Hot water recirculation system design questions

[blitziger]

I am currently living in my finished basement while we are renovating the upper floors. I just passed rough-in inspection for the upstairs baths and kitchen, and I mentioned to the inspector that we were sometimes running out of hot water for our shower when the basement slab hydronic heating had been running for any extended time. I said we would be installing a thermal mixing valve so that we could turn up the temperature of the water heater and that we would also be installing a hot water fixture recirculation pump for the basement and a separate pump for the upper floors (both systems have each been plumbed in series with a return line at the farthest fixture).

The inspector told me he had been a plumber for 20 years before he became a City inspector and very generously gave some advice. He said that we should consider upgrading our water heater elements to a 180 degree type. He also said we needed to install the recirculation line with a bypass line back to our cold water line to help guarantee the system would not still allow scalding water to enter the pipes due to cold water stagnation or something to that effect. He attempted to describe where all this should occur, but I did not comprehend what he was saying. He ended by saying that code does not require a tempering valve but we should certainly install one.

I did an internet search on mixing valve performance problems and all I could find was this video by Caleffi on YouTube called “Installing Thermostatic Mixing Valves & Recirculating Pumps in Plumbing Systems”

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My questions are these:

1. I’ve attached a diagram showing my best interpretation of the Caleffi engineer’s recommendations, and I'm hoping someone can comment on whether I’ve designed it correctly? One complication I see is that I have two separate recirculation systems, and I’m not sure whether they will adversely affect each other’s performance.
2. We currently have a Reliance 1212 55 gallon electric water heater – is it safe to upgrade the heating element in this unit?
3. Is there a better (and possibly cheaper) motion activated recirculating pump than the Taco 006-CT pump paired with Taco’s 554-4 motion sensor? I liked the fact that the pump could be set to turn itself off when the return water reached the set temperature.

Thanks.

 

[Smooky]

On the upstairs return and on the basement return the line has two paths one back to the cold side of the hot water heater and the other path to the cold side of the mixing valve. I understand going back to the cold side of the water heater. Why make a path to the cold side of the mixing valve?
This can cause adjustment of the mixing valve to be difficult with tempertures all over the place. If the cold side has a consistant temperature it is a lot easier to make adjustments. Also you show the returns but where do these lines start?

 

[Sponsor]

 

[blitziger]

On the upstairs return and on the basement return the line has two paths one back to the cold side of the hot water heater and the other path to the cold side of the mixing valve. I understand going back to the cold side of the water heater. Why make a path to the cold side of the mixing valve?

I had to watch the third quarter of the video four times before I got what the engineer was saying. If you pipe back through the cold inlet of the hot water HW tank only, then there is no back pressure to allow cold water to enter the system at the mixing valve, then the system can heat up... I can see that there possibly are other ways around this, but his system is so simple. My diagram may show a flawed design, so first understand the engineer's design, and any critique of my system diagram will be appreciated.

This can cause adjustment of the mixing valve to be difficult with tempertures all over the place. If the cold side has a consistant temperature it is a lot easier to make adjustments.

I'm not completely familiar with how a mixing valve adjusts - if there is a problem with warm water coming through the cold side, please explain the dynamics to me.

Also you show the returns but where do these lines start?

The returns start at the ends of the hot water distribution pipes - it's a re-circulation system, Google it.

 

[hj]

From his comments, I can see why he is no longer a plumber but became an inspector, (most of them are failed plumbers).
1. There are NO "180 degree elements"
2. You DO NOT install a "bypass line to the cold water"
3. If you raise the heater's temperature, you will also need a tempering valve for the heating system.

 

[blitziger]

From his comments, I can see why he is no longer a plumber but became an inspector, (most of them are failed plumbers).

Quite honestly, I could not recall exactly what his wording or recommendations were. I do know that his concerns included not installing a hot water recirculating without any mechanism for allowing tempering cold water to enter the recirculation flow. However, this concept was all new to me, and I might be completely misinterpreting his advice.

1. There are NO "180 degree elements"

I just did a Google search and it appears you are right - he might have been advising that the thermostat be replaced. The only 180 degree thermostats that I could find clearly state that they are for commercial applications only. I checked my water heater's top thermostat, and it is set to about 125 degrees currently and can go as high as 150 degrees. Both settings are too high for scalding protection, so I'll wait until I install the mixing valve before I adjust the temperature settings.

2. You DO NOT install a "bypass line to the cold water")

Again, I clearly did not interpret the inspector's advice correctly, so don't critique my paraphrasing of him. However, I'd be interested in your take on the Caleffi video engineer's recommendations. He seems so convinced of his own logic. I read on several other website plumbing forum discussions where people had inconsistent hot water temperatures coming from tempering valves when paired with recirculation pumps, and the advice was always that the tempering valve must be defective. But replacement did not resolve the problem and the discussions ended there...

3. If you raise the heater's temperature, you will also need a tempering valve for the heating system.

The heating is for an in-slab pex hydronic system. Is there a reason the water temperature can't be higher? We have a dual thermostat http://tekmarcontrols.com/products/zoning/552.html that shuts off the hot water when the slab at the thermostat reaches temperature, in order to prevent the air temperature swings that result from slab heat transfer lags - it allows for more comfort too, because the slab remains at a more constant temperature with more pump cycles. I understand that we could achieve similar results by pumping lower temperature water for longer periods of time, but we get into the same complication as with the fixture water tempering - how do you introduce modulating water if the return line goes only to the HW tank's inlet?

 

[blitziger]

I'm about to order the parts for our hot water recirculation system and thought I'd make another inquiry for feedback here. From what others have already posted, it's apparent that I should disregard most of what the builder inspector said (but, again, it was about a one minute conversation, so I may have misinterpreted what he was saying). What I did take away from his advice was the need for a tempering valve. So, three of my questions remain:
1. Is the mechanical engineer on the video by Caleffi wrong? YouTube “Installing Thermostatic Mixing Valves & Recirculating Pumps in Plumbing Systems”

2. Are there any obvious problems with my system as I have it diagramed?
3. Is there a better way to introduce cooler water back into the system than what I have diagramed? If so, does it address the issue of needing backpressure for cold water introduction?

 

[hj]

Circulation systems and tempering valves do not get along together. Since there is no "Flow of cold water" through the valve the only way to get "tempering water" through it is to connect the "cooler' circulation line water to the valve's cold inlet, but that only works until the "recirculated water" exceeds the valve's setting, then the hot water temperature will continue to climb until it reaches the heater's temperature.

 

[blitziger]

Thanks HJ, what you say makes perfect sense. I've chosen the Taco 006-CT pump because it supposedly turns itself off when the return water reaches the set temperature. Would the tempering valve eventually shut off water flow from the HW tank if the tempering water became too hot? It would be nice to know that the system would have a redundant protection ability.

 

[Jadnashua]

A tempering valve works by mixing cold water into the outlet stream...the max it can go is as full cold mixed with the hot or the setpoint of the internal control valve, whichever is higher in temp. But, to get cold water into the stream while pumping it back to the source, there's no new water being introduced to the system, and therefore, no possibility of cold coming in. As I understand the tempering valve, it always runs the hot full, while adding cold as necessary. It would internally have the port to cold open, but since there's no flow out the end of the pipe (it's being pumped back around!), no cold added. Might be all wet, but that's how I think it works.

On my system, I have an adjustable aquastat, and have the pump turn off when the water at the sensing point is just warm...for my setup, that means hot is at all of the earlier fixtures, and I don't waste as much energy getting really hot at that last fixture which happens to be the vanity sink where warm is fine, but hot is nearby. Since I'm using the cold water line as a return, I've found doing it this way, flushing the toilet purges all of the warm water in the cold line for that sink, so I can get either hot or cold.