What's my best option for cast iron boiler, primary/secondary piping?

AMservices

Weil-mclain gold, 4 sections, oil fired, 2 zones, cast radiators on first floor with original direct return piping, fin tube baseboard on second floor I'm piping with a reverse return.

What is my best options for protecting the boiler from cold return water temperatures and heat transfer to the system?

Do I pipe it the way weil-mclain has in the manual with the boiler as the secondary loop and bypass line (#7 on there drawing)?





Or do I do what Caleffi shows with the boiler as the primary loop?






I dont like weil-mclain's way, mostly because they don't even show where they want the expansion tank. I called W.M. technical support line, just because I like picking those manufacture, insider brains and asked why is there no expansion tank on your boiler drawing? " I talked to people and neither of them could tell me where they want it or why its not included in the drawing.

But then a third option would be a hydronic separator.






Is this the best option for boiler protection and rapid heat transfer?
Does the added cost of the separator make it that much better of a system compared to the other options?

EBEBRATT-Ed

They all have their advantages and disadvantages. I would go the Weil Mclain route. Bypassing the primary loop is a sure thing to prevent cold return water in the return. As far as the ex tank I would go where #4 is on the drawing so your pumping away. That puts the boiler on the main pumps suction so there is no possibility of blowing the relief valve

Just my opinion I am a primary secondary guy

AMservices

Thank you @EBEBRATT-Ed
In my minds eye, I see the boiler on the secondary loop as better protection from cold return temperatures, with the bypass as extra temperature control. When its piped as the primary loop, i see the system getting hot faster.
Ill go by the WM manual and your recommended placing of the expansion tank.

SuperJ

I would do a bypass like the WM drawing, but control the flow with a 3 way thermostatic valve on the return instead of just a two way ball valve. (assuming your pump is inside the bypass loop).

With a manual bypass the flow will be constant hot or cold, and you may find you hit your high temperature cutout prematurely once warmed up. It also doesn't provide the same level of protection on starts since it won't allow 100% bypass like the thermostatic valve would.

Another option would be to use some sort of aquastat type control to shut off your zone pumps until the boiler is hot. With no zone pumps running your primary loop will recirc until hot. The pumps might go on and off a couple times before stabilizing.

AMservices

The hydro stat that comes with the boiler can delay the boiler pump from running. Ill make sure thats set up.

Do you think a 3 way valve is necessary with primary/ secondary piping?
Or when using a 3 way valve, P/S isn't needed?

At the end of the day, the goal here is to protect the boiler from condensing and heat the system ASAP.

hot_rod

Any idea what proportion of the heat load is the cast rad loop? One rule of thumb would be the high mass zone should be 20% or less of the total load and return protection should not be needed, the boiler should be able to catch that load and be above condensing temperatures.
Although in my mind cast rads are not true high mass, like a large concrete slab:) 4000, lbs per yard of concrete, approx 110 square feet in a yard of concrete. Maybe CI rads are more of a medium mass.

Really none of the methods you show offer 100% return protection. The ball valve bypass is a guess at best. Under what condition would you adjust it?

A temperature sensing devise is absolute protection. A 3 way thermostatic as @ SuperJ mentioned.
\
That pump aqua stat is another, less eloquent way to protect the boiler. I call it the bang bang method, although Viessmann had, maybe still does that provision on their basic cast boiler.
A thermostatic device is much smoother, like slipping the clutch, compared to turning the ignition on and off, bang bang

If you system is running above 140SWT, why bother with P/S or a sep? Just pipe it with a 3 way at the return. PS and seps will always be mixing your SWT, even if you do not want it. P/S and Seps are more for the hydraulic separation function.

Idronics 19 goes into a number of way to protect boilers.

AMservices

So i should save my tee's on this one and just get a 3 way valve is what your saying?

Is there a model valve that has been proven to be reliable over time?
Electronic better then thermostatic?

SuperJ

AMservices said:

So i should save my tee's on this one and just get a 3 way valve is what your saying? Is there a model valve that has been proven to be reliable over time? Electronic better then thermostatic?

I like thermostatic for the simple elegance.  No wires.  You can get them in a variety of temperatures.  Keep in mind they have a throttling range so you will likely select one that is rated a bit below 140f so that at 140f and above it is no longer recirculating. You also want to make sure it is designed for the purpose with a low pressure drop.  DHW tempering valves have too much pressure drop at the flows you will running at. Caleffi and Danfoss make thermostatic valves.  Taco makes the highly regarded I series electronic valve. If you like to tinker you might like the Taco since it has some bells and whistles. If you’re a set it and forget it guy, go with thermostatic since it’s a bit simpler. 

As hot rod said the smooth seemless opening as things warm up is a nice feature. 

AMservices

So i went with the P/S piping weil mclain shows in the manual.
honestly, I didn't like any of the piping diagrams I found for the mixing valves. They were all showing the mixing valve being used for mixing the zone, not the boiler.
Piping in a mixing valve wouldn't have save me anything on time and it would have cost more in materials. And it throws another part in there I have to worry about breaking, sticking or someone else fiddling with. 
P/S is easier, with less to worry about.
Even though the P/S piping is always going to be mixing, Its not hurting the boiler or heat distribution. 
I put the bypass line in but with the boiler pump delayed, the boiler pump doesn't run if the water in the boiler i less then 140°F. 

Here's the before pic's

Tinman

Siggy teaches that if you can’t sense and react to low return water temperatures, you probably won’t avoid sustained flue gas condensation. That makes sense to me. 

EBEBRATT-Ed

as long as the boiler pump moves more water than the zone pumps your fine

The Steam Whisperer

I use to think as Siggy thinks, but after seeing and understanding how by pass piping works, I have yet to see it sustained flue gas condensation.

On a cold start, there will be very little heat transfered to the main loop because there is almost no temperature differential between the boiler water and the system water. Little by little the heat transfer will increase as the boiler water gets hotter and hotter relative to the main loop. With this reduced heat transfer, the boiler comes up to temperature about as quick as it does in a baseboard fin tube system.

Upon shut down, the boiler pump should be shut off. This allows the boiler to heat soak, so if it was running a bit cool at the return, the condensation is very quickly baked off. This may occur in very warm weather, much like it does with baseboard systems.

I used to install thermostatic valves or variable speed injection for boiler protection on all high temp boilers used in gravity conversions, but when I began to understand how the bypass piping inherently limits heat transfer to the main loop until after the boiler comes up to temp. I stopped. The old guys knew what they were doing when they recommended boiler bypass pipings.... and it still works.

hot_rod

The 140 control on the boiler is the return temperature protection mechanism. Since it is sensing and responding to temperature it is providing the protection you need
I’m not seeing the value of the primary secondary piping? I think it may force you to run the boiler hotter to get adequate SWT. With a wide open sectional cast boiler you shouldn’t need hydraulic separation. A single circ sized  to mimic the gravity flows may have been adequate.

Tinman

The Steam Whisperer said:

I use to think as Siggy thinks, but after seeing and understanding how by pass piping works, I have yet to see it sustained flue gas condensation. On a cold start, there will be very little heat transfered to the main loop because there is almost no temperature differential between the boiler water and the system water. Little by little the heat transfer will increase as the boiler water gets hotter and hotter relative to the main loop. With this reduced heat transfer, the boiler comes up to temperature about as quick as it does in a baseboard fin tube system. Upon shut down, the boiler pump should be shut off. This allows the boiler to heat soak, so if it was running a bit cool at the return, the condensation is very quickly baked off. This may occur in very warm weather, much like it does with baseboard systems. I used to install thermostatic valves or variable speed injection for boiler protection on all high temp boilers used in gravity conversions, but when I began to understand how the bypass piping inherently limits heat transfer to the main loop until after the boiler comes up to temp. I stopped. The old guys knew what they were doing when they recommended boiler bypass pipings.... and it still works.

And if the conventional boiler with the bypass line and manually operated valve also incorporates ODR, how does that valve move to accommodate changing SWTs? OP asked for best option. 

The Steam Whisperer

Your ODR is limited to when the boiler is operating at its minimum supply temp....On most systems with bypass piping that would probably be about 110 to 120 F system supply temp, maybe lower. With single zone high mass converted radiator systems, ODR provides little benefit since the system mass automatically limits the system temperature...the boiler shuts off on the thermostat before the system gets very hot. With the two zone system he has with low mass convection and high mass radiators, that is a little more complicated, so ODR may be some benefit to limit the temps to the convectors. The bypass piping would still probably allow system supply temps down about 110 to 120 F (depending on mix ratios) , which would cover probably about 60 % of the heating season with full reset( assuming 190F supply as design). In my climate (Chicago) ODR with this set up will bottom out in the range of 30 to 40F, providing the improved comfort and energy savings for the low mass convector part of the system for most typical winter weather.

Zman

Why wouldn't you pipe it primary/secondary with no bipass and then use a smart mixing valve on the cast iron loop. You could set up that loop with outdoor reset and have it monitor the boiler return and throttle back the mix as needed to keep the boiler from condensing.

Tinman

Taco iSeries Reset Valve. That’s exactly what I would do. 

The Steam Whisperer

If you're going to add additional equipment, I would set up the mixer with ODR for the high temp need, and then install a manual bypass loop on the low temp zone so it will reset in parallell with the high temp circuit, but just at a lower temperature curve. Used this layout many times for 3 or 4 temp systems with a single ODR control.

Tinman

There’s more than one way to approach a job, Dave. Most recommend options that have been successful for themselves. 

AMservices

Zman said:

Why wouldn't you pipe it primary/secondary with no bipass and then use a smart mixing valve on the cast iron loop. You could set up that loop with outdoor reset and have it monitor the boiler return and throttle back the mix as needed to keep the boiler from condensing.

Good question. 
Although I am familiar with ODR controls, I would rather keep things simple when I can.
I know my customer and I know they like there rads HOT. 
Last year I installed some Runtal's in 2 rooms where they removed the cast rads. The week after i put them in, I get a progress report say "the runtal's work but only when the boilers running. Its like the heat in those rooms stop as fast as the boiler shuts off". 
This wasn't a high end job that had a budget for extra T&M on mixing loops and adjusting heat curves. 
The customers main concern was not filling there oil tank every month though the winter.
My main concern was protecting the boiler from condensing. 

I see the value in having a mixing valve steadily controlling the bypass line and if anyone has an example, ill consider it next time. 

I also see the value in bypass piping because it works and its simple without to much extra T&M. 

hot_rod

Yes, those low mass panel rads will stop delivering warmth fairly quickly compared to cast iron rads. It is a tough mix sometimes, having both high and low mass on one zone, or even one building.
So either supply the CI rads with 180, and the panel rads on an ODR as @Zman mentioned. The ODR will attempt to maintain a near constant circulation so you should not notice so much on/ off on the low mass panels.

Yes it will take more piping and control, it comes down to what the customer expects and the $$ they will spend to get there.

The heat emitters are in charge of the boiler operating condition, not the opposite. So in some cases IF the boiler operates at low return temperatures, for more than 10n minutes then a protection method should be included.
To know that, either a careful heat load and assessment of radiation, or run the system and observe, which it sounds like you have done .

When possible run all the distribution at the lowest possible temperature. If the cast rads are generously sized, they may provide enough heat at lower temperatures, maybe even the same temperature as the panels, around 140F. One temperature operation simplifies piping and $$, of course.
It's true for all boilers, the lower the operating temperature the more efficient, and possibly less on/ off cycling.

We wrote this Idronics specifically to show how to design or calculate systems to run at lower SWT, when possible. This helps leverage heat pumps and mod con conversions.

https://www.caleffi.com/sites/default/files/file/idronics_25_na.pdf

Zman

I have had a couple customers complain about radiators not feeling hot with ODR. Even though the room was maintaining temp, they requested that the water temp be turned up.
Go figure...

mattmia2

I replaced an electric baseboard that heated my bedroom with a runtal that heats just the bedroom and I think that stays warm forever compared to what the forced air or baseboard did...

The Steam Whisperer

A simple solution to the variable thermal mass in emitters is to run the circulator continuously. This pulls the warm water from the high mass emitters and keep the low mass emitters at the same temperature. No ODR reset required. However, if using bypass piping, running the circ continuously will probably cause flu gas condensation, so you may need two separate loops, one for boiler with bypass pumping where the pump shuts off with the burner and a secondary loop where the pump runs all the time. When the boiler circ shuts off it stop injecting heat into the main loop and allows the boiler to heat soak to keep it dry.

SuperJ

Zman said:

I have had a couple customers complain about radiators not feeling hot with ODR. Even though the room was maintaining temp, they requested that the water temp be turned up.
Go figure...

People define comfort many ways :-)
Some need to read a special number they like on the thermostat screen, some need to feel the radiator blazing away, and just are happy as long as they don't need a sweater inside and aren't sweating.

Personally I think cast iron boilers should have a active means of low RWT protection.
This could be as crude as a pump interlock, or preferable some sort of modulating valve, or pump. I like the valves since they don't rely on more complicated control wiring, and are modulating, they just need to be piped and pumped properly.
Primary secondary loops give you a great spot to cut in a thermostatic mixing valve on the return.
A uncontrolled bypass in the primary loop could lead to excessive primary loop temperatures once heated up if too much flow is bypassing, or inadequate boiler protection if too throttled, hence the elegance of the thermostatic bypass.

jumper

How big would a buffer tank need to be to avoid thermal shock?
One minute,ten minutes,...?

hot_rod

The buffer tank really doesn't change then concern with low RWT, and could in fact make it worse as it is a big thermal mass.
However the same valves or protection methods apply.
Need to know that return temperature and be able to react accordingly.

Actually a buffer tank is one application where a properly applied delta T circulator has a good place.

Here are some examples of the right and wrong way to handle return temperature with ∆T circs.

AMservices

@hot_rod

This drawing is very close to my second option I was asking about at the start of the thread. With the exception of the variable speed circulator monitoring boiler inlet temperature.


Using a variable circulator with this P/S piping makes the most sense to me.
Its simple, does the job, don't have to make the piping look like road map of New England.
I trust a pump WAY more then a mixing valve.

hot_rod

Really your choice between a valve or circulator. Either method allows you to "slip the clutch" and have a smooth and accurate protection scheme. The circ will of course require some electrical energy. A high Cv thermostatic valve, very little additional head.

Triggering a circulator on and off with an aquastat would be like controlling the speed of your truck with the ignition switch being toggled on and off, not a very desirable method in my mind.

As for a ∆T circulator as a distribution pump in those drawings, really you are just "pulsing" energy into the distribution, injection mixing sort of.

As long as you understand the pros and cons of constrained ∆T operation. The V/S circ needs to be able to disconnect the load entirely, especially on high mass, slabs, cast rads, etc.