Boiler Return Temperature - Too Low?

DJDrew

Just as an update. The Danfoss boiler protection valve was installed. We went with the 122 degree element, as we didn't want to starve the circulation of the system by going with the 131 element. Ideally, the 131 would work, but we didn't have the money right now to re-pipe and separate the boiler loop from the radiator loop - however it seems easy to upgrade the element in the future.


Some performance notes:
- The boiler still runs for 35-45 minutes per call for heat (as I expected.)
- The hydrostat has condensation protection enabled, so the pump will only operate once the internal boiler temp hits 125, shutting off only if the internal boiler temp dips below 115.
- The hydrostat reads 122 for the first 30ish minutes of a call for heat.
- Instead of the max temperature finishing in the mid 130s as it did before this time last year, it is now finishing around 127-130 range*. (My guess is because the temp going out to the radiators is consistently higher, so the radiators are heating up more quickly and satisfying the thermostat.)
- Overall, most rooms of the house are more than comfortable (no change in performance), but as predicted by others earlier in the thread, 4 out of the 14 rads are significantly slower to heat up, two hardly heat at all now. This is probably due to the valve throttling the circulation, as these are at the end of the line or at the end of the line and on a second floor. Perhaps if I partially close down some of the closest rads, it will draw more from the others?
- *We have only had low temps in the upper 30s/low 40s, so there is not a huge demand for heat yet, only some. We had mini-splits installed for AC this year, I didn't spring the extra money for hyperheat, but the installer said that the standard mini splits we had installed will perform fine into the low 30s or upper 20s for heat. I am guessing I could just run the mini-splits until the demand for heat more matches the boiler and would allow it to run for a period of time in the 130-140 range.

One other related question: While installing the valve I snapped a picture of the tray under the burners. The boiler is 1 year old, are the rust/deposits that have fallen from the boiler about normal for 1 year of service? I am planning to get the original installer here over the next month to do a cleaning and combustion check.

hot_rod

Once you get up to the cartridge temperature the valve will be wide open, should not be much pressure drop or reduction of flow.

For instance, if it is a 17Cv valve, only 1 psi pressure drop with 17 gpm flowing. You may just need to do some balancing of the radiators, if possible.

SuperJ

As hot rod suggested, I would start by trying to balance things out:
1. Make sure the sluggish areas are wide open
2. Throttle the zones that heat up first a bit

As an alternative, It might be worth putting a couple TRV's on the zones that heat up the quickest (if your piping design allows for this). This will help divert flow to where it's needed more. I wouldn't get too carried away though since your boiler doesn't modulate.

Eventually when you have the cash, you could always cut in a hydraulic separator/closely space Tee's and a secondary pump.

@hot rod Caleffi should make a separator with built in low RWT protection since they include air, dirt, hydraulic and magnetic separation. How about adding thermal separation to the mix?

bob eck

Hot Rod are all VS pumps ECM motors? What about dirt and excessive iron that could be in his system? Would he need an Caleffi Dirtmagnet filter installed? The dirt magnet unit would help keep the system water clean and help the ECM pump last longer and help with better heat transfer to the older cast iron radiators. Doesn’t the dirt in older systems act like an insulator causing more energy to be used to heat the home?

bob eck

I thought when sidewall venting an condensing boiler and using concentric vent termination you can be within 12” of an door or window opening. If the contractor could not find an sidewall place with this clearance couldn’t the contractor suggest to the home owner we can sidewall vent here but the termination point does not meet code because of the window that can be opened. Let’s consider making this window inoperable.
I know of a job where the house has CI radiators and it is 1/2 double home. To install an condensing boiler and sidewall vent the contractor had to vent the boiler sidewall but he had to come up with the vent into a closet on the first floor and then vent sidewall and the reason for this is because the only wall to vent out there is a public sidewalk and the vent termination needed to be 7’ or higher off the public sidewalk.
An condensing boiler or condensing combi boiler would have been great for this house.

hot_rod

bob eck said:

Hot Rod are all VS pumps ECM motors? What about dirt and excessive iron that could be in his system? Would he need an Caleffi Dirtmagnet filter installed? The dirt magnet unit would help keep the system water clean and help the ECM pump last longer and help with better heat transfer to the older cast iron radiators. Doesn’t the dirt in older systems act like an insulator causing more energy to be used to heat the home?

bob eck said:

Hot Rod are all VS pumps ECM motors? What about dirt and excessive iron that could be in his system? Would he need an Caleffi Dirtmagnet filter installed? The dirt magnet unit would help keep the system water clean and help the ECM pump last longer and help with better heat transfer to the older cast iron radiators. Doesn’t the dirt in older systems act like an insulator causing more energy to be used to heat the home?

You can add an external drive to most pumps to turn them into VS operation. That is basically what tekmar pioneered years ago and you still see their technology on some circulators. Those are used with PSC type motors. Rumor has it PSC may be phased out by DOE on fractional HP pump motors.

Some of the ECM style use two or three piece assemblies, so the motor is not in the fluid like a wet rotor, thatbhelps with magnetite dbuild up.

But you still want to keep as much crud as possible out of the systems. Anything that coats out the hot surfaces reduces efficiencies, at the boiler and heat emitters.

CTG

Hi Heatinghelp,

Is it proper to bump an older thread? Should I start a new one?

I learned over the long holiday weekend I have the same issue with low return water temperature to the boiler and found this thread very helpful in explaining some of the causes and solutions.

Wondering if I could ask a question regarding location of the 3-Way thermostatic mixing valve and boiler loop circulator. I think I read a couple references indicating to have the circulator on the supply side of the boiler (do I have that right)? The examples I have found so far (including in the manual for my boiler) have the circulator on the return side pumping away from the 3-way mixing valve.

Is it preferable to have the circulator on the return pumping away from the mixing valve when used? Or the circulator should be on the supply side pumping away from the boiler?

Thank you!

Chris

hot_rod

https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Caleffi-280975A-Product Overview.pdf

A single pump or two? In some cases the one pump is adequate, or with a sep and two Pumps if needed

CTG

Hi Hot_rod,

Thank you for the reply and link! I will try to digest that a bit

I think the answer to your question is 4? ...but I probably don't understand the question!

Attached is a diagram I did of the existing system and picture.

I've also added temperature supply/return temperature sensors/displays a few days ago so I could "see" what was happening.

Thank you!

Chris

STEVEusaPA

How's the overall performance, comfort?
The one thing I may change is not taking the indirect off of the primary loop with closely spaced tees, and take it directly off of the boiler outlet and return on the inlet, rather than have it run all the way around the loop, past the zone heating, then back to the boiler.
But if the indirect tank is recovering fast enough (or it's on priority) I guess it doesn't matter.
It also looks like your injection piping is incorrect.

CTG

Hi Steve,

Could you elaborate on how the Injection Piping is incorrect?

Regarding performance…

The existing controls do not provide Priority for the Indirect DHW. This I now know has caused us issues randomly, but until recently I couldn’t connect the dots. Last Wednesday evening my wife informed me we had no hot water after she had run the dishwasher following dinner and drew what turned out to be a cold bath. I went to the boiler to find 3 zones calling for heat as well as the DHW. The gauges on the piping showed 70-80 degrees. I manually shut the heating zones down to give the hot water priority till it recovered. I ordered another controller that provides priority for the DHW and it should be here today. I also installed supply/return temperature sensor displays Thursday to help me understand what was taking place.

My speculation of what happened Wednesday evening:
We live in CO at 9000’ and get a lot of sun in our large south facing windows. Wednesday was mild I think in the 30’s (maybe 40’s) and sunny, the sun kept the house thermostats satisfied all day. Coincidently, the garage and two spare rooms (all three on the north side of the house) we keep the thermostats at 55* started calling for heat at the same time the DHW did and that overwhelmed the system.

Fast forward to Saturday morning, we wake up and the house is colder than normal. Take my coffee with me to the boiler to find several zones & DHW calling for heat, but the boiler hasn’t fired. Following the trouble shooting guide in the manual I find a blown fuse on the 24v control circuit. With the boiler running I again interrupt zones till the DHW is satisfied then since I installed the temperature sensors I was able to “watch” what would happen to the temperatures as zones came on. Basically I could watch the return temperature on the MixLoop plummet, then right behind it the return & supply temperatures of the Boiler would plummet, and that would cascade then to the supply side of the MixLoop. All the zones in the house were cold, it was mid teens all day and cloudy so I was able to watch this take place for several hours repeatedly by manually shutting off zone valves and then staging them in one by one over time.

Trying to understand why and what for I found this thread

One thing I don’t understand is that the controller indicates it provides boiler protection and would reduce the mix injector speed, but I never saw this happen. Boiler and mixing loops would both plummet in temperature and the controller was showing the output to the injector pump still dialed up?

Thank you!

Chris

STEVEusaPA

The problem is with the logic of the Tekmar boiler control + boiler protection + injection mixing-as you seem to have figured.
Usually trying to protect the boiler (via a sensor) stops the injection process until the boiler comes up to temperature. Then it immediately injects, supply temp drops and stops injection.
With my oil boiler it causes short cycling when only a radiant zone is calling, the boiler shoots up to the reset temp, and shuts off. Then in a few minutes, back on. I've spent years staring at it, adjusting, talking to Tekmar, staring adjusting.
Best to protect the boiler mechanically with a 3 or 4 way mixing valve, turn the boiler protection off on the Tekmar.

Here's the Tekmar essay on mixing:
http://www.tekmarcontrols.com/images/_literature/e0021_06.pdf

Idronics has many diagrams on proper piping methods with indirect tanks, priority/non-priority, etc.
https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_19_na.pdf

All Idronics:
https://www.caleffi.com/usa/en-us/technical-magazine

DJDrew

Just as an update to the original start of this thread, I used a mechanical mixing valve with a 121 degree element. It does a great job at keeping the boiler temperature in the happy zone.
The only downside I have noticed on my system is that the radiators at the end of the line & far away seem to get minimal flow, so they only heat up toward the end of the t-stat cycle.
I have tried to balance the system in response by restricting the flow on the rads that heat up first and installed a remote thermostat sensor to average the temperature readings over the house. It's been slightly better, but still annoyingly colder in the end-zone rooms.
I thought of two possibilities to address the issues the mechanical mixing valve introduced:
1) With the way the system is setup, it would be relatively easy to modify the piping to primary/secondary, which would probably then allow for better flow since adding a second pump would then not be not restricted by the mechanical mixing valve at the boiler.
2) I could also probably solve the problem with TRVs on some of the rads.
Since both fixes/mods are fairly pricey, we have been going the other route to the problem and beefing up the insulation in stages around the house - the payback seemed more solid. Then when something else bigger needs to happen with an addition or other impacting the heating system, we'll probably then tackle additional modifications to the heating system.
Summary: The mechanical boiler protection, as suggested by others, works great - but it did slow the flow down some on our system with only a single pump.

CTG

Steve,

Thank you for the links!
Will read up on these and Hot_rod’s links to see if I can discern how my layout is off.
I’ve gathered the mechanical mixing valve is necessary and naturally would want to attend to any other areas if warranted while the system is open.


DJDrew,

Thanks for prompting this informative thread and sorry if I hijacked it!


Best regards,

Chris

DJDrew

CTG said:

DJDrew,

Thanks for prompting this informative thread and sorry if I hijacked it!

Best regards,

Chris

No worries - it's related and all good info to expand upon.

Zman

The 356 will take care of the boiler condensing during heat call if you set it up correctly. The sensor needs to be on the return side and the parameters need to be changed. It does not stop the mixing, it just backs off when the boiler temp gets too low.
For domestic priority, the easiest this to do is install a double pole double throw relay in place of the SR501 (post a picture of the inside of the 501 to be sure it is not capable). With this strategy, you would break the low voltage connection between the ZVC 404's and the 356 any time there is a call for DHW.
This is all an easy fix.
What part of Colorado?


I just looked at an old SR501 I have. It looks like you could set it up for priority. I would just put the power to the 356 on the NC side of the DHW circ. That way the 356 loses power anytime the DHW circ is operating.

Zman

How is your radiant installed? In slab? Underfloor with plates?
Your settings seem off.
Room is usually 70
Outdoor design would be more like 0
Term units and mix temps depend on the above question.
Boiler sensor should be on return.
WWSD is usually more like 65.
https://s3.amazonaws.com/s3.supplyhouse.com/product_files/356-install.pdf

CTG

Hi Zman,
Thank you for your help!!
We are in Dillon.
Please see attached the requested picture of the SR501 along with a sketch of the wiring I did earlier this summer when I was trying to understand how the system functions and traced out the wiring.
Your solutions are WAY easier and WAY cheaper than installing the 374 I ordered! I can easily implement either solution. Would it be detrimental to the 356 over time to power cycle that often? If not, then yes the N/C on the SR501 is there for the taking After reading your initial suggestion I was wondering if it might make sense to break the low voltage between the ZVC404 and 356 with a timer to limit how long the DHW can have priority over heat?
I can easily relocate the sensor to the return side and adjust the settings. Should be pretty straightforward to see if the controller backs off the injector pump when the return temp drops.
For the term units, we have 2310 sqft in slab on the ground floor and 1080 sqft of underfloor upstairs.
My notes from earlier this summer indicated I found a chart listing -10 for Outdoor Design, but I think the closest local was Leadville, so it would make sense for -0- at our location relative to there.
Thank you again!

Best regards,

Chris

Zman

Barry Engleman for GTS did lot's of those designs here in the late 90's and early 2000's. They work pretty well once you get rid of the glitches. I wonder if they installed plates on the underfloor. If they did not, you may eventually want to go with a 2 temp system. A plateless staple up requires very high water temps, In slab does not. If this is the case, you will have trouble getting enough heat to the upper zones.

CTG

Ah, ok, I see. So the 374 might have more merit in that case.

We are the second owners and all the ceilings are finished so I do not know how the underfloor was installed. I guess I have to do some homework on how the installation was completed. I think there is a storage area I might be able to get access to in order to see how the underfloor was done.

I guess for the time being I can try setting the TermUnit for the slab while I am experimenting and see how the upstairs responds.

I found a relay and base in my collection, so I can install that shortly, make the changes to the 356 settings, and monitor this afternoon since I'll be in the garage working on another project.

Part way through the very informative links Steve provided!

Thank you,

Chris

CTG

Still early in the observation of changes, but brief update:
Relay install functions to interrupt 356 and provide DHW priority. The Zone valves of course stay open and initially there was some reverse flow into the mixing loop as shown by temperature increasing on return side sensor. That seemed to stop/stabilize relatively quickly.
Relocating the Boiler temperature sensor to the return side and updating settings in the 356 works to provide boiler protection as Zman indicated once up and running. With single digit temperatures this morning and several zones calling at the same time the injector circulator would adjust between 0-20% to maintain the target temperature in the mixing loop. Very different from what I saw on Saturday.
More observation needed, but it seems that if the system has been off and then a call for heat, the startup process can drive the boiler below temperature with circulation starting followed by the time it takes for the flow switch, fan proof switch, and boiler to fire and build heat. Not sure if this is terribly important or not, but have observed a few instances where the boiler loop was above minimum return temperature then dropped below briefly when startup was initiated. Appears this would be addressed with the prescribed bypass loop and thermostatic mixing valve.

Zman

Dipping into condensing temps at startup is normal. It is important that the boiler runs hot long enough to dry the condensate on the boiler and vent. When you drive around you can see rust on the vent terminations that have oversized atmospheric boilers. Some are like yours where they are running low temps, others are boilers that short cycle and never dry out.