Pressure loss curves

[LukePDX]

Nortiz publishes pressure loss curves for their products. I'm trying to understand the practical implications of this. For example, http://support.noritz.com/download.php?file=Literature Page/Spec Sheet/NRCP SPEC.pdf&field=SpecSheet.

For the most part there appears to be a somewhat nonlinear drop in pressure across the unit relative to the flow rate. During low-flow operation (large delta T, or low flow operation like sinks) the pressure drop is quite low and seems like something I can just ignore. During higher flow operation (two showers, tub fillers, etc), the unit above claims that pressure will drop 15 to 30 psi across the unit.

Presumably I should se a bit of a pressure increase at the mixing valve for the shower/tub fillers because the cold water should still have roughly the original pressure for the system.

Does anyone have any rules of thumb about pressure loss in these units? I think we get about 65 PSI from the city, and I think we want something like 45 PSI at the tub filler mixing valve (not sure about the showers). If I lose the maximum (8GPM flow is 30 PSI drop) then the hot line would be at 35 PSI (ignoring other losses). Recombining with the 65 PSI cold line to mix 120 degrees hot line down to like 110 degrees should boost this a bit, right?

My main concern is low pressure showers and very slow tub fill rate (if, e.g., I have a 10GPM@44 PSI mixing valve, but I'm only getting low 30 PSI at the valve due to system-wide losses). Should I look for a unit with better pressure loss, or am I likely to be okay?

 

[Jadnashua]

IT might get more complicated...a typical pressure balanced shower valve expects equal pressure on both the hot and cold lines, or the balancing valve will start to limit the maximum flow through both lines.

Some of the flow restriction may be the unit trying to ensure it can achieve the desired temperature rise. This will be worse in the winter when the incoming cold water can be significantly colder than say in the middle of the summer (in my home, the difference in inlet temperature can be over 40-degrees summer-winter). That's all more heat that needs to be applied to the water that just may not exist, so it slows the flow down so it has to heat less water or on some, it just lets the outlet get colder...think about dropping your normal 120 or so water 40-degrees or dropping the outlet volume from 10gpm to 4 or so! That's easily the difference between a comfortable shower and a cold one, or one with a decent flow and a dribble.

Tankless can work, but it's a bit more complicated than running water through a tank that took a long time to heat up gradually, but is already the temperature you want.

 

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[LukePDX]

IT might get more complicated...a typical pressure balanced shower valve expects equal pressure on both the hot and cold lines, or the balancing valve will start to limit the maximum flow through both lines.

Some of the flow restriction may be the unit trying to ensure it can achieve the desired temperature rise. This will be worse in the winter when the incoming cold water can be significantly colder than say in the middle of the summer (in my home, the difference in inlet temperature can be over 40-degrees summer-winter). That's all more heat that needs to be applied to the water that just may not exist, so it slows the flow down so it has to heat less water or on some, it just lets the outlet get colder...think about dropping your normal 120 or so water 40-degrees or dropping the outlet volume from 10gpm to 4 or so! That's easily the difference between a comfortable shower and a cold one, or one with a decent flow and a dribble.

Tankless can work, but it's a bit more complicated than running water through a tank that took a long time to heat up gradually, but is already the temperature you want.

Thanks Jim.

Certainly in the winter my 80 degree change in temperature is going to limit my system GPM (the unit I linked is marked as 4.7 GPM@80 dT. This is going to be right on the boundary for me to run two 2.5 GPM showers in the winter, but 80 dT is for 120 degrees and the showers will mix back to 110-112 degrees, so I think I have just enough capacity for the coldest time of the year. 4.7 GPM correlates with 10-15 PSI pressure loss on the chart, and I don't think that's going to cause an additional problem.

The summer is where I get confused. My summer dT is more like 45-50 degrees, and that puts me in the 8ish GPM range... clearly no GPM problems in the summer. What concerns me is that 8GPM corresponds to something like 30-35 PSI loss on their curve.

If I'm running two showers I can't draw more than 5 GPM anyway, so really the concern is a single high GPM tub filler, or a tub plus shower, **in the summer only**... the winter is always limited by the max GPM rate, not the pressure.

I'm comparing with their top of the line unit https://support.noritz.com/download...pec Sheet/NCC199CDV Specs.pdf&field=SpecSheet, which has dramatically better pressure loss across the scale (as well as better efficiency and thus better winter performance).

I don't think I understand what the practical implication of this pressure loss rating is... maybe it doesn't matter for shower comfort and just impacts tub fill speed.

 

[Dana]

The shower head is only ~105F, no more than about a 70F delta T from the incoming water, even in places colder than Portland OR. It's more like 65F in Portland during the coldest weeks of winter. Even if you set the tankless to 105F so that ALL the flow to the showerhead was through the tankless, with no mixing of cold at the shower, you'd be good for more than a pair of 2.5 gpm showers. With 40F incoming water and the tankless set to 120F , at 5 gpm of showerhead flow and only a hair more than 4 gpm would be flowing through the tankless, the other ~1gpm would be the cold side of the shower mixer.

A 2.5gpm showerhead only delivers that much at 80 psi (which you don't even have coming in at the meter, let alone at the shower head). At ~50-55 psi at the showerhead a 2.5gpm head will be delivering about 2 gpm, so you're looking at a ~4 gpm total flow to a pair of 2.5 gpm heads, with only ~3.3 gpm of that flow running through through the tankless set to 120F.

You can probably even be able to cover three simultaneous showers, since the pressure (and flow) would drop. Every 2gpm is 1000lbs/hr, and with a 65F rise that takes 3 x 1000lbs x 65F= 195F BTU/hr of tankless at 2 gpm each, but with the bigger pressure drop and 1.75 gpm per shower you'd be only needing ~170,000 BTU/hr of tankless output.

But you're right, it primarily affects tub filling rates.

 

[LukePDX]

The shower head is only ~105F, no more than about a 70F delta T from the incoming water, even in places colder than Portland OR. It's more like 65F in Portland during the coldest weeks of winter. Even if you set the tankless to 105F so that ALL the flow to the showerhead was through the tankless, with no mixing of cold at the shower, you'd be good for more than a pair of 2.5 gpm showers. With 40F incoming water and the tankless set to 120F , at 5 gpm of showerhead flow and only a hair more than 4 gpm would be flowing through the tankless, the other ~1gpm would be the cold side of the shower mixer.

A 2.5gpm showerhead only delivers that much at 80 psi (which you don't even have coming in at the meter, let alone at the shower head). At ~50-55 psi at the showerhead a 2.5gpm head will be delivering about 2 gpm, so you're looking at a ~4 gpm total flow to a pair of 2.5 gpm heads, with only ~3.3 gpm of that flow running through through the tankless set to 120F.

You can probably even be able to cover three simultaneous showers, since the pressure (and flow) would drop. Every 2gpm is 1000lbs/hr, and with a 65F rise that takes 3 x 1000lbs x 65F= 195F BTU/hr of tankless at 2 gpm each, but with the bigger pressure drop and 1.75 gpm per shower you'd be only needing ~170,000 BTU/hr of tankless output.

But you're right, it primarily affects tub filling rates.

I measured our current cold and hot temps at the existing shower head and get 40F on cold and 110 at preferred hot mix, which is right in the 70F dT range that you've described (I was looking at 80 dT for the unit set at 120F... I'm not sure if it's better to run the heater at 110 or 115, or to run it at 120 and use more higher pressure cold at the mixing vave). This was with a thermopen, so not great but not terrible accuracy in these ranges. The utility claims we have water that varies from 38-75, so that's consistent with the observation as well.

I'm only running two showers on this unit, so that seems fine.

I didn't realize that shower heads are rated at 80PSI... that seems quite high. Is that from https://www.epa.gov/sites/production/files/2017-01/documents/ws-products-spec-showerheads.pdf section 3.1.2? How do showerhead flow limiters impact this... if you're already below 2.5 due to PSI does removing the limiter make any difference?

 

[LukePDX]

The shower head is only ~105F, no more than about a 70F delta T from the incoming water, even in places colder than Portland OR. It's more like 65F in Portland during the coldest weeks of winter. Even if you set the tankless to 105F so that ALL the flow to the showerhead was through the tankless, with no mixing of cold at the shower, you'd be good for more than a pair of 2.5 gpm showers. With 40F incoming water and the tankless set to 120F , at 5 gpm of showerhead flow and only a hair more than 4 gpm would be flowing through the tankless, the other ~1gpm would be the cold side of the shower mixer.

A 2.5gpm showerhead only delivers that much at 80 psi (which you don't even have coming in at the meter, let alone at the shower head). At ~50-55 psi at the showerhead a 2.5gpm head will be delivering about 2 gpm, so you're looking at a ~4 gpm total flow to a pair of 2.5 gpm heads, with only ~3.3 gpm of that flow running through through the tankless set to 120F.

You can probably even be able to cover three simultaneous showers, since the pressure (and flow) would drop. Every 2gpm is 1000lbs/hr, and with a 65F rise that takes 3 x 1000lbs x 65F= 195F BTU/hr of tankless at 2 gpm each, but with the bigger pressure drop and 1.75 gpm per shower you'd be only needing ~170,000 BTU/hr of tankless output.

But you're right, it primarily affects tub filling rates.

Oh, I ran into another one of your posts that discusses drainwater heat recovery. That seems like a great way to preheat the cold water when we have two showers running, (the downstairs shower in on the slab, but the master shower has 8ft to drop) which could really help with the worst-case winter dT for the most common case hot water use (4 daily morning showers, 2 at a time).

I'll ask my plumber about the cost of this option.

 

[Dana]

In OR there are rebates available for drainwater heat recovery based on their NRCAN tested return efficiency. But it looks like that will only be available until 31 December 2017 (unless they re-up it.)

http://www.oregon.gov/energy/At-Home/Pages/Water-Heaters.aspx

The unit itself runs about a grand purchased direct from the manufacturer, more if purchased through the big orange box store. Installation is DIY-able if you have plastic drains and some rudimentary plumbing skills. (The more adventurous can DIY cast-iron drains too.)

It has to be mounted pretty close to perfectly vertical to work well, since it's relying on the surface tension of the water to spread into a thin layer over the interior side of the drain. You won't be able to recover the heat from the slab-level shower, but it would work fine for the master shower. If it's common to have both showers running simultaneously there will still be a benefit to feeding the cold side of the slab-level shower mixer with the output of the heat exchanger though. The net amount of heat recovered from the outflow of the master shower is improved if the potable water coil has a higher flow than what's going down the drain.

Removing the flow limiter will of course increase the flow, which increases the pressure drop across the heat exchanger in the tankless. Measure the flow rate using a bucket and a stopwatch to see what it really is.

 

[LukePDX]

In OR there are rebates available for drainwater heat recovery based on their NRCAN tested return efficiency. But it looks like that will only be available until 31 December 2017 (unless they re-up it.)

http://www.oregon.gov/energy/At-Home/Pages/Water-Heaters.aspx

The unit itself runs about a grand purchased direct from the manufacturer, more if purchased through the big orange box store. Installation is DIY-able if you have plastic drains and some rudimentary plumbing skills. (The more adventurous can DIY cast-iron drains too.)

It has to be mounted pretty close to perfectly vertical to work well, since it's relying on the surface tension of the water to spread into a thin layer over the interior side of the drain. You won't be able to recover the heat from the slab-level shower, but it would work fine for the master shower. If it's common to have both showers running simultaneously there will still be a benefit to feeding the cold side of the slab-level shower mixer with the output of the heat exchanger though. The net amount of heat recovered from the outflow of the master shower is improved if the potable water coil has a higher flow than what's going down the drain.

Removing the flow limiter will of course increase the flow, which increases the pressure drop across the heat exchanger in the tankless. Measure the flow rate using a bucket and a stopwatch to see what it really is.

Oh, interesting. I was envisioning running the input line into the Noritz unit through the potable water coil in order to preheat the groundwater when the master shower is running to reduce dT in that case, but you think that preheating the cold lines into the two shower mixing valves would make more sense? The drain is certainly closer to the showers than the tankless heater, which would make plumbing it that way much easier... will I get full efficiency during master-only operation if the potable coil flow rate is only feeding the single mixing valve?

 

[Jadnashua]

The pressure loss can play hell with the pressure balanced anti-scald valves in a modern shower...they expect the pressure to be about equal on both the hot and cold. IF one side's pressure drops, it starts to decrease the other to keep the balance even. At the worst case, it can literally shut off the flow entirely (ask people that have tried just hooking up one supply to their new shower valves and wondering why nothing comes out!).

 

[LukePDX]

The pressure loss can play hell with the pressure balanced anti-scald valves in a modern shower...they expect the pressure to be about equal on both the hot and cold. IF one side's pressure drops, it starts to decrease the other to keep the balance even. At the worst case, it can literally shut off the flow entirely (ask people that have tried just hooking up one supply to their new shower valves and wondering why nothing comes out!).

Delightful. I guess that I need to get information from the valve manufacturer about this. Hopefully they have some competent people answering the phones.

 

[Dana]

Oh, interesting. I was envisioning running the input line into the Noritz unit through the potable water coil in order to preheat the groundwater when the master shower is running to reduce dT in that case, but you think that preheating the cold lines into the two shower mixing valves would make more sense? The drain is certainly closer to the showers than the tankless heater, which would make plumbing it that way much easier... will I get full efficiency during master-only operation if the potable coil flow rate is only feeding the single mixing valve?

The drainwater heat exchanger feeds BOTH the water heater AND the cold side of the shower mixer. The warmer water on the cold side means less flow is required through the tankless to mix to ~105F, and the warmer water on the inlet side of the tankless means it's firing rate to achieve ~120F output is much lower. Both save on fuel, both are necessary for the drainwater heat exchanger to hit it's % heat recovery performance marks.

This diagram is drawn a tank-type water heater, but it can really be any type of water heater:

Typical tankless + drainwater heat recovery installation: