Boiler PSI gauges not equal, plus PSI setting for Viessmann Vitocrossal CU3A

b25urgandy

Hey everyone. boiler in questions (Viessmann Vitocrossal CU3A-125)

I have 2 questions regarding the PSI for my boiler as I'm gearing up for heating season. I've never owned a boiler and I want to get as familiar as possible with its operation and temperament for operation. So bear with me.

I have an Axiom Hydronode where the water pressure reducing valve enters the closed system, and off that is my pressure tank (factory charged to 12PSI) and above that is a new pressure gauge I installed. I also have a second pressure & Temp gauge right at the supply exit of the boiler.

Well my pressure was set to 15PSI, but I moved it up to 18 to see the change in pressure for each gauge. As my pressure reducing valve is now set to 18 PSI, however the new pressure gauge directly over the pressure tank (on the hydronode) is showing 5 PSI. The pressure gauge at the supply (exit of boiler) is showing ~14 PSI.

Is the pressure reading lower above the expansion tank due to the PSI differential between the pressure reducing valve setting and the expansion tank PSI charge?

If I have the PSI on the pressure reducing valve set to 18PSI, my gauges are both reading lower than the valve setting, so does this mean either my valve is incorrect or my gauges are messed up?

I have pictures below for reference.

My second question is in regards to the manual when I was reading into the PSI concerns I had.

First the manual states the minimum operating PSI is 14 PSI, then goes on to state this:

"1. Check the pre-charge pressure of the diaphragm expansion vessel when the system is still cold.

Note: If the pre-charge pressure of the diaphragm expansion vessel is lower than the static pressure of the system, top up with nitrogen until the pre-charge pressure 1.5 to 3 psi (0.1 to 0.2 bar)is higher than the static system pressure.The static pressure corresponds to the static head.

2. Close the gas shutoff valve.

3. Open any installed shutoff valves.

4. Fill the heating system with water and vent it, until the fill pressure 1.5 to 3 psi (0.1 to 0.2 bar) is higher than the pre-charge pressure of the diaphragm expansion vessel using the boiler drain/fill valve."

So is the manual telling me to charge my expansion tank to say 13.5-15 psi (1.5-3 psi over the factory set 12 psi)? My house is 2 floors and boiler is in basement, with baseboard cast iron radiators. And after that, does it want me to charge the system to 1.5-3 psi higher than the expansion tank pressure?

This is whats confusing to me, as they want you to charge the pressure tank to 1.5-3 psi higher than the static psi of the system, and then charge the whole closed system 1.5-3 psi higher than the pressure tank. From all my reading, it sounds like you want your pressure tank and closed loop to match, not be different as I'm reading the manual as.

Also when I unscrewed the "InSight" service indicator cap, it was very slightly hissing air? Is that normal or is the scherader valve not fully seated and I need to correctly charge it?

I'm figuring I'm miss-understanding somewhere which is why I'm coming here for help

Jamie Hall

Could be a bit confusing…

The expansion tank air precharge should be set to your system operating pressure. In this case, I'd say 15 psi would be about right. However. This must be done with the tank drained of all water. I hope there are valves available to allow you to do that… Perhaps a valve between the expansion tank and the rest of the system at least, so you could close that and unscrew the tank enough to drain? The Hydronode may have the correct combination of valves — the ETV-1 and HN-3 look as though they do (they also look as though you could unintentionally isolate the tank from the system… check your manual).

Now. Pressure gauges. Odd. The gauge in the first picture is reading 5 psi — at the Axiom. But I can't see what the Axiom is connected to, or how, nor what combination of its valves is open so I honestly have no idea what that pressure refers to. A picture showing how that is connected and to what would be helpful. Thes picture in the second photo reads 18 psi, which is what you said you set the pressure reducing valve to (set it back down to 15, by the way), and that is a good gauge — so it is probably accurately reading system pressure. The last gauge is reading about 14 psi, but those combination gauges are notoriously inaccurate, so I wouldn't pay much attention to it.

You don't have to use pure nitorgen to precharge the tank. An 80%/20% mix of nitrogen and oxygen is quite acceptable and is free…

The Schraeder valve on the expansion tank should not be hissing. At all.

b25urgandy

https://forum.heatinghelp.com/discussion/comment/1811169#Comment_1811169

Thank you for the reply. The Hydro-node I have does have isolation ball valves on all the ports except the pressure gauge port. All the valves were open. I reduced the fill valve to 15 PSI, thank you.

Here is where I think the problem might have lied. I mentioned how the "insight" cap would hiss when I took that off to inspect the valve core. Well I took my tire pressure gauge to it and…nothing, absolutely no reading, maybe 5psi but my slide gauge didn't budge, and that was due to the "insight" gauge holding 5psi worth in, since it does have a rubber seal. Well I tried charging it, and it would hiss back out. So I got my valve core removal tool out (motorcycle addition comes in handy). And guess what? The valve core "tip?" for lack of better term, that you grab with the tool, was sheered off, and the core stuck in the tank. So….I'm not sure if that was a factory install error, or contractor installation error. Anyway, I doubt I could get the install company to admit fault, or Amtrol to say it was their doing.

So… I ordered a new amtrol ex-90pro (factory charged to 15 psi vs 12 psi with the normal ex90). I also then ordered a second pressure gauge, because my Caleffi pressure reducing valve, is the type without the gauge. So I'm going to add a gauge to the vacant port, I'd rather have redundancy with PSI gauges, trust but verify.

Also I had an idea that maybe the port on the top of the Hydro-node, that the pressure gauge mounts to, might have had a pocket/bubble of air between the neck/threads and the inlet of the pressure gauge? causing the air to compress and give the PSI gauge a false reading? I uninstalled it, blew out the port and will re-install it when I install the new pressure tank, but this time I'll fill up the hydro-node with water until that port is weeping water out the top, then I'll install the pressure gauge.

So those should be here Thursday, and I'll update the post once I get everything installed and re-fill the hydro-node with water.

GGross

For future reference don't check the tank pressure while it is hooked to the system, you have to drain it and remove from the system first, and be careful screwing those insight covers back on. I have seen a few tightened down hard enough they let some air out. The ex-pro tanks are built more like a domestic expansion tank with a liner so they are better for dealing with bad water, but either tank could be charged to 15 psi if you are able to salvage the old tank as a backup. And always check the charge on a tank before installing it, they may leak some pressure in transit to you from the factory.

b25urgandy

https://forum.heatinghelp.com/discussion/comment/1811415#Comment_1811415

Thanks for the reply, I did isolate the pressure tank, removed it and took it to the garage to test. I took the insight gauge off and it started hissing air, when I put my pressure gauge on it, it didn't read any. So I believe it was faulty since install, 2 years ago. The rubber gasket on the bottom of the insight gauge kept the last 2-4 psi in the tank lol. I was going to charge it back up with a valve core, but the valve core was broken and stuck inside the valve. I tried getting it out with an tiny bolt extractor tool, but it wasn't working. So I bit the bullet and bought a new tank, and that's why I got the ex-pro tank, I liked the lining idea as we have well water and cast iron radiators and want it to last.

b25urgandy

So for an update…

I installed the new pressure tank, checked that it was charged to 15 psi. I also got 2 new gauges, to check to see if maybe my Winters PSI gauge from my Original post was faulty. And I believe that's the case with my original issues above. Yes, I did vent the gauges too, so it wasn't that the gauge wasn't vented.

To recap, I had 2 gauges, one reading from the boiler that showed 13psi when i set the pressure reducing valve to 15 psi, but my gauge close to the valve was only showing ~5 psi.

I took the low reading gauge off. replaced it with a new gauge, and then added another gauge to the Caleffi pressure reducing valve. So I could have 2 gauges near the reducing valve (see photo below) to have redundancy.

And Voilà!

The gauges are reading within 2-3 PSI of the pressure gauge. To get a reading of 15psi on the gauges, I have to set the pressure reducing valve to 17-19 psi if I remember correctly. Is this okay long run? Boiler is stone cold, so its as accurate of a pressure I can get. The pressure reading on the combo pressure/temp is reading like 13-14 psi.

Picture of my Hydronode setup after I replaced the faulty gauge, all ports/valves open and bled.

Also here is picture of my boiler setup (Ignore the low pressure reading from the gauge on the Caleffi PRV, that was before I changed that gauge and verified it was bad).

Green is intake/makeup water.

Blue is return water

Red is supply water.

I made sure to follow the "pump away piping" rule of thumb when plumbed. As the original installers placed the pump before the boiler, and the expansion tank right after the boiler as shown above. You can actually see the pro-press fitting to the right of the expansion tank where I made them remove the pump and place it after the boiler.

My only concern now is that I have to have the pressure reducing valve set to like 17-19 psi to get 15 psi to show on the gauges (which are now equal to each other since i verified a bad gauge). I also trust the gauges more than the manual dial on the caleffi, so I think I have a good logic at setting it to the gauges versus the dial.

Let me know what you all think.

Also, learn from my mistake, my faulty Winters psi gauge I got from ebay, supposedly new open box. From now on, I'll be getting from a supply store if i need anything.

Jamie Hall

The pressure reducing valve is likely to be less accurate than your nice new gauges — so I'd go with them.

Not, mind you, that a couple of psi off one way or the other is critical!

I don't have much experience with buying parts for heating off eBay — but I do for cars, and it almost seems as though by the time you get one which works reliably you have spent almost what you would have had you bought OEM parts from the manufacturer… and have a nice trash can full of duds.

b25urgandy

https://forum.heatinghelp.com/discussion/comment/1812176#Comment_1812176

Thank you. Yeah, I was able to return the gauge no problem.

Do you see any issue with my current plumbing layout? It appears to be properly "pumping away" from the point of no pressure change, and the pressure tank being after the boiler.

On the return line, out of picture, I do have an Adey Magnaclean professional 2XP. To get that to fit, the copper pipe goes from: 1-1/4" reduced to 1" inch, into filter, and then from 1" exit from filter, to 1-1/4" increase.

Do you believe that will be much of a bottleneck? All the zones are 1' copper, and they all run into 1 1/4" copper return going into the boiler, and then supply into 1 1/4" to the zone valves which are reduced to 1" until the zones return again. In my photo you can see the tee's are 1 1/4" x 1 1/4" x 1".

I don't believe it'll be much of an issue seeing as the inside of the taco zone vales themselves appear to be quite restricting too.

Jamie Hall

That short a stretch of 1 inch isn't going to make much difference.