When running the bath water the volume goes to almost nil to acceptable. Initially I thought the problem was with the circ system check valves since nothing else made any sense. I did a test yesterday and found the problem was the line going into the hot water system. I was able to stop the water flow by covering the pipe with my finger so obviously there is a pressure problem. After stopping the flow for a while I would get some volume and after a while it would reduce to a slow flow. I was an apprentice plumber back in the day when copper was a novelty and plastic waste was just getting accepted so I know a little about plumbing. The piping in my house is the soft plastic with the non glue type fittings. I understand they use manifolds these days. I don't understand how the HW pressure could be almost nil when the cold water is just fine. I assume the line to the HW would come out of the manifold. What in the manifold would cause a reduced pressure/volume? Also why would the volume to the tube be acceptable at times and other times almost nil. A couple of times it quit completely. Any ideas?

 

Is the reduced hot water pressure only happening at one location ?

 

No. The lack of volume/pressure is common to every hot water outlet. Although buried under insulation I tried to follow the line going into the hot water heater. The line diverts to supply water to the washer and the other side goes to the WH. The only fixture from the washer and WH is a bathroom. I haven't been able to find where the cold takeoff is from that line to the bathroom. The hot takes off close to the WH. The furnace is under the bathroom with the duct close to the ceiling so it is hard to follow the line. The cold water pressure is fine going to the bathroom so the problem has to be somewhere between the tee going to the washer & WH. There is plenty of cold water pressure going to the washer. There is a 3/4 inch line going to the gate valve which supplies the WH. I don't know if the cold line takes off from the line going to the washer or to the WH. I can understand something plugging the line creating a loss of volume, but what I can't understand why the lack of pressure.

On the title I should say the volume not pressure varies from nil to satisfactory.

I was able to find the line going to the bathroom which takes off the line going to the WH located in the furnace room.

 

Has any work been done lately that involved closing the valve at the water heater?

If it's all hot water at all fixtures, I would look at that valve. The gate may have separated from the stem.

 

It sounds like there is a restriction somewhere. Most often it's a valve that's mostly closed but it can also be sediment or debris clogging the pipe. If you do have a valve turn the tub faucet on full hot and open and close any valves in the hot water piping several times. If there is debris caught in the valve it may break it up and allow it to pass. Make sure you do this with only the tub faucet open. You want the chunks to safely exit. If those chunks get into the dishwasher or clothes washer you could be in for more problems.

 

I had a similar problem at a house back in 2008 that lead me to replace the 80 year old galvanized piping with copper and replace the unbranded water heater with a Bradford-White. In the end the restriction turned out to be a large hard chunk of calcium probably from the cheapy water heater that was lodged at the hot water stop under the kitchen sink.

 

What I don't understand is if I have a restriction it would reduce my volume, but why is it also reducing my pressure? I can stop the flow with my finger. I just checked it out and I have a 1" soft plastic line reducing to 3/4" copper just before the gate valve that goes to the HW heater. I had previously checked out the gate valve so I am pretty sure it is OK. About 10 feet before going to the HW heater the 1" line has a 1X1/2 tee going to the bathroom. When the HW at the tub is on with little volume the volume to the bathroom appears to be normal. Since the house is plumbed with the soft plastic piping I can't see a calcium buildup causing the problem. The house is about 15 years old. I have to believe the problem is where the reduction is made from the 1" plastic to 3/4 copper, but what bothers me is why the lack of pressure if this is the location. Could a blockage affect the pressure when the flow is somewhat normal for about a minute after being released. I can get volume for a short period, but no pressure. Checking where the line is reduced is not an easy task. I would have to move the air chamber and recirculation pump to pull the piping out of the wall. Re soldering is always a problem with water dripping. Before attacking that I want to be sure that is the problem area.

 

You described it in your first post. Pressure and volume go hand in hand when you have a faucet open. If you held your finger over the pipe or faucet long enough you'd feel the pressure build until you could no longer hold it back.

Initially you have pressure and flow volume but it quickly diminishes. There is a restriction that only allows a small amount of water through. When you are not using water water flows through the restriction and pressurizes the pipes on the other side. When you open a faucet you get an initial surge of water that was stored in the pipe on the downstream side of the clog. Soon that water and pressure has been released and you are left with the trickle making it past the clog. When all your faucets are closed the pressure will eventually equalize on both sides of the clog.

The buildup or mineral crud is likely not from your piping. It is formed by the heating process in the water heater. Normally chunks sink to the bottom but occasionally one can make it's way out of the tank. Then any restriction or sharp bend in the piping can potentially catch the chunk forming a clog. Gate valves are the worst but a change from a larger to smaller pipe size can also be a clog point as can 90 degree elbows and even regular fittings.

First I open the tub hot water and exercise all the valves I can find. If that doesn't work I leave the tub valve open and start banging and shaking pipes. It that doesn't do it it's time to get mean. You'll want a helper as you'll be cutting pipe on a fishing expedition. Have plenty of fittings (couplings) on hand to reconnect the piping you'll cut.

Turn off the hot water shutoff at the water heater. Open the tub hot to bleed the system pressure. Then pick a spot and cut the hot water line. Have your helper turn the hot water shutoff at the heater back on. A bucket to catch the water is a good idea unless you need a shower. If the flow is strong the clog is somewhere further downstream. If the flow is weak the clog is upstream toward the water heater. Repair the pipe you cut and move to the next test point. Eventually you'll map out where flow is good and bad and the clog will be in between.

If you have a thermal camera (FLIR, just the brand I use but Fluke also has good ones) you can look at the pipes and sometimes spot the problem area. Turn the tub faucet hot on full. Then go looking at the pipes with the FLIR. If lucky you will see the entire pipe uniformly hot on one side of the clog and a temperature drop or irregularity on the downstream side. This works best on horizontal pipe runs and when the water flow direction is down. When the flow direction is up, especially vertically this method doesn't work very well.

 

I fully agree with what you are saying. The clog is before the water tank. I unhooked the water supply going into the HW heater. I turned on the gate valve and got decent volume for a short period. I, then stopped the water flow with my finger. I held it there for several minutes and the water pressure never increased to the point that it was difficult to stop the water flow. Basically this is what I have.
Water supply to tank initially has volume.
After volume is reduced I can stop flow with my finger.
pressure never returns
when I release my finger volume returns for a short period.
Last night the HW stopped completely at the tub.
I shut off the valve and the flow returned.
With hot water running at the tub the cold water into the bathroom with supply about 10ft from the HW on the supply line going to the HW has normal volume..
This is the last fixture supplied from the cold water line going to the HW heater.
The HW to that basin is almost nil due to the tub HW running.
After the TEE that supplies CW to the bathroom the line extends about 10ft and reduces from 1" soft plastic pipe to 3/4 copper & 3/4 copper L and then to the 3/4" Gate valve . The only thing between the gate valve and where I checked the water for volume and pressure is the air chamber tank (whatever you call it). I had checked the gate valve previously and it appeared to be OK so the only place that I can see as the problem is where the line is reduced. So my problem is:
Why is there never any pressure after the initial flow of water?
Why would there ever be something in the supply line when there is only plastic pipe coming to it?
Why would the water shut off completely at times and other times the flow has been normal.
My conclusion is the problem has to be at where the pipe is reduced because everything before that seems to be normal
Assuming that is the problem is where the pipe s reduced I am baffled why sometimes the volume is normal and other times the water stops completely and why is there no water pressure?

 

The type of pipe does not always matter. The debris is likely a mineral chunk from the water heater or even possibly from the city if they knocked a chunk loose when working on the water line. Those chunks can become stuck in any type of piping. Where the pipe reduces in size is a perfect place to check for a clog. Hopefully you have easy access to it.

Clogs can be funny especially if it's a hard chunk that doesn't solidly wedge in place. When water is moving the flow can push the chunk against the pipe reduction and block the flow. When the flow stops there is nothing holding the chunk in place so it can fall back. The next time water is used the chunk can hit the restriction in a position and might not block the flow as much.

 

How long ago and how did you check it?

 

It was a couple of years ago when I first had the problem. I checked the gate valve and check valve. I found the flapper on the check valve had come loose so I replaced it. I figured I had solved the problem. I checked the gate valve at the same time and it was OK. I believe I checked it by pulling the valve off, but not sure at this time. I know I confirmed that it was OK though. Like I say I can understand the lack of volume. It is the lack of pressure that makes the problem hard to detect. Logic is that it would be where the 1" pipe is reduced to 3/4 copper, but if that was the case why does the pressure not increased when I stop the flow with my finger.

 

Just an FYI.. If you have a faucet open, hot only ,and get low pressure because of said issue then you can get increase pressure in the hot when you open another faucet. such as a single lever sink. The cold water will travel from the cold to hot line and it would seem like flow is restored at the tub. When all you are really doing is mixing cold into the low pressure hot line.

Try turning the gate valve at heater ccw to open it. Does it stop, or does it just keep turning?

If it just keeps turning the shaft is broke. And we don't know what position is broke at..

Put a full port ball valve in its place.

 

When you checked the gate valve, did you disassemble it? It's not too hard to take it apart after you turn off the main water and drain the hot water down. The reason I mention it is that I've seen broken gates doing just what you describe, on more than one occasion.

But if that's not it, and you have all copper/plastic water lines, and you think there's something blocking a reducing tee, you can try to back-flush it with cold water pressure.

You've been putting up with this for years?

 

I'm pretty sure I did take it apart, but I don't remember now. I know I did with the check valve which I found to be bad so I replaced it. I was sure it would fix the problem, but it didn't. Backflushing is a great idea. I never thought of that. That would be the easiest solution. Thanks for the idea.

 

The gate valve does stop when it is fully opened.

 

I finally got the time to check on the problem. I suspected it was the check valve since that was the only thing that made sense. I have a hot water circulating system. The flapper on the check valve on the supply like going to the HW heater had come detached and was flopping around. I should have gotten to this sooner, but dreaded the project due to problems in trying to removing the fittings and re soldering them with water draining into the line from the air chamber. I tried to remove the air chamber and gave up because it was pretty well sealed. I got a super hot torch and was finally able to free the pipe from an ell. From there I was able to take everything apart. I checked the stop and waste to find the flapper wandering wanderlust in the valve. I replaced that and the check valve on the recirc pipe which was OK. I soldered everything together leaving only the main line to solder. I used a trick that my brother had told me they used to use was to put some bread to absorb the moisture while soldering the joint. That worked. My only fear is that the bread would solidify and clog the system. My brother said that it wouldn't!! It took forever to get water to the tub, but it finally came. I think that the bread got caught in the check valve. After more than half an hour I finally got water. I got volume, but I won't be sure the volume will maintain throughout filling the tub. I did not want waste water so will check it out this evening.