanks for your advice as well. I will definitely have someone else come and "re-fix" the boiler wiring and ASAP. The limit switch seems to be adjustable from 140F up to 200F - currently sitting at 170. From some googling on the limit switch, it looks like it's supposed to have a 30 degree differential... I know you said 10-20 degrees is better but it looks like the designed differential is 30... I hope that's OK.

                      The boiler manual says that it needs to be protected from cold water input of less than 130 degrees... it seems unavoidable to me that when it hasn't been running for a while that it will cool down to near room temperature (maybe a bit warmer due to the standing pilot light). Am I correct to assume that sub-130 degree water is OK as long as the temperature of the water at the exit to the boiler is also only slightly warmer? The bypass piping should protect the boiler from any large damaging differential from the water inlet to outlet....

                      Given that radiant heat in the floor should run around 125 degrees and has a three-way valve to mix with cold whenever the input hot water is >125 degrees to protect the floors from overheating, maybe what I need to do is set the limit control to whatever it takes to have 125 degrees as the low-end of the heating hysteresis... this way the burner will fire until it is outputting 155 degrees (which takes a while due to the bypass and open zone valves), and then will be shut off by the limit controller until the temperature drops to 125 degrees (and then will fire until 155 again) - unless of course the "call for heat" goes away due to the thermostats.

                      Comment


                      • #12
                        Re: Do I need a low-limit set on my non-condensing boiler if it is piped with a bypas

                        most aquastats i have seen have a range of 20 to 30 degrees differential. sub-130 degree water is normal at startup. they dont want 130 degrees or less for the condensation reasons and that's why there is bypass piping. the aquastat should be set to 180-190 degrees and the mixing system should handle the rest. you really do need the higher temps in this cast iron boiler to have the longest lifespan of the cast iron.
                        ~~

                        ... it was plumbed by Ray Charles and his helper Stevie Wonder

                        Comment

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