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Burnham V-73-T tankless coil size help

ilya980ilya980 Posts: 10Member

I need to replace leaky tankless coil in my Burnham V-73-T boiler. What size coil should I use? 5GPM? Would 6 GPM coil fit? I have a rectangular plate 5x12-1/4 with 8 bolts. The boiler is 105000 BTUs. The current marking on the tankless plate says V1-2. I called multiple plumbing stores and suppliers, but no one can tell me if 6 GPM coil will fit. Are there any spec sheets with measurements available? I also can't find anywhere how large the coil compartment is. The boiler manual has all dimensions, but no diagram for the coil compartment. So, will either 5 or 6 GPM coil fit? Is there a reason to choose 6 over 5?



  • ilya980ilya980 Posts: 10Member
    Thanks. I called them. They are nice and helpful, indeed, but they could not answer my question whether 6 GPM or 5 GPM coil will fit. They don't make 6 GPM coils. They offered me 5 GPM coil priced higher than from most local suppliers. They told me to call Burnham about 6 GPM option. I already did. Burnham said they don't know. I was hoping that someone has dimensions of the 6 GPM coil or knows whether it fits into V73T boiler, or at least has dimensions of the coil compartment.
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,630Member
    also tryTOLL FREE PHONE: 1-800-654-8045 TFI Everhot

    if your inlet water temp is 50 degrees and you heat it to 120 deg 70 deg rise in temp it would take 174,300 btu/ hour to do that at a flow of 5gpm.

    so a bigger coil if available (and it probably isn't) won't do much.

    If you need more hot water install an indirect storage tank
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,630Member
    @Hatterasguy , Oh, I agree you can almost never get rated capacity. I was just trying to show that the boiler won't support a larger tankless anyhow.

    We used to put flow restrictors in the cold water feeding the tankless

    Also, if I am not mistaken tankless coils are rated @200 deg boiler water which no one maintains but is possible with a steam system in the winter
  • GrallertGrallert Posts: 305Member
    ilya980 said:


    I need to replace leaky tankless coil in my Burnham V-73-T boiler. What size coil should I use? 5GPM? Would 6 GPM coil fit? I have a rectangular plate 5x12-1/4 with 8 bolts. The boiler is 105000 BTUs. The current marking on the tankless plate says V1-2. I called multiple plumbing stores and suppliers, but no one can tell me if 6 GPM coil will fit. Are there any spec sheets with measurements available? I also can't find anywhere how large the coil compartment is. The boiler manual has all dimensions, but no diagram for the coil compartment. So, will either 5 or 6 GPM coil fit? Is there a reason to choose 6 over 5?


    I would suggest you look into replacing the tankless coil with an indirect water heater. There will be a greater upfront cost but after that you will realize only benefits. Your fuel consumption will drop dramatically and you comfort will increase. You'll have an almost unlimited hot water supply that will be a stable temperature. You'll notice that your boiler isn't coming on every ten minutes in the summer time to poorly heat its self up in case you are going to use the hot water. Have a pro look into this option for you.
  • mark schofieldmark schofield Posts: 142Member
    A long time poster (Ice Sailor) would suggest using an inexpensive electric water heater tank, not hooked up electrically, as a storage medium. It would be fed from the tank-less coil output with a circulator pump.
  • HVACNUTHVACNUT Posts: 2,036Member
    Like the other guys said, an indirect is the way to go, but
    Diversified Heat Transfer Inc.
    800-221-1522 shows your V73 6 gal. coil.
    Hope you have a good tap and dye set.
  • ilya980ilya980 Posts: 10Member

    thanks for the comments and ideas. I bought the diversified 5 GPM coil. Outside looks exactly like mine, haven't seen inside yet - too cold to drain the system. Where do you see the picture or measurements of Diversified 6 GPM? I called Diversified, but they could not tell me the length of 5 GPM vs 6 GPM. There was only one plumbing store told me that 6 GPM definitely will not fit.

    With indirect heater the truth is that it would take many years to recoup the upfront cost. My gas bill in Summer is around $40, and that incudes gas range and hot water. Even if indirect cuts it in half (which I doubt), considering that I am not using heat for 4 months/year that would be $80/year savings. It will take at least 15-20 years to recover the cost of the indirect system + labor. By that time the system will be old, or I might not even live in this house.

    I also don't like reducing boiler temperature in Summer because it starts to leak everywhere and corrodes. Bad, I know, but what can I do? It is an older boiler.

    Why do I need tap and dye? Is it not possible to just swap the coil? I see that all connectors to it are threaded. If I break them while unscrewing or damage the threads, I'll get new once. And there are only 2 connectors: 1 on the inlet and another on the outlet. Is it impossible to get them out?

  • HVACNUTHVACNUT Posts: 2,036Member
    The new coil will come with new bolts and gasket.
    Like Hatterasguy said, the 8 bolts holding the coil plate to the block are not coming out with a simple twist. Do what he said. Have a good 3/8 hex ratchet. I think the bolts are 9/16.
    What I do when I know it's gonna be a bitch. Take a flat cold chisel and a lump hammer and tap (semi hard) the bolts (all around. Top, bottom, left, right) where they meet the plate, being careful not to damage the bolt face. Then go EASY with the ratchet. An
    1 1/2" spackle knife is good to scrape the old gasket.
    I'm doing this over 30 years and a coil replacement can be an hour and a half or 8 hours.
    If you have to re tap, have new metal drill bits. Start small and enlarge 1 size at a time. Pick up a zoom spout electric motor oil to lube and cool your drill bits.
    Scared yet?
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,630Member
    Getting the bolts out is a challenge that's for sure. It's usually about 50% break 50% come out. The only boiler I ever saw done right was the older Smith boilers had a flange cast on the boiler so the tankless heater was a nut and bolt job.

    I have also drilled and tapped some oversize and used 7/16" diameter bolts when the boiler had enough meat. 7/16-20 takes a 23/64" drill so your really just taking out the 3/8" (24/64") threads and tapping a little larger. You can use brass bolts which are easier to drill out the next time

    If only people used "never seize" on the original install
  • ilya980ilya980 Posts: 10Member
    Thanks, great ideas! The bolts you are talking about were unscrewed by a plumber when the system was converted to gas 4 years ago. They are cap screws. He broke 1/8 and had to drill and heat to get it out. He replaced the gasket at that time because it was leaking. Several months later I noticed that the gasket started to leak again and also noticed that those 8 bolts were loose. So I re-tightened them by about 1/2 turn each. The leak stopped. I haven't seen any leaks since then. Those 8 bolts have some corrosion over 4 years, but not much. My plan is to try to loosen them with a socket before I drain the boiler and disconnect the aquastat. I'll have an idea whether I need to drill and heat or not.

    But I am concerned about brass couplings, at the inlet and outlet. They have no caps, just threads. I probably can unscrew them with wise grips, but that will likely damage the threads. Is there a good way to unscrew those fittings? Can I use a strap wrench? (like for car oil filters). Or should I just buy new couplers and not bother unscrewing them?

  • ilya980ilya980 Posts: 10Member
    I just tried to brake loose all 8 bolts and it was very easy. Just regular 14 mm wrench was enough. I also see there are flat parts on the inlet and outlet couplings? Can I just grab them with wise grips and twist? It will score brass a bit, sure, but as long as I don't poke a hole it should be ok, right? Is there a better way? Also, do you use Teflon tape or thread sealant on those connections?

  • HVACNUTHVACNUT Posts: 2,036Member
    Can you post a pic?
  • ilya980ilya980 Posts: 10Member
    Two photos are attached.

    Yes, I am talking about fittings from the ports (and the aquastat fitting). How can I not take them out? The new coil has ports, but it does not come with fittings. Are you saying I should get new fitting and elbow at the top and fitting, T, and valve at the bottom? Can I reuse any of the existing parts? If yes, should I use Teflon tape or thread sealer?

    I labelled the desoldering points. Top one seems easy, but the bottom looks harder, because there are 2 other soldering points right next to it. Any suggestions on how not to damage the other 2 connections?

  • ilya980ilya980 Posts: 10Member
    Thanks. Should I use Teflon tape or thread sealer or something else on the new couplers?
  • ilya980ilya980 Posts: 10Member
    What is the best tool for unscrewing the fittings from the boiler? Large vise grips? Is there a special wrench for these fittings? Thanks.
  • russiandrussiand Posts: 73Member
    Gosh if you have gas, it may be simpler to install a regular gas water heater
  • ilya980ilya980 Posts: 10Member
    Hi everyone,

    just wanted to give you an update. I finished the project. Heat is on, hot water is on, no leaks. Everything seems great!

    Big thanks to those who helped with valuable advice.... and shame on those (not on this forum) who gave me wrong information.

    Now a long rant about my experience. Maybe, one day another DIYer will be reading this and find it useful.

    It is not that difficult of the project I think, after all. But there are some pitfalls. After asking 10 different plumbers and plumbing store sellers no one could tell me what kind of coil should I buy, or where to get OEM coil, or what size (in inches!) of the coil I need, or what is the size of the coil compartment. One guy told me that I should buy diversified 5 GPM and 6 GPM will not fit, because he is working for 40 years and he knows for sure. Well, I bought 5 GPM diversified, eventually it fit, but I don't think it is the best choice.

    After taking everything apart I found that the original coil is bigger than Diversified-5. I was disappointed. I still cannot identify who made the original coil and how many GPMs is it? Do you know (see photo)? The second problem is that D-5 coil only fits UPSIDE DOWN. I hope water flow direction does not matter, but it is still very odd. The separator inside the compartment is not halfway between top and bottom! See pictures. The OEM coil has 3/4'' ports, but D-5 apparently has 1/2'' in and out pots, but 3/4'' aquastat probe port. Meaning that I had to run to the store in the middle of the job to get replacement couplers and other connecting hardware. But the funkiest part is that the aquastat no longer fits as it was on the original coil. The only way to fit it, yes, you guessed right, is upside down. Fortunately, I have 100% electronic modern aquastat. But what if I had a circulator relay that is disconnected by gravity? I would not be able to make it work at all! Also, if the aquastat wires have almost no slack, it could be a problem to flip it. On top of that was it pure luck that the "gap" in the windings in the D-5 coil was just where the compartment separator was? If it wasn't, I would not be able to install the new coil. So, D-5 coil is marketed as compatible with Burnham V-7 water boilers, but in my opinion it is a stretch. Thankfully, it worked after all.

    In terms of hot water production - there is not much difference. When I took the shower I feel the moment when the "initial" water stored in the coil is ending. But I felt the same with old coil too. It lasted longer though. I am not running out of hot water, but I definitely would chose OEM coil over D-5. (unless OEM is no longer made)

    Some advises for DIYer that I learned hard way. Always remember the correct direction to wrap the Teflon tape on a thread (so it does not bulge). Otherwise good soldering job means nothing if Teflon connection is leaking. Do not desolder, cut instead (thanks for this advice, was very useful). Much more heat is needed when the soldering seam is next to other big things - they consume all the heat.
  • HVACNUTHVACNUT Posts: 2,036Member
    It's quite possible, because lack of demand, they only manufacturer the 5 gph
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,630Member
    @ilya980 , you were successful so good job. Changing a tankless heater can be a challenge, more so if the bolts snap off. Lucky you did not have that issue.

    In most cases with smaller boilers the capacity is determined not only by the size of the coil but also by the boiler capacity.

    In you case while a larger coil is always better the smaller coil may have little effect on it's performance.

    In this business, it's common to find differences in equipment when you "open the box". When someone says to me that the replacement is" exactley the same" that's when I get scared

    Thanks for the update
  • Chris_110Chris_110 Posts: 3,056Member
    This is a funny thread.. Considering a boiler w/a tankless coil operates while the heat is on a 5gpm, 6gpm even 10gpm rated is just a number.. Coil is nothing but a HX whose capable output is dictated by available btu/hr and it's sq surface.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
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