Need to decrease pressure on the cold side

[Todd Werner]

We have a complicated plumbing system in our new home. There are two hot water tanks, the first being preheated by a geothermal system. There is also a recirculating loop with a taco pump that runs hot water from the powered hot water tank to the loop and returns it to the holding tank. The water returned from the loop gets reheated with waste heat from the geothermal systems.

It's very efficient, but the plumbers who installed it didn't consider pressure drops. The loop runs all the way to a semi-detached garage with a bathroom, so it is almost 300' long. But the cold runs are straight shots in the main house.
With the Taco pump and two showers going, the pressure drop in the hot water line is so great compared to the cold line that the second shower in line gets no hot water. What I think is happening is that the shower mixing valve (Delta) doesn't have enough hot water pressure. I estimate that by the time the hot water gets to the second running shower, after 300', that even without incorporating bernoullis principle due to the hot water being pulled past the second shower by the pump, the pressure drop must be at least 15psi.

Other than adding a 200' coil of pipe in front of the cold line, is there anything available that could decrease the cold side pressure when the system is flowing?

Thanks,
Todd

 

[Reach4]

A partially closed ball valve would be simple.

There are shower valves that are called "pressure balancing". I don't know how well they work.

 

[Sponsor]

 

[Widgit Maker]

If this is occurring through out the house, put a pressure reducing valve on the cold water line after the connection to the water heaters. If it is occurring only in the semi-detached garage bath put the PRV on the cold water line to the garage.

 

[Jadnashua]

FIrst, static pressure should be identical in both the hot/cold lines. But, working pressure during use, does vary based on internal friction of the delivery pipes. It sounds like one of two (or maybe both) things are happening: because the pipes are not properly sized, the friction during use is way too high for things to balance out; or, you have a cross-over somewhere (probably in the return line piping) that is allowing the hot water to bypass that fixture that is furthest (?) away. A return line should have a check valve in it (especially when using a pump which can open it while operating), to prevent the water from flowing backwards from the cold inlet of the WH tank array, diluting the hot water supply. The water will flow along the path of least resistance.

 

[Todd Werner]

Thanks guys. Lost our internet for a while.

I think it's that the pump pulling on the hot water loop, plus a fixture being open in front of the last 2 showers, drops the pressure to much for the delta mixing valves to make the water hot.

I will try to remove any unnecessary check valves or places where the hot water loop might be restricted. There's one between the holding tank and the on tank, which isn't needed. I'll look into adding a pressure reducing valve on the cold side. Will that do the same thing as adding a long coil in front of the houses cold water supply?

Todd

 

[Reach4]

I'll look into adding a pressure reducing valve on the cold side. Will that do the same thing as adding a long coil in front of the houses cold water supply?

No. A partially closed valve would come closer to matching the effect of a long piece of pipe.

If you have a thermostatic mixing valve at the water heater which intentionally adds cold water to the hot water line, 300 ft away from the shower, that could be limiting your performance a lot. A system where any thermostatic mixing occurs at the bathroom would give better performance.

How is the powered WH powered? If electric, maybe you could consider adding a tank closer to the bathroom. It could have its own feed from the non-powered WH. You would not be keeping 300 ft of pipe fully hot.

 

[Todd Werner]

There's really no way to add another water heater for those showers at the end of the loop. It's a dedicated loop, sort of like a hotel would run, and I've insulated as much of it as I can, probably the first 200 feet are insulated. The rest is in the walls after it's trip out to the garage/bonus room. Of course, that means the part that cools fastest is what's right by the fixtures inside the house, so when the loop turns off at 11:30, it cools pretty fast. By 11:45, it's lukewarm. Annoying that the plumbers didn't insulate all of it, and more annoying I didn't realize it.

Anyway, the heated tank is electric, but I don't think it's running that much since the holding tank gets preheated by two separate geothermal units. I'll have to look at the electric bill or better, put a monitor on it. Right now I just don't want the kids' shower or guest shower to go cold because of the pressure difference between hot/cold when it's running.

 

[Reach4]

Why are the showers drawing any cold water at all if the shower is not hot enough?

 

[Todd Werner]

My best guess is that the pressure difference between the hot and cold side of the mixing valves is too great. They're delta single lever valves with a little lever on the volume control that let's you set the temperature. Guessing I need to drop the cold water side water pressure.

Maybe I should hook a gauge onto hot and cold supplies in those bathrooms under the sinks where they're accessable, and watch the numbers as we turn stuff on upstream.

 

[Jadnashua]

Are you sure that you've set the limit stop inside of the Delta valve to allow it to get hot enough? Depending on the model, it's often a cam that can be moved that will limit the maximum hot. This may help:

https://terrylove.com/forums/index....emperature-limit-stop-on-shower-valves.64760/

The bigger hassle is, if you set it in the summer to a max temp, it may not get hot enough in the winter when the incoming cold water is colder. And, if you do it in the winter, it might be too hot in the summer, which is why I prefer a thermostatically controlled valve. Depending on the Delta valve you have, you might be able to swap to that type with a new trim/cartridge package.

 

[hj]

The valve AUTOMATICALLY "drops" the cold pressure to match the hot pressure, so your diagnosis is faulty somehow.

 

[Smooky]

My guess is there is a cross flow somewhere in the system and cold is getting into the hot water. Reducing the cold water pressure is not going to solve your problem. Proper placement of back-flow preventers is important in this type of system. If you have any extra shut off valves at the shower could cause this problem. Some pictures of how the plumbing is set up at the hot water heater might reveal some clues. Is there a separate dedicated return line? Is there a mixing valve at the hot water heater?