Help requested sizing residential recirculating hot water pump

[ScaldedDog]

The hot water recirc system in our (circa 2003) home hasn't worked great since we bought it in 2020, and it's been making a racket if the pump runs very long for the last year. I thought it might be air in the return line, but after installing a vented 90* fitting in that return I don't believe that to be the case. So now I'm thinking of replacing the pump. It's a 3yo Grundfos Comfort PM. When I took it apart, the surface area the impeller rubs against was rusted and pitted. I got the rust off, but the surface is still pitted.

I need help sizing a new pump, as I believe the current one is too small. Here's a photo of the system:



The system is made up of three loops of cold supply, hot supply and hot return, in order from right to left. (The rightmost lines support outside hose bibs and can be ignored.) The hot supply line out of the tank is 1" copper, and it serves the three 3/4" PEX supply lines. The three hot water return lines on the left are 1/2" PEX, feeding into the 3/4" copper line leading to the Grundfos pump.

The three hot and return loops are:

Loop 1 (Kitchen) - 35' of 3/4" PEX and 35' of 1/2" PEX return. The Taco Comfort calculator (http://apps.tacocomfort.com/wizard_dhw.html) I used calculates 3.39 feet of head for this loop, at 1.1GPM (which the calculator says is the max recommended for 1/2" PEX at 140*).

Loop 2 (Master Bath) - 49' of 3/4" supply and 1/2" return line. Calculated head is 4.72.

Loop 3 (Guest Bath) - 72' of 3/4" supply and 1/2" return line. Calculated head is 6.96.

The utility room supply and return lines have a calculated head of .64 at 3.2GPM (the limit for 3/4 copper).

My question is, what GPM and head figures do I use to size the pump? It seems to me I'd need 3.3GPM from the pump to get 1.1GPM through each of the return loops, or does their differing lengths make it more complex than that? I have no idea what head number to use. I know not to simply sum them, but I'm not sure just using the figure from the highest one (6.96 + .64 from the utility room loop = 7.6) is right, either.

The current Grundfos pump appears to have a max head pressure of 3.9', at which it flows next to nothing. Sure enough, it only works marginally well if running all the time, and if the Guest bath loop is closed. I'm considering the Grundfos Alpha 15-55 HWR-T, but am concerned it's way too big. For example, at 7.6 feet of head (from the paragragh above), it flows nearly 9GPM, way more than allowed if I expect the pipe and tube to last.

I've been reading, watching and ciphering all day, and am coming to you guys for expertise. School me, please.

Mark

 

[PoorUB]

GPM on a recirc pump doesn't really matter. It will just take a little longer to get the how water to the fixtures with a lower flow. I looked at the Taco calculator and am surprised at it. I have installed a few circulators and never worried about head pressure and not one complaint. Also, the head pressure is non-existent became you are pumping from and back to the same pressure. I suppose they are calculating some line friction, but roughly 1 GPM in a 1/2" line I would think it would be nothing.

I agree the 15-55 is too much pump and wonder if the existing pump is too small. Even at zero head the little Grunfos pumps only a couple gallons.

If we believe the Taco calculator you need to size for the worst head pressure, the lower head loops will flow more water.

 

[jmiller_2308]

DOH! I wrote the comment below in a blurry eyed early morning confused state. My comments were about hydronic heat and not recirculating hot water.

--

Wow... I'm surprised that the water heater can keep up with feeding that much tubing especially if you start pumping 9 gpm with the 15-55.

I have tempering valves on my basement slab loops and if I open them too much they can literally **** all the energy that my 120000 btu boiler can put out. I do have more linear feet than you but I suspect that if you actually start pumping as much as 9 gpm through the system that at some point you are likely to exceed the recovery rate of that water heater.

You mention that the system "hasn't worked great" but what does that mean? Are some of the loops warmer than others? Is there not enough heat or too much heat in some places? It is early and my eyes are blurry but I don't see evidence of zone controls or tempering valves on each of your loops. My system has both zone control and tempering valves. The zone controls allow for the obvious of controlling temp in different zones. The tempering valves allow me to fine tune each of the loops in a given zone. Using the tempering valves to balance the loops is really essential when a zone spans multiple living areas that have different needs.

 

[ScaldedDog]

Thanks guys! You've both been really helpful. @PoorUB's post gave me the idea to partially close the valves on the two shorter return loops in an attempt to equalize flow between all three. I did that yesterday, and the system has worked pretty well since. Hot to warm water very quickly in all three locations, and it doesn't seem to have affected the temperature of the MBR shower, which has been a problem for a long time. The Grundfos pump I have claims to figure out it's cycling time based on need, but I think I might do better with a pump with a little more flow and a simple 5-on, 10-off control strategy. (No where near 9g/min, though. I'm thinking more like 3g/min, so that each of the 1/2" return loops could get 1g/min if I'm able to equalize the flow. Am I thinking about that correctly?)

@jmiller_2308, there have been a few problems with this system since we bought the house. The previous owner had a Taco pump of unknown size that ran 100% of the time, and it was replaced with the current Grundfos pump when we replaced the water heater shortly after moving in in 2020. It, too, has been setup to run 100% of the time. Also, until last week no insulation was on any of the pipe/tube in the system.

The issues have been:

1) If all three loops were full open, the first shower in the MBR would be warm, not hot. Whoever showered next would get a nice hot one. Strange, and I never figured out why. Our solution was to leave the guest (longest) loop off, since that bathroom is rarely used. That helped noticeably, but not completely.

2) Related to the first issue, there has always been a lot of variability in the temperature of the water at any fixture anytime the system was running. Even in the kitchen (shortest loop), sometimes the water at the faucet is really hot, but oftentimes it's not. If the recirc pump is off, the water is always hot, if tardy.

3) After being gone for three months last year with the system off, it started to make an intermittent rattling noise in the utility room after the pump had run awhile. I thought it was air in the system, but purging using the purge valve below the pump didn't help. Last week I plumbed in a 90* fitting with a vent above the pump, but have seen no air anytime I tried it. When I took the pump apart, I found pitting on the surface the impeller faces. I'm assuming that's from cavitation, but it might just be from our hard water. (We are on a well, which I should have mentioned, and installed a water softener a couple of years ago. The pump ran for a year solid before that, though.) If the pump only runs intermittently, like we have it set to do now, it never makes a peep.

I'm a simple guy. All I want is consistently hot water reasonably quickly. Thanks to you guys I'm closer than I was, but not quite there. Further suggestions are welcome!

Mark

 

[Shiftless]

“I'm a simple guy. All I want is consistently hot water reasonably quickly. Thanks to you guys I'm closer than I was, but not quite there. Further suggestions are welcome!”

I too consider myself a pretty simple guy. My 1951 vintage house with no circulation system is not huge at 2600 sq. ft. but the shower in the bath furthest away from the water heater gets hot after running water for 15 seconds. This “wastes” about one gallon of water flowing down the drain. Is that so bad?
And it never goes less than hot for unknown reasons. The relatively new 40 gallon gas water heater provides 3 showers in a row with no problems.

 

[ScaldedDog]

My 1951 vintage house with no circulation system is not huge at 2600 sq. ft. but the shower in the bath furthest away from the water heater gets hot after running water for 15 seconds. This “wastes” about one gallon of water flowing down the drain. Is that so bad?

Ours is not that good. It takes a minute or two for the closest shower to get hot if the system is off, and longer for the guest shower. Being on a well (which I have run dry a time or two) in an arid climate, running water down the drain isn't the greatest strategy.

Mark

 

[Shiftless]

Ours is not that good. It takes a minute or two for the closest shower to get hot if the system is off, and longer for the guest shower. Being on a well (which I have run dry a time or two) in an arid climate, running water down the drain isn't the greatest strategy.

Mark

Those are excellent reasons to have a recirculating system. I am trying to figure why it takes so long to get hot. If your shower runs for a full minute before it’s hot, that’s what…2 gallons of water? Is there something crazy like 100 feet of tubing between your water heater tank and the shower? Or is the tubing really large in diameter to hold that much water? My house is plumbed with half inch copper.

 

[ScaldedDog]

Those are excellent reasons to have a recirculating system. I am trying to figure why it takes so long to get hot. If your shower runs for a full minute before it’s hot, that’s what…2 gallons of water? Is there something crazy like 100 feet of tubing between your water heater tank and the shower? Or is the tubing really large in diameter to hold that much water? My house is plumbed with half inch copper.

The supply lines are all 3/4" PEX, fed by 13' of 1" copper. The closest shower is 13' of 1", 49' of 3/4", plus 8-10' of 1/2 copper, which I assume is what's in the walls. That's about 2g in the lines, so if the shower flows 2g/minute, a minute would be about right for it to start to warm. Maybe it seems like two minutes when one is standing there naked waiting...

Mark

 

[Shiftless]

The supply lines are all 3/4" PEX, fed by 13' of 1" copper. The closest shower is 13' of 1", 49' of 3/4", plus 8-10' of 1/2 copper, which I assume is what's in the walls. That's about 2g in the lines, so if the shower flows 2g/minute, a minute would be about right for it to start to warm. Maybe it seems like two minutes when one is standing there naked waiting...

Mark

That explains it. I wonder why the plumber ran those large diameter pipes to supply a shower when the shower head has a flow restrictor? Filling a big spa tub would require pipes like that. I assume you don’t have a big tub to fill.

 

[fitter30]

There's more to design than than picking random flow and head. Even though your lines look like their piped reverse return they might not be. Return lines would add globe valves one size smaller piped in the bottom out the top for water balance. Globe valves are the next best valve for balance control. Circuit setter would be #1.

Hot Water Recirculation Systems

There’s more to designing a domestic hot water (DHW) recirculation system than just adding another pipe and circulator. John Siegenthaler, P.E., a favorite ...

www.google.com

 

[ScaldedDog]

UPDATE: I bought a timer that let's me run the current pump on a 5on/10off schedule 24 hours a day, and have been doing so for about a week. With the three circuits roughly equalized (just by partially closing the zone valves on the two shorter circuits), the system has been working great. No warm-but-not-hot showers like we would have when the pump ran nonstop, and we have at least warm water in just a few seconds everywhere in the house. The pump is still making some intermittent racket, but not enough to make we want to spend money on a replacement yet.

Many thanks to all who helped!!

Mark