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two pressures with NTI tx-151c combi unit

Hi guys.
I am new to this house, and it has a NTI tx-151 combi unit. There is a digital pressure monitor on the front panel of the main display, and a normal round one by the overpressure release valve.
I think I have some air in the system, and maybe a bad watts ext-30 expansion tank.
The round gauge is almost always at 30 (some times a little less) sometimes a little more after the system has bee running for quite a while. When it is over 30, it releases a little water (table spoons).
The digital gauge varies from 13 to 20 lbs when the system is running, and with a pressure tester on the expansion tank, the digital gauge seems to follow the tank pressure... which is a little wierd because it moves so much and so fast. (it will move a pound every second or two)
The User guide shows both gauges but doesn't explain their function. I not about to dig into it too far until the weather warms up a bit.
I need to understand it, because I don't have faith in the installer. After reading some forums, the plumbing seems way off. Also it is a replacement (5 years old) for another system that failed. A few of the oddities;
1) no way to descale, they never installed a hose fitting/tap on the cold side of the hot water heater side of the unit
2) they never installed the screen filter to keep particulates out of the boiler on the cold side of the unit.
3) they connected the hot side of hydronic floor to the return side of the hydronic floor with a 1 inch pipe (almost no flow thru the floors)
4) there is an H pipe supplied with the unit, and with this added to the existing piping, there is very little flow of floor water to the boiler. (there is a mainly stagnant loop between the boiler H pipe, and the circulation pump (which has it's own loop..
... I can fix it all, but I would like to understand the meaning of the two different pressure gauges before I start.


  • Steve_WheelsSteve_Wheels Member Posts: 15
    Craig, Not sure I understand your piping explanation, maybe a drawing or picture would help me see what you are explaining. As far as the gauge vs boiler pressures.... the gauge s $3, the sensor $50... one is more accurate when the system is static. The biggest key I have found is the GAUGE is on the outlet side of the internal circulator and the sensor is on the lnlet side of the, inherently this will cause a big difference IF the interior circ is running (see Pumping Away by Dan Holohan). The pressure should be set around 18psi when the circulatory is off and the system is cold. When running, if the pressure is climbing, maybe you have a bad expansion tank, maybe the tank is undersized... depends on water volume, temperature rise and system temperature. Of course most people here will agree the way to test a diaphragm expansion tank is to see if you have water on the air side of the diaphragm. drop system pressure to zero, check air pressure on schraeder valve on tank, it should be 18PSI if it's zero or water come out you have a bad tank (if below 18 pump it up). Of course if you want to make sure the tank is bad, drill a 1/2" hole on the air side of the tank and see if water comes out... guaranteed then the tank needs replacing (this last sentence is a joke).
  • Tom_133Tom_133 Member Posts: 684
    Nice explanation @Steve_Wheels I wanted to say the same thing but it didn't come out as nice as you wrote it
    Montpelier Vt
  • SuperTechSuperTech Member Posts: 1,303
    I don't understand the explanation of the piping configuration either, I think it would be a good idea post pictures.
    As far as the pressure problem, it sounds like a bad expansion tank. Are you pumping away from the expansion tank? I have found that if you are pumping into the expansion tank it greatly reduces the lifespan of the tank. One customer of mine has to replace his almost every year, he still doesn't want me to fix his near boiler piping for some reason.
  • ottawa_craigottawa_craig Member Posts: 7
    I have attached a few pictures.
    I have done a bit of studying, and I think I am going to call NTI to get their explanation of the two different pressures as well... What confuses me is a idle (days with boiler component off) the digital guage reads 14 pounds, and the round gauge shows 30. I would have thought that they would need to equalize at some point.
    Lots of goofyness in this piping.
    note the 'h pipe directly under the boiler. then their is a short coupled loop between that , and the primary feed. The primary feed is looped back with no pressure regulator back to the intake... Just one 1 inch loop.
    1- Either the H pipe or the short couple loop into the primary fed, needs to go.
    2- I am going to put a pressure bypass valve at the top of the piping between the pressure, and the intake, to limit pressure on the feed side into some of the smaller floor loops.
    3 - I am going to add a couple of valves at the top of the piping (one to bleed and one to cutoff)

    The Pressure tank may be gone (house from 2003) but no water at the Schrader valve.
    I have tried bleeding it down to about 10 lbs, with the pressure in the system at about 30 (according to the round gauge) and with the tap to the pressure tank closed. I still get no difference in the round gauge or the digital gauge. Then I open the tap, expecting one of them to move.. nothing.
    Almost as if the tank is blocked.

    Thanks for your time guys


  • DZoroDZoro Member Posts: 1,035
    Bad air tank, piped wrong, surprised it even works at all. Your primary loop should go through the factory low loss header under the boiler.
    The way the add on pressure gauge is piped it could have junk inside of it.
  • Tom_133Tom_133 Member Posts: 684
    Too bad, a really good workhorse boiler, and its really not piped well.
    Montpelier Vt
  • Steve_WheelsSteve_Wheels Member Posts: 15
    replace the gauge and fix the piping as suggested by DZoro!
  • ottawa_craigottawa_craig Member Posts: 7
    Spring came, so I got to work on correcting the piping. I found that the spirovent has a small plug in the inner tube of the wire cartridge. This plug slid down in the tube, blocking the pressure tank connection, then scale built up preventing pressure from escaping from that same tank. The expansion tank was Isolated from the system, causing pressure blowout. Along with the piping correction, and adding a differential presssure regulator the system is acting like it should now.
    The two pressure guages still read miles apart (one at 15 - digital the other at 30 - the analog one ), and the analog gauge is greatly affected by temperature. You can REALLY see that as if you blow off a bit of water, the analog gauge goes up.
    I am going to replace the cartridge in the spirovent an NOT INSTALL that stupid PLUG. It is hard to get the core in Canada. I think I will have to order it to a border town, and go pick it up. Here they only want to sell the whole thing for $300 rather than a core for $13. Go figgure.
  • Tom_133Tom_133 Member Posts: 684
    Montpelier Vt
  • ottawa_craigottawa_craig Member Posts: 7
    Thanks Tom.
    I am going to give them a try, but they only sell to US customers. Last time I used them, they complained, but they still sent the stuff to a border town where I could get it.
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