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how to get temperature of water returning to boiler

timo888 Member Posts: 137
How to measure the temperature of the water returning to the boiler? Is there an inexpensive jacket-style thermometer that can be placed around the return pipe?

I am trying to see if the circulator on our system is right-sized, using this document as a guideline:

<a href="http://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf">http://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf</a>

Our remotest radiators tend to stay tepid. The outbound pipe is too hot to hold. The return pipe is tepid.


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    edited May 2011
    Non surface contact Infrared thermometer...

    Point and shoot.

    As it pertains to delta T, in my many years of slogging around mechanical rooms, I have seen the perfect "20 degree delta T" one time. It was on a hot water baseboard system, that was brand new, and was brining a house from a no heat condition to a heated condition, AT design conditions outside.

    Other than that, I've only seen a 7 to 10 degree F differential under real world, normalized conditions.

    What is more important, is to protect your heat source from long term condensation production. The Buderus cast iron has more silica in it, which makes it MORE resistant to the corrosive tendencies of condensate, but does not completely protect it. That requires the use of a non electric 3 way anti condensation valve.

    As for the cold radiator, remember, water is like my ex brother in law. Wet lazy and stupid. Given its own choice, it will ALWAY follow the path of least resistance, meaning those radiators that are the furthest from the pump will see less flow than those that are closer to the pump. Chances are, that you can balance the flow out, by restricting the flow to those radiators closest to the pump. This can be tough to do, depending upon how the system was piped.

    The alternative to this is to change the piping system from a direct return configuration to a reverse return configuration, which can be nearly impossible to do in some cases (finished ceilings).

    There are some inexpensive strap on dial type thermometers available on line, but the IR is better because you can use it to balance out your radiators too. Less than $100 bucks for a decent IR thermometer.


    ANother word of caution. In the Taco article, tehy assume a 20 degree differential, and don't go on to explain that high mass distribution systems typically see a significantly higher DT. High mass cast iron could see as much as a 50 degree differential, whic significantly affects the formula they are using. From all the research I have done, the use of a 20 degree DT is done to make the subsequent math easier, and has no real basis in real world applications. It is just a theoretical number used in determining the required flow rate. Basically, 1 GPM at a 20 degree DT = 9,960 btuH (rounded up to 10,000) delivery capacity. So, the theoretical heat loss calculation, divided by 10,000 = required flow rate. It is overkill.

    100 GPM @ a 1 degree F DT delivers the same amount of energy as 1 GPM @ a 100 degree F DT.

    THe other component in sizing a pump, as spelled out in the article, is pressure drop, and the higher the flow rate, the higher the pressure drop, the higher the parasitic cost of operation.

    The Europeans typically use a 30 degree DT (celcius) in their basis of design. That is much more than our 20. They laugh at our 20 degree designs.

    EDIT: I forgot to add, that the use of non electric TRV's will automatically balance the hydraulics and thermal out put. A worthwhile investment in my professional opinion.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • timo888
    timo888 Member Posts: 137
    anti-condensation valve non-electric

    Thank you for the detailed reply. So, I must research a topic new to me -- non-electric anti-condensation valve-- to see where they get installed and how they work. Things I don't know now. When you say non-electric are you meaning to distinguish them from the baffles that Taco has on their pumps? That's the only other place where I saw anti-condensation recently. Or is that something completely different?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Completely different....

    In fact, I am not familar with any baffles they have in/on their pumps, but you want to protect your cast iron boiler.

    The anti condensate valve is installed on the boiler return, and has a bypass from the supply to the return through this valve. Untill the return water is above the minimum setting, no flow is allowed out to the system. As the return is warmed up, it proportionally allows only the excess water to flow to the load. You can use the lower setting due to your boiler design (Buderi')

    Danfoss makes one.


    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • timo888
    timo888 Member Posts: 137
    edited May 2011
    MNPLUMBER Member Posts: 28
    Mark can you explain

    Why does this valve come in different operational temp settings on the return such as 113 and 140? Can you explain the difference and the uses?  Let's say you're running a standard fintube set up with 180 degree water in a loop and a cast iron boiler, would you use the 140? Shouldn't it be closer to 160? I'm confused. Thanks.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    The difference...

    Natural gas flue products condense at temperatures below 140 degrees F.

    The European casting technologies use more silica in their design, which makes them inherently more resistant to the corrosive tendencies of condensation. Hence, the lower return water temperatures allowed. Note the use of the term "resistant". I have seen many Bluderus boilers with their share of condensate corrosion in the combustion chamber... They are resistant, but not impervious.

    North American cast iron boilers must not be allowed to see a long term RETURN water temperature of less than 140 degrees F, or destructive condensate will occur. Big difference in the casting technologies, from what I have been told, by knowledgeable German boiler makers (Buderus).

    EVERY boiler condenses when it is first fired. It is a matter of HOW LONG it is allowed to stay in the condensing mode that counts.

    In the case of a low mass distribution system, like copper fin tube, personally, I would not worry about long term condensation production. What little condensation that is generated is evaporated about as fast as it is produced, lessening the potential for damage.

    It's the HIGH mass distribution systems, with large pipes and heavy cast iron emitters that cause long term condensation production that are problematic. This includes radiant floors poured in cementitious materials...

    If its not a German casting, I'd use the 140.


    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
    MNPLUMBER Member Posts: 28
    Got it

    Thanks Mark I get it now.

    When out on my regular plumbing service calls lately I've been taking a close look at any boiler I can get at. There are SO MANY variations of piping it's starting to make my head spin.

    I saw one this morning that had a bypass. Had the pump down low on the return side and had a one size smaller bypass above that going to the supply with only a gate valve in between? It looked they were were attempting to bypass with adjusting the gate valve to get warmer water into the return?

    I thought about this and it seems like it could work.  I don't think it would work with the pump on the supply side because wouldn't you just be sucking cold water from the return into your system?

    Wouldn't that also be a catch 22 since if you are limiting the amount of water in the system which limits flow wouldn't that drop the temp to the return even more?

    As you can tell I've got boilers on the brain lately.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Yeah, as many different piping methods as boilers....

    The best suggestion I can make is that you follow the manufacturers installation instructions as closely as you can. No two systems are the same.

    There are essentially two different methods of bypass used in the field. One is a system bypass and the other is a boiler bypass. The latter is usually used for protecting the emitters from too hot of a temperature, and the other is an attempt to protect the boiler.

    Neither of these methods are a fit all, fix all solution, but are better than no effort at all, which is what usually happens in the field.

    As you start learning more about these ultra high efficiency modcon boilers, you will note that boiler protection is not required. They are MADE to condense, in fact LOVE to condense. But you may find yourself still having to incorporate some means of high temperature protection if you have multiple and varying hot water demands. I.E., radiant floor needs 110 deg F water, but baseboards require 170 degree water, then the boiler will obviously have to be operated at the higher of the two temperatures, and some means (non electric or electric mixing valve) to protect the lower temp loads from seeing too high a water temperature.

    I've seen some older systems that were SO screwed up, that it astonished me that they could even get ANY heat out of the system, and it had been that way for YEARS...

    Just when you think you've seen everything, something new pops up on the radar screen....

    As for limiting flow creating greater DT's, yes, to a point, but once the return comes back adequately hot, and the boiler is hot, its all mute points anyway. The systems is going to warm up and everything will shut down anyway.

    As for boiler brains, we call it being a wet head. Welcome to the club. We need to start a self help group for addicted wet heads, like myself. I can hear the meeting starting now.

    "Hello, my name is Mark, and I am a wet head...."

    "Hello Mark, how many days have you been wet?". blah blah blah... 36 years...

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    multiple and varying hot water demands

    I have that, but my boiler controller can manage all that, partly because I am lucky.

    My boiler controller has three thermostat inputs, each running at a different priority. The zones are controlled by circulators. Normally, if a high priority heat call is being attended to, all the lower priority circulators are turned off. However, the way it is wired, if the baseboard circulator wants heat, its circulator runs. There is a timeout in the priority system so no priority can hog the boiler.

    Each priority selects its own reset curve, and which circulators are to run.

    So Priority 1 (for the indirect hot water heater) sets 170F and runs only the circulator that supplies the indirect.

    Priority 2 (for my big in-slab radiant heated zone)  sets 75F to 120F for the radiant zone and runs the boiler circulator and the circulator for the radiant zone.

    Priority 3 (for some Slant/Fin baseboard in a small zone) sets 110F to 135F for the baseboards. It runs the boiler circulator and the circulator for the baseboard zone.

    It should be obvious what happens as long as only one thermostat calls for heat. When both the radiant priority 2 zone and the baseboard priority 3 zones call for heat at the same time, the boiler circulator and both heating circulators run, using the reset curve for the radiant zone (the lower temperature one), so the radiant zone gets the heat it wants, and the baseboard zone gets some heat, but not as much as it would normally get. Because the baseboard has a small heat load, this is enough to keep that zone comfortable until the timeout gives it the full temperature it would otherwise want, or until the radiant zone is satisfied.

    The luck part is because if the baseboard zone had a much higher heat loss, I would need to put more heat up there, possibly running the boiler at the upstairs temperature and using a temperature mixing valve to keep the radiant zone under 120F.
  • timo888
    timo888 Member Posts: 137
    edited May 2011
    buderus return temp

    We have a Buderus Logano 115/4, 98K BTU. We don't have a Logamatic controller, just a small Honeywell Aquastat type L8148A.

    I find the attached document less than clear.

    What does it mean in the "Boiler ShutDown" column: "Possible, provided the boiler operates after total shut-down for at least three hours intermittently." You can shut the boiler down to work on it, but it must be made to come back on every few minutes for three hours afterwards?

    In the summer our boiler is off all day. I am confused. It does not fire intermittently for three hours in the evening when son takes a 30-minute shower after soccer practice. It might fire for only 20 minutes and then be off again.

    At rest (when it's been off for several hours) our boiler temperature gauge reads about 118F. Is that in violation of the Minimum Boiler Temperature requirement of 150F? Or is the minimum boiler temperature measured when the burner is firing?

    It's not clear to me whether the boiler temperature is not supposed to be allowed to drop below 150, or whether it can go as low as it wants to, as long as it can reach 150 within 10 minutes.

    In column #2: A mixing valve is "necessary". What kind of mixing valve is this? Where does it go? Is this mixing valve another name for the ACV discussed above?

    Our system uses large heavy iron piping (about 2" diameter) for its main return/supply lines, with 1/2 iron supply/return branches to 12 convector-type radiators. Is ours therefore a system "with very large water content" per the Buderus tech doc, so that the minimum water temp of 130F applies to us?
  • timo888
    timo888 Member Posts: 137
    edited May 2011
    motorized 3-way

    Is my understanding of this diagram more-or-less correct? The annotations in red are mine.

    1. The (motorized) 3-way valve directs some of the approximately 180-degree water from the boiler back to the low-temp radiant zone's return line, where it mingles with that 100-degree return water to become some temperature in between, say approximately 140-degrees? There's also approximately 160-degree water from the baseboard zone joining that return flow.

    2. The goal of this valve setup is to keep the return water temperature high-enough to minimize condensation in the boiler.

    And is something like this Italian-made valve the non-motorized equivalent?

  • Charlie from wmass
    Charlie from wmass Member Posts: 4,216
    No the water flow is going

    from the radiator return into the radiator feed to limit the temp to the radiator not increase the temp ion the return. The red horizontal arrow should be pointing the opposite way to indicate the actual water flow.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • timo888
    timo888 Member Posts: 137
    edited May 2011

    BTW, I'm gathering these documents here for my own benefit, and for the benefit of those who are as much in the dark as I am. I hope that's clear.

    Now I am trying to interpret the attached diagram (from Buderus manual) with a by-pass in light of this document from Bell and Gossett. I am in the information business, but this document from B&G has me totally stymied. It seems they deliberately went out of their way to be less than clear. Why not A B C D instead of A B and A B? :(

    But here's the section I find most interesting:


    Now, why would you want to raise the temperature of the water returning to the boiler? Well, suppose you had a high-volume system and a low-volume boiler, like an old gravity system. If the returning water were cool (less than 140 degrees for a cast iron boiler), the flue gases would condense inside the boiler and cause corrosion. There’s also the possibility of thermal shock, although this is usually less of a concern than condensation.

    Also, without the bypass, the fuel bills will usually be much higher than necessary, because the low-volume boiler will find it difficult to reach high-limit and shut off. Piped this way, the bypass lets you avoid these common problems.


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Boiler bypass versus system bypass...

    Tim, you have two different fixed valve methods shown, neither of which are completely dependable.

    Neither of them have any kind of self compensating operator to reflect changes in demand, so IF you happen to be so luck to be onsite during design conditions, and get the system bypass adjusted so the return is above 140 degrees F,at any condition of LESS than design condition, there will be too much bypass occurring. And conversely, if you happen to adjust it at less than "design" conditions, when design conditions arrive, your return water temps may be too low. Hence the need for the Termomix non electric thermostatic bypass valve. It doesn't care what the load is, it simply guarantees a return water temperature of 140 degrees F or greater under ALL load conditions.

    Location of the pump is critical to insure this works right. Otherwise, you will lower the water temperature going to the load, but not protect the appliance from long term condensation production potentials.

    Follow the piping instructions that come with the ESBE Termomix and you will be fine.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • timo888
    timo888 Member Posts: 137
    edited May 2011
    Danfoss : pump location

    Again, thanks, for the guidance.


    If the solid triangle inside a circle is the icon for pump location+direction, I could have a pump on the supply pipe (right where is it now), before a tee which branches over to the return.

    This seems to be minor modification with a major benefit.

    The matrix in the Buderus G115 manual cited a few posts earlier distinguishes between systems according to the type of controller. When an aquastat is controlling the system, it says return temperature must not drop below 130F. So I would order this Danfoss thermostat: 193B1703 131°F (55˚C) with the ESBE VTC511 Series #193B1700 Cv10 (the 1" valve).

    Instructions for the Danfoss anti-condensation valve:


    Danfoss actually specifies the torque requirements when seating the thermostat :)

  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    A little secret....

    Yes, triangle with circle is a pump.circulator.

    Danfoss owns ESBE. Really.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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