Pressure drop with hot water demand

I'm having real trouble figuring out how a hot water demand, such as from a shower or the like, could affect the pressure in a presumably closed hot water heating loop. Something does not add up here — the pressure in the hot water heating loop should be well below the minimum pressure in the domestic hot water at all times.

if it is the pressure in the hot water heating loop which is falling (and tripping a low water/low pressure code) when domestic hot water is being used, something very peculiar is happening.

Piping diagram or photos?

The PRV, pressure rducing valve. That beige Caleffi valve you posted a picture of. If you let some water out a drain, does the pressure drop then return to more or less what the pointer is set to when you close the drain?

You may have a leak or issues with the expansion tank. I would close it again and see if the pressure stays. If it loses pressure you need to figure out what is going on, either it is leaking somewhere or the expansion tank isn't working and it is seeping out the pressure relief vavle when it is hot. It will lose a little pressure as the air from the fresh water you added works out through the automatic vent. You could also leave the makup water supply on for a few days to let it work most of the air out then turn it off and see if it holds pressure over a few days.

This is why having that additional pressure gauge on the Caleffi fill valve is useful! It's always nice to have a little redundancy to double check if something is working correctly. (edit: noticed there isn't even a tapped port there! I guess they didn't come with one back in the day?)

What didn't make much sense was this

"but I would bypass the LWCO by shorting the wires and system would build up the pressure back to normal during water heating cycle. Lately, though, the pressure started to drop more often with hot water demand and if the TT is idling and builds up back again when TT is heating DHW."

What you have is an automatic fill valve set at what looks like 16-17psi. This valve does not care what your boiler is doing. It can be off. It will feed water into your closed loop if it drops below the set point.

So when you say it only builds up pressure again when heating the water, something is already wrong here. Unfortunately, the only gauge you have that I can see is on the boiler itself.

What I believe Alan was asking you to do was to shut off that fill valve after it oddly drops pressure from using your domestic hot water. Not manually draining the system. What we want to see is if the pressure returns without the use of the Caleffi fill valve after losing it to running the domestic hot water.

Also, you're lucky the vent that copper pipe on the top of your boiler is burning through is the air intake and not the exhaust.

Ah yes, I suppose a hot pipe wouldn't leave burn marks like that!

You have hard water. Look at the hard water deposit (scale) on the atmospheric vent which was (is) dripping on the double check before the Caleffi auto fill. You can delime the caleffi and the double check or any other valve (thermostatic mixing valve) that has moving parts. The thermostatic mixing valve can scale up and not function correctly and mix more cold water in with the hot from the tank coil giving you lukewarm water. Then use conditioned water in the boiler.

A problem is, hard water in the coils of the domestic hot water supply can scale the interior and exterior of the coil reducing heat transfer from the boiler water to the domestic hot water.

I don't think domestic hot water demand side is the problem execpt with a scaled up mixing valve. You have a indirect tank circuit with a pump, maybe on priority setting. I woud look at that pump and any check valves on that circuit.

Are you on well water?

The knob on the bottom of the Caleffi fill valve is a shutoff knob and should be in the open position so water can flow to the boiler sys should the sys need water. That valve has a cartridge in it that can be cleaned. https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Caleffi-553542A-Product-Overview.pdf

I don't see the mystery — other than your domestic water static presure is low. If you open a hot water tap upstairs, and the pressuren on the heating side of the boiler drops… you have a leak between the boiler and the hot water. Either a baclflow nfrom the boiler fill to the hot water line (you mentioned the backflow preventer vent drips? if so, it needs repair or replacement) or in the Turbomax coils. Those are the only two hydraulic connections. All the electric stuff is irrelevant to the pressure problem.

You have a ball valve (red handle valve on the wall) before the Caleffi fill valve/backflow combo. You can re-run your test using the ball valve to shut off boiler filling instead of the valve on the Caleffi to rule it out.

I also had questions about the piping, but didn't really think it was pertinent. Could you let me know which way the arrow is facing on the circled pump?

A faulty lwco switch is possible, but does that answer why pressure drops in both the indirect tank and the boiler? It seems the lwco is functioning as his pressure is indeed dropping.

It also doesn't answer why he suddenly gets brown water after running hot water for a while as if he was using up all the water from a tank. His hot water is direct from the cold main, through the coil, and then to his fixtures.

It does seem it would be a hole in the coil and the tank's boiler water is getting sucked in while using it.

If this is the case, the only question remains is why his relief valve doesn't go off as a leak is a two way street. And why then during repressuring does it stop at exactly 17.5 psi and not continue to rise? And why it also returned to 17.5psi when the fill valve was shut off.

I think we had another post here recently where pressure would go missing and come back by itself and he did not even have a fill connection. Wonder if that was solved..

But in both cases, feels like something very simple is being missed.

Does he have galvanized iron domestic water piping (brown rusty water) ? What is the street pressure? Too, high a flow for copper pipe size leading to pipe erosion?

What ever the Caleffi fill valve is set for the Expansion Tank air cushion charge should be the same pressure. The gray tank. The white tank air charge should be the same as street pressure.