Low Pressure on Hot Water Circuit

[Lowmiler]

Over the last several months, we have noticed water pressure dropping while showering. Further investigation shows the pressure drop to be throughout the house and isolated to the hot water circuit.

Attaching a pressure gauge to the municipal water supply, after the inline pressure regulator, shows about 58psi, and drops to around 53psi when any kind of water use happens, hot or cold. This seems normal.

Same pressure gauge when attached to the hot water return line (we have a circulator), shows 58psi and drops to around 10psi when two or more hot water sources are used (shower and kitchen sink for example).

There is no easy way to tap into the cold water supply to the water heater or the outlet of the water heater. Test was done while circulator pump was running, so that was not a restriction.

The water heater is an indirect design, and is about 16 years old. It is a Weil-Mclain brand and should be stainless steel and good for 25-30 years.

Our water is just slightly hard, and there is no evidence of scaling or build up anywhere in the house (I even cracked open one of the flow check valves on the cold water supply and it was clean as a whistle).

I have noticed that there is a corrosion build up on the inlet and outlet unions of the water heater. I am not familiar with these types of unions, and thus not comfortable cracking them open to see how extensive the corrosion may be internally. Photo attached.

I am kind of at a loss where this restriction may be living on the hot water circuit and looking for suggestions.

Thanks,

Todd

 

[GReynolds929]

Is there a thermostatic mixing valve on the outlet of the water heater?

 

[Lowmiler]

Is there a thermostatic mixing valve on the outlet of the water heater?

I do not see one anywhere visible on the about 20' run from the heater. I did search for one thinking that could be the issue.

 

[MicEd69]

What does your circulating loop look like? Where is you pump in relation to your water heater? Does it take suction from the tank and pump to the fixtures, or does it take suction from the loop and pump the water back into the water heater?

Those unions are dielectric unions that are supposed to keep corrosion due to galvanic reaction between dissimilar metals from occurring. They don't work as well as they should. Using a stainless or brass 6" nipple or longer in place of those works much better.

 

[Lowmiler]

The pump is within a couple feet of the tank and it is on the return side. It pumps into the cold water feed about a foot from the inlet to the tank. I did replace it recently since the old one was making a "bad bearing" noise when running, but that had no positive or negative effect on the pressure issue.

Could the dielectric unions be corroded up internally? What's the risk with disassembling them to take a peek?

 

[Lowmiler]

Thinking this through further on my own, I think I am chasing a ghost using the pressure readings I got on the hot water side.

Correct me if I am wrong, but if I am measuring on the RETURN side of the circulator, with faucets open, then the pressure is going to the faucets, and I will inevitably see a much lower pressure on the return line for the circulator.

The circulator is just for temp control at the point of use, it has no pressure function, so when you open a faucet, the pressure naturally drops on that line.

I guess I need to find a way to measure the pressure on the main hot water feed line to see if this pressure issue is truly limited to the hot water circuit.

 

[MicEd69]

Yes, the insides of those dielectric couplings could be pretty rusty. They are often hard to disassemble, and once taken apart. they are pretty much trash in my experience. So, if you decide to take them apart, be prepared to do some soldering. And from your picture, it looks like you will need to do some replumbing on the cold-water side if you want to install a stainless or brass nipple in place of those old couplings.

And yes, the pump is only to provide "instant" hot water. From your description, it appears you run your plump 24/7. Many people only use their pump on a usage demand cycle, a timer for when hot water is normally needed, or by a separate switch, which is how mine is setup. You could turn your pump off and see if anything reacts differently. Your check valve that should be near the pump may also be sticking. I'm not sure if that would impact the pressure as much as it would the water temperature, but it's worth checking.

 

[Lowmiler]

I think I’ll leave that inspection and replacement job to a pro, thanks.

The circ pump is temp triggered, in line with the return run.

I did disassemble both circ and main line check valves, and they are clean and functioning fine.

 

[Rossando]

Sounds like you need to descale your combi boiler unit. Look up the service schedule/procedure from the manufacturer website. After reading it call a professional or attempt to do yourself. Usually there's an annual service for these things.