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Cold water turns on combi boiler

JPL941JPL941 Member Posts: 46
edited April 28 in Gas Heating
Hello everyone, I have a quick question.

I've noticed recently that when I turn on cold water ie faucet, hose etc, it trips the combi boiler to either turn on or leave heating mode and go into DHW mode. I attached some pictures from the manual and a sketch of my set up.
I've already tried a shark bite check valve a couple feet (maybe 3) while it's going to the HW heater but it still trips the machine. If I close the hot water valve entirely and call for cold water, it doesn't turn on, like it's supposed to. Any idea where I should place this check valve? Is there one in particular to use? Should it be mounted closer?

Thanks.




Comments

  • Jamie HallJamie Hall Member Posts: 12,952
    The pressure relief valve is not an option. If you don't have one on there, you're way out of code. Furthermore, it must be located between the hot water outlet (port 2) and any valve or other flow restriction.

    In most cases, when the installation instructions use the word "must" (see the notice in the diagram you provided), they mean "must" -- so I have to admit I fail to see what the question is?
    Br. Jamie, osb

    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
  • EdTheHeaterManEdTheHeaterMan Member Posts: 636
    edited April 29
    OK tell me if I understand your query.
    You turn on cold water (not hot water) faucet in the house and the combi boiler is activated to operate as if it was a call for hot water.
    Is that right?

    Assuming that is correct... there is a flow switch in most tankless water heaters. Many of these switches are some type of propeller that moves faster as the flow increases. This change in speed sends a signal to the CPU (computer geek-speak) to tell the gas valve how much gas is needed. Faster means more water so you need more gas to heat the water. Slower means less gas to heat the water .

    Now, you have a 40-gallon tank (this defeats the purpose of the tankless water heater by the way) with an air cushion expansion tank. As the water heats and cools in the system this air cushion takes up the slack. As water temperature increases it expands and needs a place to go. This tank is where it goes. As the water cools, the air pressure in the tank pushes the expanded water out as it shrinks.

    That said, when you open a faucet any water in the air cushion tank will be released. This pressure change may be rapid and cause a momentary acceleration of the water past the tankless water heater flow control. The flow may be back and forth like a spring bouncing back and forth, but only for a few seconds.


    The location of the air cushion tank might be the answer to your problem. Place it on the cold water pipe before the Vega distribution block.

    Just a thought...
    if you leave the tank in the same location then change the check valve location to the place in the boiler instructions

    Pick one or the other, not both. check valve only needed if the tank is in the hot water line. It MUST be used if the tank is in the hot water line.

    Next question. When did the problem start?
    A. Always had problem
    B. After I installed the vega distribution block
    C. After we replaced / installed / repaired the (Xxxxxxx)

    You need to think like Sherlock H2Olmes
  • JPL941JPL941 Member Posts: 46
    edited April 29
    Yes, @EdTheHeaterMan , you are understanding my question. The minute a cold faucet, hose or toilet is used/flushed, it kicks the combi boiler on momentarily. I do understand what you mean by flow switch and that is what it has installed. Regarding the hot water heater, my old combi failed over Christmas 2 years ago and we were without hot water. Should it ever go down again, at least I have a backup. You’ve never met my wife without hot water. Consider it a safe redundancy - happy wife, happy life.

    The easiest thing for me will be to place the check valve in a closer position to the boiler rather than move the expansion tank. If that fails, I’ll move the expansion tank to the beginning of the manifold and remove the check valve. Do I have that correct? I wasn't quite understanding what you meant by this,

    "Pick one or the other, not both. check valve only needed if the tank is in the hot water line. It MUST be used if the tank is in the hot water line."

    To answer your other questions:
    This has been a problem since I moved into my house several years ago. I didn’t install the manifold.
    Since I’ve had this combi, it’s been tripping on cold water. It was even going off before the hot water heater and expansion tank were in place. I just want to get it fixed now.

    Also closing either one of the isolation Valves stops this from happening as well. So hopefully i installed it too far away and it will be fixed.

    Thank you very much for your very detailed explanation.
  • Jamie HallJamie Hall Member Posts: 12,952
    I'm sorry. I must be missing something quite essential. What, exactly, is the problem with piping that poor thing the way the manufacturer says you should?
    Br. Jamie, osb

    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
  • HVACNUTHVACNUT Member Posts: 3,413
    Could single lever faucets cause that?
  • Jamie HallJamie Hall Member Posts: 12,952
    HVACNUT said:

    Could single lever faucets cause that?

    Easily, if they aren't completely on the cold side when operated.
    Br. Jamie, osb

    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
  • EdTheHeaterManEdTheHeaterMan Member Posts: 636
    OK Somewhere in your home this is happening. A hot water branch and a cold water branch are connected together, then after the connection, there's a valve that is closed. This can be on a washing machine or a shower valve or appliance that has a need for mixed warm (hot and cold mixed) water.
    Maybe, if you wash your car with warm water, you have one of these.


    If you can't think of anything like that, then use the process of elimination. Close each hot water branch one at a time. Then test your system to see if the problem goes away.

    If that does not locate the problem, then try the cold water branches one at a time. If that does not locate the problem then the cross over is between the un-valved main hot and cold connections at the top (in the picture)

    You really do need to solve it. Every false start is another real start cycle on the components causing wear and tear on the components of the tankless water heater.
  • JPL941JPL941 Member Posts: 46
    I appreciate everyone’s input. To make it easy and not run throughout the house, I turned off every single hot and cold valve on the manifold and it didn’t change anything. That is after piping in the check valve On the hot side like we discussed earlier.

    Water comes to the meter, from there it makes kind of like a “U” and one goes to an outside hose and the other goes to an in-line filter, to the manifold, and as described in my pictures. I did forget to mention off the cold line that runs to the boiler there is a tee to feed another outside hose with a valve very close to the manifold that Is off. I opened that up, left the air out, closed it and it still did it. I can’t find a crossover. Even by the water meter when I open up the outside hose, it triggers the combi.

    I’m not quite sure what to do anymore. I did read that in the UK they put the check valves on the cold side and people say that works but I didn’t do that because it wasn’t in the manufacturers directions.

    Does my water pressure of around 80 make a difference?

    Thank you all.
  • Jamie HallJamie Hall Member Posts: 12,952

    I'm sorry. I must be missing something quite essential. What, exactly, is the problem with piping that poor thing the way the manufacturer says you should?

    OK, so I'll post it again. Look at the control device labelled 4 in the manufacturer's Figure 43 and just do it.
    Br. Jamie, osb

    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
  • JPL941JPL941 Member Posts: 46
    Jamie, it’s there. I put it in already and the problem is still happening.
  • Jamie HallJamie Hall Member Posts: 12,952
    Then it is a draw on the hot water line somewhere, somehow. Close valve 6. Then the phantom flow can't happen -- in either direction. Then track down the location of the phantom flow -- as has been mentioned, it may be a single handle faucet anywhere in the house.
    Br. Jamie, osb

    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
  • JPL941JPL941 Member Posts: 46
    Interestingly I had my wife turn the faucets on and off .. fast and slow. If it was fast, I could see a bit of movement in the pex. If it was a nice slow turn off, no movement In pex and the boiler didn’t turn on. Makes me wonder if the pressure is too high coming in and to make life easier, could i just add another expansion tank to one of the open cold ports on the manifold or a shock arrestor to see what happens?
  • hot_rodhot_rod Member Posts: 13,519
    Add a pressure gauge near the heater. Buy one with an indicator hand and see if you get pressure fluctuations. 80 psi is the most pressure you want, plumbing codes indicate 25- 89 psi
    A regulator and thermal expansion tank may be needed?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • BillyOBillyO Member Posts: 199
    I have never used this manifold before but is there a chance its faulty? just looking for most obvious first
  • JPL941JPL941 Member Posts: 46
    Hello everyone, I wanted to update you on my boiler. Plumber came today, installed a Pressure reducing valve, dropped to 60 psi. Left check valve in place. It no longer turns on with cold water taps, hoses, toilets. Thanks for the help.
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