Advice on Fine-Tuning Primary/Secondary Pumps

[Davie]

A few years ago, I got some great help here doing heat loss calculations, choosing a boiler, and designing a radiant system for my home. The system is installed now, and it ran well all last winter. I'm very happy with how efficient it has been, and how comfortable it is. Thanks to everyone who helped me, but especially to @Dana, whose many comments were invaluable.

I'm now interested in fine-tuning the system to see if I can get it to run a bit more smoothly.

I am running an IBC boiler, the SL 10-85 G3 on outdoor reset mode, with a primary/secondary loop, and 3 zones controlled by zone valves. All three zones have cast iron rads, and two of them also have radiant floors. I have a grundfos alpha2 on the primary, running on speed "I", and a taco VT2218 on the secondary loop, running on delta-t mode, with the temp-diff set to 9 degrees celsius (16F).

Here's a look at the primary/secondary piping:



And here I've drawn over it to make it a bit clearer:



I've noticed that the primary loop always remains hotter than the secondary. On warmer days, this can result in the boiler shutting off while the system continues to circulate, and then firing back on when the system temperature has dropped sufficiently. (This particular circumstance seems particularly prone to happening when only Zone 2 is calling for heat.)

Today, for example, the temperature outdoors was 10 celsius and only Zone 2 was calling for hear. The "target" boiler temp was 34C, based on the reset parameters, but actual supply temp rose to 39C (primary loop), while the temperature in the secondary loop was only 34C.

I think this is because my primary (boiler) loop has a higher flow rate than my secondary (zone) loop. So, some of the hot water from the boiler is bypassing the closely spaced tees, and heading back to the return side of the boiler.

I know that it's impractical to expect to have identical flow rates through the two pumps, but it seems to me that if one pump has to run more flow than the other, the more efficient option would be to have higher flow through the secondary loop. My thought is that this would help prevent the primary loop from overheating when demand is low (ie when outdoor temperatures are quite a bit above freezing) by sucking all the newly heated water into the zones, and ensuring only the coolest water in the system is heading back to the boiler return.

So my question is about how to set my pumps.

The lowest flow rate I seem to be able to get from the Grundfos Alpha2 is about 3.5 GPM (the display switches back and forth between 3 & 4). I don't believe there is any change in head in the primary loop as zones open and close on the secondary, so I think that flow rate is pretty constant. The flow through the Taco VT2218 varies based on which zones are open. It seems to be something like this:

Zone 1 ...........2 GPM
Zone 2 ...........1 GPM
Zone 1 & 2 .....3 GPM
Zone 0 & 1 .....4 GPM
All zones ........5 GPM

So, in three of these 5 cases, the flow rate through the boiler is higher than through the zones.

It's too late now for me to buy different pumps, so I'm going to work with what I have.

I'm experimenting with Constant Speed mode on the Taco, but preliminary testing seems to show that the Delta-T mode was operating as though it was in Speed 1. I never actually get the 15F temperature drop that I designed the system to operate with -- it's usually more like 9F (measured on the secondary loop). So, I think the pump is probably just trying to run as slowly as it can to increase the delta-t.

I'm now trying it out on Speed 2, and while I like the higher flow rate in the secondary loop, I imagine the temperature drop will reduce.

Should I be trying something else here? Am I looking at this in the right way? I am thinking I could try swapping the pumps, one for the other, and see what that does to the flow rates. The boiler manufacturer states minimum flow rate at 2GPM, so I could theoritically cut back by 40% or so, but I'd rather not go to the trouble of all that before hearing from you folks.

Any thoughts or suggestions would be much appreciated. Thanks!

 

[John Gayewski]

What was the actual delta t in the primary loop? Did o read that right and your not having any temperature drop in the primary loop?

 

[Sponsor]

 

[Davie]

The delta t in the primary loop is comparable to the secondary, when temperatures are higher. Yesterday the delta t was 9F in the primary and 9F in the secondary. When the weather gets colder, the primary delta t will increase.

I have data from a day in March, for instance, the outdoor temperature was 17F, and the primary delta t was 18F, but the secondary was only 7F. Flow through the secondary was 3 GPM at the time.

 

[Fitter30]

Looks like your running the cast radiators and floor radiate heat the same temp water? What the max temp of outdoor temp reset?

 

[Davie]

Looks like your running the cast radiators and floor radiate heat the same temp water? What the max temp of outdoor temp reset?

That's right, it's all one temperature.

Design system temperature is 140F (at 1F outdoor temp), and max temp is 154F.

Minimum system temperature is 93F.

 

[John Gayewski]

What kind of flooring do you have?

 

[Fitter30]

Floor radiate normally runs 5-10*f over thermostat setpoint because of the floor coverings and being to not for your feet. Have a floor temp sensor built in to the thermostat?
https://www.ibcboiler.com/support/TIPortal/Content/Guides_and_manuals/SL/SL_G3_residential_PDFs.htm
Controller manual.
Pg17 temp differential 20*f
starting at pg 23 express setset- up
resetting the load type. Doesn't give a very good explanation but it did show high mass for cast iron radiators. Thermostats for zones brand and model and If all different types of emitter? Looking at your reset design supply 60*f can heat your house at this temp at -17? Summer shut down 16c not 18*c?

 

[Davie]

What kind of flooring do you have?

It's hardwood flooring (3/8") with tongue and groove subfloor (7/8").


(NB: the actual insulation used is R22, and I didn't use a reflective foil.)

Floor radiate normally runs 5-10*f over thermostat setpoint because of the floor coverings and being to not for your feet. Have a floor temp sensor built in to the thermostat?

I don't have a floor sensor, but I designed the system maximum temperature with this in mind. At design temperature, with the effective R1.85 insulation the flooring provides, the floor surface is meant to be 85F. I'd have to dig up my math for this, as I can't recall the formula I used, but I did base the temperatures on this reality.

I know it's less than ideal to have the rads and floors running at the same temperature, but it was a tradeoff I decided to make in exchange for a simpler system to design, build and maintain.

Doesn't give a very good explanation but it did show high mass for cast iron radiators. Thermostats for zones brand and model and If all different types of emitter?

I do have my supply differential set to 20F, FWIW.

When I was setting the load profile, I called IBC and spoke to them about my specific situation (ie hybrid of low-mass radiant and cast-iron rads). They recommended I use the Cast Radiator profile. Which I have.



I will say, while I understand theoretically why it is important to have different profiles, I don't have a great grasp on what tangible difference I could expect between that profile and the High Mass Radiant, for example. Thus, I don't know if switching might help me.

Thermostat for Zone 2 is LuxPro TX9100E


Zone 1 is this Honeywell CT87K4776


Zone 0 is this old White Rodgers unit


(NB: Zone 0 doesn't have the ability to call for heat on its own. The thermostat only controls the zone valve, but has no connection to the boiler. If the basement gets cold, it waits until one of the two living space thermostats is calling for heat.)

As for all being different types of emitter, I think you're asking if all the emitters are identical, and they are not.

Zone 2 has three cast iron rads (all different sizes), one towel rack rad, and 160 square feet of heated floor.
Zone 1 has 2 cast iron rads, one towel rack rad, and 475 square feet of heated floor.
Zone 0 has 2 massive cast iron rads.

 

[Davie]

Looking at your reset design supply 60*f can heat your house at this temp at -17?

60C (140F) is what's required to heat my house at -17C (1F), based on the heat loss for my house, and the emitters we installed. In the past winter, we only had a few days that got close to that temperature, around 5F (-15C), but the house was cozy on those days. Actually, more cozy than usual, because it's only when it's really cold outside that the heat from the floors is noticeable underfoot.

Summer shut down 16c not 18*c?

I tweaked this in the springtime. I had originally set the summer shutdown to 18C, but the system was cycling a wee bit more than I liked. The change probably wasn't necessary, but I'm happy with it. And, in the spring, when it's 16C, it's usually because it's daytime and sunny, and the heat isn't really needed. We don't really have overnight temperatures above 16C until the heat isn't needed anymore.

 

[Fitter30]

Difference in heat emitters. Fin tube has very little mass heats up fast cools fast. Cycles per hour 3-4.Hi mass concrete radiate floor takes long to heat up and long time to cool. Cycles per hour 1-2. So if a space over heats or doesn't heat enough this is just another way fine tune a space. One concern is your radiate floors and the type of floor coverings and is the piping under plywood or cement slab?

 

[Davie]

One concern is your radiate floors and the type of floor coverings and is the piping under plywood or cement slab?

The piping is under 7/8" tongue and groove subfloor and 3/8" hardwood.

 

[Fitter30]

https://hardwoodfloorsmag.com/2020/...radiant-floors-with-the-2019-nwfa-guidelines/
85* max temperature. Contact flooring manufacturer for their recommendation.

 

[Davie]

Could changing the primary and secondary pump flow rates be a way to get more heat out to the zones? I'm confused as to why there always seems to be a temperature drop between the primary supply and the secondary supply.

There often seems to be a ~10F drop between the supply from the boiler and the supply in the secondary loop. It seems that typically the secondary supply line, measured just inches from the closely spaced tees (i.e. where it hasn't had time to radiate) is much cooler than the supply side of the primary loop.
I can feel this difference when I touch the pipes.

Here's an example of what I'm seeing.

The flow of the primary loop in this example is 4 GPM and flow in the secondary loop is 3 GPM.

The Primary supply is a lot hotter than the secondary. And by the time the secondary return mixes back in to the primary loop, the primary return is almost as hot as the secondary supply. It's not uncommon for these two temperatures to be exactly the same.

Is this normal? What kind of temperature drop do folks generally see from primary to secondary?

Again, my system is working fine. The house is warm, the boiler isn't short-cycling; I'm interested in fine-tuning a system that is already working quite well.

Thanks.

 

[Fitter30]

Moving way to much water on both loops. 20*f is normal under full load. Boiler short cycle?

 

[Davie]

Moving way to much water on both loops. 20*f is normal under full load. Boiler short cycle?

The boiler isn't short cycling (currently 15-45 cycles per day).

Any thoughts as to how I can get a slower flow rate, if that's what you're saying I need?

What mode would you use on the Taco VT2218?

 

[John Gayewski]

Could changing the primary and secondary pump flow rates be a way to get more heat out to the zones? I'm confused as to why there always seems to be a temperature drop between the primary supply and the secondary supply.

There often seems to be a ~10F drop between the supply from the boiler and the supply in the secondary loop. It seems that typically the secondary supply line, measured just inches from the closely spaced tees (i.e. where it hasn't had time to radiate) is much cooler than the supply side of the primary loop.
I can feel this difference when I touch the pipes.

Here's an example of what I'm seeing.
View attachment 78781

The flow of the primary loop in this example is 4 GPM and flow in the secondary loop is 3 GPM.

The Primary supply is a lot hotter than the secondary. And by the time the secondary return mixes back in to the primary loop, the primary return is almost as hot as the secondary supply. It's not uncommon for these two temperatures to be exactly the same.

Is this normal? What kind of temperature drop do folks generally see from primary to secondary?

Again, my system is working fine. The house is warm, the boiler isn't short-cycling; I'm interested in fine-tuning a system that is already working quite well.

Thanks.

Your water is mixing in the primary loop. The cool return water from the zones is injected back into the primary loop. If this didn't happen your boiler would short cycle. I'm not convinced there's a problem here. There is some simple math one can use to figure out what temperature should be what across tees with thr corresponding flow rates.

It is very out of the norm to mix radiating types inside of a loop. Generally they are separated.

 

[John Gayewski]

I think the only way you could get more efficient would be if you could separate your radiators from your radiant flooring.

 

[Davie]

The cool return water from the zones is injected back into the primary loop. If this didn't happen your boiler would short cycle. I'm not convinced there's a problem here.

I agree that if the cool return water from the zones heading to the boiler is crucial. In the image above, the cool water returning from the zones is at 84F. The problem that I see is that the water returning to the boiler is hotter 88F. More to the pount, I'm not happy with the 9F split between primary and secondary supplies.

If there were no primary/secondary split, the water returning to the boiler would be closer to 84F, and the water heading into the zones would be closer to 99F.

 

[John Gayewski]

I agree that if the cool return water from the zones heading to the boiler is crucial. In the image above, the cool water returning from the zones is at 84F. The problem that I see is that the water returning to the boiler is hotter 88F. More to the pount, I'm not happy with the 9F split between primary and secondary supplies.

If there were no primary/secondary split, the water returning to the boiler would be closer to 84F, and the water heading into the zones would be closer to 99F.

It will be hotter returning to the boiler in almost any case. Your could turn the primary loop pump down to try to get them closer.

 

[John Gayewski]

Your boiler wants the coldest water possible returning to it. You'd be better off returning more cold water to it, but the faster you return it the warmer it'll be. That's why separating the emitter types can help you when you build a system. Mixing emitters in a loop doesn't give uppity much control.