Tankless water pressure questions

[Brian80]

I had a Navien NPE-240A installed a few weeks ago. It replaced an 11 year old 40 gallon storage tank that could not handle peak demand (4 adults, 2 kids all showering in the evening). I was taking lukewarm showers on a regular basis, sometimes even cold showers...

It's working well and definitely solved the problem of running out of hot water, but we noticed that all our plumbing fixtures (single-handle w/ mixing valves) need to be dialed significantly further towards the hot side than before to get the same water temperature as before.

Is it correct to say that this is due to the inherent pressure drop in the heat exchanger, which means the hot water pressure is less than on a traditional storage tank? In other words, if you mix the same ratio of hot/cold you now get colder water than before, so you need to mix in more hot water to compensate?

This is not a problem for us, but the shower in the rental suite has a safety stop so the handle can only be rotated so far towards hot (to prevent scalding) and now our tenant is complaining that the water is not hot enough with the handle rotated as far as it'll go. I have some options:

1) raise water heater temperature from 120 F to 130 F. Not ideal since I have young kids and don't want them to accidentally get scalded by 130 F water.
2) bust open the offending shower valve and set the safety stop further towards hot.
3) I noticed that the installers added some (seemingly) unnecessary 90 degree elbows on the cold and hot side. I think they did this to be able to clamp the pipes to the wall. I read that each PEX elbow reduces water pressure by around 2 psi. Would removing these 4 elbows help in this situation?

Here's what I mean about the unnecessary PEX elbows. The red lines are how I would redo it. Is this helpful or futile?




Separate question: what is the right way to measure hot water pressure of a tankless water heater? Can I attach a water pressure gauge to my washing machine's hot supply line, then turn on a nearby faucet as hot as possible and expect to get a valid reading?

Thanks!

 

[Bannerman]

It appears that the final sections of PEX connected to the WH maybe smaller diameter than the PEX lines below them. PEX already has a smaller internal diameter compared to the same size copper pipe and the PEX internal fittings only further reduce the internal diameter which will further restrict the flow rate supported. When using PEX, best to utilize larger diameter piping, especially as the flow to and from the WH will not be supplying a single fixture but will likely be supplying hot water to multiple fixtures at the same time.

A tankless WH must take the entering cold water that maybe 40-50F, and almost instantaneously raise that temperature to the desired temperature. Your 120F setting represents 70-80 degree temperature rise. The NPE-240A has a maximum heating capacity of 199,900 BTU. Obviously, slower water flow through the WH will allow the WH to more easily heat the water compared to when water is flowing faster. When water is flowing faster than the WH is capable in heating to deliver the desired temperature in consideration of the temperature rise, then the resulting hot water temperature will be lower than desired.

Although you have programmed the WH for 120F, what is the actual temperature arriving at fixtures under usual usage conditions? Perhaps the tenant maybe attempting to shower at the same time someone else is running the dishwasher or the washing machine or while other showers are running. If the combined flows are exceeding the WH's capacity to deliver 120F at 80F temp rise, then the hot water exiting the WH maybe only ~115F or some other temp.

One test would be to measure the water temperature from each faucet one at a time using a candy thermometer. Maybe due to heat loss occurring from pipes while flowing to each fixture, the actual hot water arriving at the tenant's bathroom is not actually 120F but is somewhat cooler. The temperature test could be repeated while hot water is being utilized as usual to determine if there is any difference.

If both the hot water temp test results are essentially equal, then the WH is likely keeping up with demand. Perhaps raising the WH's water temp setting to 125F will be sufficient to provide 120F water to the furthest location. If you are concerned that water from some fixtures nearby to the WH is too hot, antiscald devices are available to retrofit individual fixtures.

 

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[wwhitney]

2) bust open the offending shower valve and set the safety stop further towards hot.

You say "bust open" but it should be doable non-destructively. I.e. unscrew a few covers to get to the adjusting mechanism, make the adjustment, put the covers back on. The details should be in the manual for the shower valve.

So once any unnecessary sources of pressure drop are eliminated (like those elbows you point out), adjusting the shower valve is the final step.

When water is flowing faster than the WH is capable in heating to deliver the desired temperature in consideration of the temperature rise, then the resulting hot water temperature will be lower than desired.

I believe many (most? all?) modern tankless heaters have an internal throttling valve they use to prevent that from happening.

Cheers, Wayne

 

[Bannerman]

I believe many (most? all?) modern tankless heaters have an internal throttling valve they use to prevent that from happening.

Yes, but I'm not certain how much temperature reduction would need to occur before a throttling valve will reduce the flow rate. I understand Navien's Water Adjustment Valve is solenoid activated so there will be 2 flow rates possible. Also, reducing the flow rate will result in lower hot water pressure to fixtures which is what the OP was concerned with initially.

A pressure gauge on both the hot and cold supply lines leading to/from the WH should provide a method to compare and determine pressure loss through the WH. The pressure should be essentially identical under no flow and moderate flow conditions but if a throttling valve is restricting flow at higher flow rates, then a pressure differential should then be apparent.

Best to understand how the unit functions. If possibly reducing hot water demand such as rescheduling showers or laundry time will resolve the issue, that maybe a workable solution.

 

[Brian80]

Hey guys, thanks for the feedback! Here are a few clarifications:

• I'm on city water
• The city water main is 3/4" polybuetylene (house built in 1990). Piping buried behind walls/floors is still poly-b, although I'd like to replace it someday.
• All piping to and from the WH is 3/4" PEX. The old WH also used 3/4" PEX, so no change there.
• Cold water is 46 F at this time of year where I live.
• The tenant's bathroom is close to the WH. Same floor, nearly adjacent room maybe 12 feet away, although the pipes may take unnecessary detours behind the wall.
• There was no other water usage when I confirmed the tenant's shower was lukewarm at max temp setting.

I tested the water pressure at the service valves under the WH.

• No water usage: cold 44 psi, hot 44 psi
• Nearby faucet using 1.2 GPM (according to WH diagnostics): cold 36 psi, hot 36 psi.
• 2 nearby faucets using 2.4GPM: cold 38 psi, hot 34 psi

I did a hot water test at the only faucet in the house with separate hot/cold handles. The result: 123 F. The WH is set to 130 F and is outputting 130 F according to its diagnostics. I assume it's normal to lose a few degrees in the pipes?

I'm surprised to see fairly accurate hot water temps at the faucet, equal or nearly equal hot/cold pressure yet we still need to mix a much higher ratio of hot/cold water compared to our old storage tank WH. I recall that our old WH had its temperature dial set to "normal" which I assume means 120 F, but maybe it was actually heating water to a higher temp due to malfunction or "normal" actually means 130+? That might be all that's going on here?

Random question: is there any reason to have redundant shutoff valves on the hot/cold pipes? There's already a main shutoff built into each service valve, but the installer added smaller shutoffs too. I don't get it. See here:

 

[wwhitney]

Random question: is there any reason to have separate shutoff valves on the hot/cold pipes? There's already a main shutoff built into each service valve, but the installer added smaller shutoffs too. See here:

Those smaller shutoffs are unnecessary, and are obviously not full port. I suggest getting the installer back out to eliminate them and eliminate the unnecessary elbows. If they want to strap the pipe, they can put a block on the wall where the pipe comes out and strap it there.

Once that's done, then just adjust the shower valve as required.

Cheers, Wayne

 

[Bannerman]

The ball valves directly below the WH are utilized when descaling the WH.

With both the Hot and Cold valves shut, the caps will be removed from the side ports above the valves to connect a pump using a short hose so as to circulate a mild acid such as white vinegar through the WH heat exchanger. A second short hose will be connected to the Hot water outlet side port to return the vinegar solution to a pail to allow the pump to continue circulating the same solution over and over.

 

[wwhitney]

That's all correct, but the OP is referring to the valves that are down lower, against the plywood. Which I did not at first notice.

Cheers, Wayne

 

[Brian80]

Wayne is right, I was referring to the 2 shutoffs well below the service valves:

 

[Fitter30]

Wayne is right, I was referring to the 2 shutoffs well below the service valves:

View attachment 69161

Total agree those shut off are not needed and aren't full port. Do they have a brand and model #?
Water heater at 5 gpm has a 18lb pressure drop. Get a gauge 0-100 with female hose fitting check pressure at washing machine connections cold and hot with shower running.

 

[Jadnashua]

One of the benefits of pex is that it bends, and you can avoid extra fittings most of the time. Pex-A with expansion fittings has less restriction than those compression ring ones. A bend support can make a right-angle more manageable and doesn't restrict the flow.

If you were using say thermostatically controlled shower valves, they would adjust themselves based on the setting you have unless the incoming water is too low for it to allow it to adjust any more (the incoming hot needs to be slightly above your desired setting as it can't shut the cold off entirely).

The dynamic pressure will change depending on how much friction there is along the path, but the static pressure should be the same except for the effects of elevation (you'll lose about 0.43#/foot of elevation rise).

 

[Brian80]

Total agree those shut off are not needed and aren't full port. Do they have a brand and model #?

I can't make out a brand or model number:



It does appear to be full port though. When open, there is no restriction. This is the best picture I could take (my camera could not focus properly)



I think I know why the installer used a shutoff valve on the cold side. It isolates the water heater's cold water from the rest of the house's cold water, which means he only needs to shut off water to the whole house for the few minutes it takes to install that valve. That way the customer can still flush their toilets, wash their hands, etc., throughout the day even though the tankless install takes several hours.

I think the shutoff on the hot side was done for aesthetic purposes to match the one on the cold side, and serves no useful purpose.

 

[Brian80]

It was tricky working around a gas line and several other pipes, but I got er done.



Unfortunately after setting the WH temp back to 120 F, my tenant's shower still did not get hot enough to her liking so I set the safety stop in the shower valve all the way to "hot" and now it can get up to 112 F.

Not much change in hot water pressure though. With 2.4 GPM flow, it used to be 34 psi and now it's 36 psi. Removing 4 elbows did not have as much effect as I had read that it would. Oh well!

 

[Jadnashua]

Depending on where you measure pressure, keep in mind that elevation will create a hit, too, at about 0.43# of elevation change.

There can be a big difference between dynamic pressure and static pressure. The static pressure won't change when taking out some of the fittings, but the dynamic pressure will as there'll be less lost due to friction.

 

[Fitter30]

Heater rated at 67* rise @ 5.6 could measure cold and hot with your thermometer not the unit and what is the temp difference. Are you on a well?

 

[Aiman]

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