Pumps/ Flow/ Boiler Temp. What is the goal?

TAG

I'm still confused when it comes to pump choice and settings with Mod Con primary loop and indirect water heater loop. Alpha and typical multi speed pumps.

Is there an ideal operation window?

I brought this up a few years ago but I'm still confused. Covid had me saying my home building/ rebuilding days were over — but here we go again. I'm about to start another project.

My last project used a Vitodens 200 w/ a Viessmann LLH and 300 indirect. Out from the LLH is an Alpha 2 on auto and it seems to be the perfect fit to the 4 manifolds feeding the radiant.

What confuses me is the boiler loop. Playing with the Alpha pump settings on the boiler loop. More flow drops the boiler water temp. What is the goal? More flow w/ lower boiler temp or less flow and higher temp? More flow uses a bit more electric obviously … does it matter? I most likely don't need/ want the Alpha 2 in the boiler loop. Have played with it on Auto (assuming that's not ideal) … but, I don't understand if it should be on flow or speed setting. Is not some constant flow best?

The same question with the indirect water tank … don't I want the highest setting on the simple 3 speed pump? Is not more flow going to give me faster recovery ?

One last question. What is the ideal flow through radiant tubing? Mine all seem to be around 1.5L min using the flow meter on the cross manifolds.

I never really thought about this because for years you just used simple one speed pumps.

The new project is not as big so not as many manifolds/ loops but I'm thinking same equipment w/ propane

GroundUp

Yes.

TAG

https://forum.heatinghelp.com/discussion/comment/1836231#Comment_1836231

My power went out as I was typing … the body of the post was lost

Jamie Hall

It's really all about design tradeoffs. There are some constraints — you don't want the velocity of the water in the pipes too high, for instance, as that is noisy and uses more pump power than needed (your flow rate, by the way, looks about right). If you are dealing with a condensing boiler, you want the return temperature as low as possible — implying a lower flow rate — but on the other hand you need the temperature of your emitters to be high enough throughout to deliver the amount of heat needed with that size emitter.

The temperature of the return to the boiler can't be lower than the lowest temperature of the returns from the secondary loops. The temperature of the supply from the boiler, however, can be as high as you like, if you are working primary/secondary with tempering mixing valves. The amount of heat you can transfer out of the boiler with the flow is set by the delta T across the boiler and the flow — that good old hydronic equation!

For an indirect, you really want the boiler side hot, so as to get decent heat transfer across the heat exchanger and good recovery. Many systems using outdoor reset and modulating burners will ramp up the output temperature to get good recovery when the indirect is calling.

TAG

I'm only using the main ODR program curve on the 200. The boiler is doing the magic w/ outdoor and LLH sensors. Single Alpha pump is clearly providing enough flow out to the 35+ loops although I don't know if there is a "best" flow to shoot for. I'm not going to be at that house this week to see system performance w/ really cold temps coming.

I'm surprised that Viessmann does not have a pump/ flow recommendation for the 300 indirect mated to a Viessmann boiler … or make it easy to understand what flow they want through the 200 boiler's primary loop.

The 200 does ramp up to heat the indirect

hot_rod

The higher the flow rate the higher the heat transfer rate. Regardless of the boiler or heat emitters.

But too fast can lead to boiler short cycling.

Faster flows more pressure drop, bigger circs needed.

A good guideline for hydronic fluid flow rates is the velocity, in FPS feet per second. 4- 5 fps for hot water hydronics.

Above 5 fps you start to hear velocity noise, more potential for pipe and tube erosion.

This chart helps see the velocity to flow rate. Use the 4 fps column. Unless you piped with larger than 6"?? :)

Jam as much gpm to the indirect tank as you can. If you want fast recovery. It's a short run time so you could exceed the 4 fps rule of thumb.

I agree the Alpha on the boiler loop is over-kill. But is does save some electricity.

Fixed speed is fine, unless you have a boiler that can drive and vary the boiler circulator speed.

Screenshot 2025-01-19 at 1.53.22 PM.png

Kaos

Heating an indirect is never efficient anyways and since you generally want to get quick recovery, so more flow there.

With modcons and heat pumps, delta T is your friend, so you want the lowest flow you can get away with while still delivering enough space heat.

With some emitters if you turn the flow down too much, you get uneven heating (ie series baseboards) so there you are limited by comfort constrains. For floor heat 20F seems is a good starting point, much higher than that you tend to get some striping.

Primary flow should match emitter flow at full load. This means the delta T on both is about the same which is you the lowest return water temperature, thus most condensation.

LRCCBJ

https://forum.heatinghelp.com/discussion/comment/1836271#Comment_1836271

I will explain what to shoot for:

Very few installers understand this and it is not explicitly explained anywhere.

The Alpha pump defines the flow rate and the DT for the secondary. Whatever temperature water returns to the T's………..you want to go directly to the boiler. Simple………correct?

But, that can only happen if the primary flow is equal to or less than the secondary flow. If the primary flow is HIGHER than the secondary, the water from the primary will just turn around and head back into the boiler thereby increasing the RWT to the boiler and killing the efficiency somewhat.

So, your job, if you decide to accept it, is to carefully measure the RWT from the secondary flow and compare it to the RWT to the primary. If the primary is higher………….throttle the primary until they match.

Of course, this only works if you DO NOT have a variable speed pump on the secondary. In that case, there is only equilibrium at one single flow rate. Both the secondary and primary would need variable speed pumps and there is no assurance you can get them to behave together under all conditions.

Regarding your indirect question…………the highest speed on the pump transfers the maximum amount of energy in the shortest time. DONE. Nothing more to discuss on that.

TAG

The Viessmann 200 boiler does not have an internal circulator. Just the primary loop pump into the LLH …. in my case an Alpha 2. When I was ordering parts for the system I just ordered two Alpha pumps for the primary secondary and one of the 3 speed 15-58 Grundfos pumps for the indirect.

Looks like I should have the indirect 3 speed pump on high … I think that is where I do have it set.

Did they stop making the Alpha 2 … don't see it online at supply house.

looks like I used 1" copper for the primary and 1 1/4 for the secondary — but the boiler internals are smaller. I still don't understand what is ideal … I know when I put it on auto it settles around 3 or 4 GPM. From Hot rods graph … looks like 3 GPM is ideal for 3/4" pipe and that's what the boiler looks to have

LRCCBJ

https://forum.heatinghelp.com/discussion/comment/1836294#Comment_1836294

It doesn't matter which pumps you utilized on the primary and the secondary within reason. Be assured that the headloss for the primary will NOT match the secondary. Nobody usually cares about this but, if you want the most efficient system, the flow rates need to match. You don't want to screw around with different pumps………..if the flow rates are close………..just install a butterfly valve on the one with the higher flow rate (the PRIMARY) and get them to match. I explained how to do that above…………….

The boiler looks to have sufficient flow as defined by the manufacturer………………it's output is determined by the modulation in most cases. If you failed to provide it with sufficient flow it will just modulate down to a flow rate that it can accommodate. This rarely happens because the flow rate on the primary is usually much more than required to heat the building and the boiler is running at 50-60% or less.

The mod-cons do not have any "magical" flow rate provided is is sufficient for the output that you need.

The Alpha 2 can offer 8g/hr at 15' head at it's maximum output. But, you don't know your resistance in the primary loop so the Alpha might be flowing much more than that. You obviously must run the Alpha 2 at a fixed speed to match the Alpha 2 on the secondary.

hot_rod

Your boiler manual will generally tell you the circulator requirements, or actual model numbers.

That boiler, if it is a coil typeHX, probably has the large diameter coils, the INOX style? If so the Alpha may be enough circulator.

The smaller tube boilers also shown need higher head circs.

I think the Alpha 15-58 is the latest version or the Alpha

Screenshot 2025-01-19 at 3.13.25 PM.png

LRCCBJ

Your boiler manual will generally tell you the circulator requirements, or actual model numbers.

Sure it will.

It will provide the circulator necessary for the MAXIMUM OUTPUT of the boiler. If you have a 150K mod-con, it's going to specify a circulator that can deliver 14 g/minute at whatever headloss is in the HX plus a safety for the piping.

Now, consider that this mod-con is installed in dwelling with a HL of 60K on the design day.

The secondary piping is typical and returns a DT……….of……….say 15.

Now consider the disparity between the primary and secondary flow rates.

You're sending and estimated 50% of the primary supply back to the boiler.

This makes absolutely NO SENSE!!

Somebody once said that the radiation governs the boiler…………..they were correct.

TAG

LRCCBJ: Thought the reason for the LLH was to eliminate the pump differential problem ?

Hot Rod: I did a quick scan of the manuals and did not see anything … in typical German style they give you a library of information.

That new Alpha is at least another $100 bucks more. They also sold an Alpha w/o the "auto" for about what the 3 speed goes for now.

LRCCBJ

Your boiler manual will generally tell you the circulator requirements, or actual model numbers.

Think about this for a moment.

The pump specified by the mfr. will do 14 GPM.

The system, in April, has a requirement of 15,000 BTU. The DT on the secondary is down to about 7° with a constant speed pump.

The secondary flow rate is about………..say 5 GPM……….on 3/4…………which satisfies most 2500 square foot dwellings.

Now, think about the DT on the primary under this condition.

What happens?

The boiler shuts down on limit!! Guaranteed.

LRCCBJ

LRCCBJ: Thought the reason for the LLH was to eliminate the pump differential problem ?

Sure it eliminates the pump differential problem FOR THE MANUFACTURER. He wants the maximum flow rate necessary for the full output of the mod-con.

You want a flow rate that MATCHES your radiation requirements.

You have no need for a flow rate of 14 G/min if your building requires 60K at design. You need 6 G/Min on the primary……….not more.

Of course, if you had a 50K mod-con, we wouldn't need this discussion………….the primary requirement would only be 5 GPM and the secondary would be close.

But, nobody has a mod-con that matches their radiation.……………………..

It is an interesting discussion because the manufacturers demand P/S piping to fulfill their maximum need……….say 14 GPM……….which the secondary is unlikely to deliver. But, in reality, nobody needs the full output of the mod-con and in MOST cases you can pipe it direct unless you have a multitude of small zones that result in a flow rate below the minimum for the mod-con.

The manufacturers are simply protecting their asses to prevent a low flow situation in the mod-con which can damage it in short order (because it cannot modulate fast enough on zone closures).

hot_rod

 But, in reality, nobody needs the full output of the mod-con and in MOST cases you can pipe it direct unless you have a multitude of small zones that result in a flow rate below the minimum for the mod-con.

Then isn't the boiler oversized? Limit the firing rate, lock the fan speed so it cannot over-fire.

That's the beauty of the seven fan speeds in the controls. And lower the flow?

We've sold hydronics on the beauty and selling feature of unlimited zoning. Now you want us to single zone so we can direct pipe a boiler that was meant to modulate and cover a wide range of small and varying loads. Sounds like one step forward, two steps back.

Sure we can pump control our way out of this. At what cost? All the manufacturers have released pricepoint, low feature mod cons to stay in the game. Now you want them to add 500 bucks worth of smart circs?

LRCCBJ

Then isn't the boiler oversized? Limit the firing rate, lock the fan speed so it cannot over-fire.

That's the beauty of the seven fan speeds in the controls. And lower the flow?

Just about every mod-con is oversized. That's what people do.

Sure, go ahead and limit the firing rate.

And, lower the flow…………absolutely. Without lowering the flow you end up with the situation that I described above.

I don't want you to do anything. And I never suggested you'd need to run a single zone. Just realize that the current design as given by the manufacturers is woefully wrong for most systems.

I told you how to get out of the problem with a butterfly valve if the primary circ is too large.

I don't believe you fully understand the problem.

hot_rod

I see the mod cons as way beyond what a fixed output cast, steel or copper tube boiler has to offer? Efficiency, modulation, a large amount of control adjustments, venting options, weight physical size. Possibly hundreds of brands, models and sizes. If you can’t find a 10-1 turndown boiler close to what you need, what exact are you asking for. I doubt a boiler manufacturer will offer boilers in 5,000 btu/hr steps.

Jumping across brands I find 100, 110, 120, 125, 133, 142, 150K models out there. Turndowns in the 8,000 btu/ hr range. I think my Knight had 7 steps in the burner firing range, it can be locked at any one of the in a time period or permanently. My circ modulates with the firing, factory built solution, not an aftermarket workaround.

As long as the load remains dynamic, so should the heat provider.

Is it the perfect machine? No but what are you offering as an affordable mass market option?

I understand your concerns. If the solution is so simple, why can’t you get the manufacturers to add the butterfly fix?

Build the prototype, show it to the industry.

LRCCBJ

I'm not quite understanding your ongoing defense of mod-cons.

I explained why the primary flow rate is totally wrong for most P/S installations and how the DT on the primary can basically evaporate under many conditions.

You respond by suggesting that I go and design a new one or otherwise teach the manufacturers something they already know and couldn't care less.

Many of them provide a mod-con that cannot come close to their claimed output without a DT of 40 on the primary. I've explained this to you as well and how it cannot possibly work for any typical secondary system, and it was basically ignored.

They have done a woefully inadequate job of matching the primary flow rate to the load and very few installers or owners understand anything about it.

You can continue to defend them and otherwise ignore the problem(s).

I'm done.

hot_rod

https://forum.heatinghelp.com/discussion/comment/1836364#Comment_1836364

gripes and no workable solution, how is that “Heatinghelp”

I’m not defending mod cons, I’m embracing them and the voyage getting from MZ to where we are today. Looking forward to the future.

Same with heat pumps.

LRCCBJ

gripes and no workable solution, how is that “Heatinghelp”

Sorry but I cannot help it.

I did provide a detailed explanation of the problem (which does not translate into a "gripe") and a perfectly workable, and inexpensive solution as stated above. Please read it.

Kaos

One simple setting change you can do is limit the space heat firing rate of your boiler to your actual max load.

If you get your fuel use for the past season in therms and you multiply it by 42 to get your rough space heat load. Why 42? Besides being the answer to life universe and everything, that math works out to about that when do a more detialed fuel use calc.

From there you now have a max boiler firing rate, so you can adjust your primary loop pump to get the correct flow through it at least on your design day. No need to run the pump at higher flows if you don't need the extra heat.

LRCCBJ

@Kaos

From there you now have a max boiler firing rate, so you can adjust your primary loop pump to get the correct flow through it at least on your design day. No need to run the pump at higher flows if you don't need the extra heat.

It's worse than "no need".

When the primary pump is moving 2X the secondary (which would be the typical situation with a big mod-con and the use of the suggested pump),, 1/3 of its flow goes directly back to the boiler reducing the DT significantly. Boiler's response is to modulate down.

The two flows don't need to be perfectly matched but it is a benefit to get the primary flow down to a reasonable level.

GGross

https://forum.heatinghelp.com/discussion/comment/1836308#Comment_1836308

The viessmann manual actually will show a chart with pressure drop and flow rates at various different point. I have never known Viessmann to just spec a specific pump

TAG

I'm not sure how you can match the flows?

The majority of my projects over the years have needed high temp water someplace …. most were rehabs of old structures where the heat load was greater than what radiant floors could provide everywhere or there was just no way to do the floors w/o too much demo. I always used high mass natural gas Buderus boilers w/ ODR… set them up using the old "pumping away" piping module as in Dan's book in the early 90's w/ continuous circulation. The systems always worked to my amazement due to the old cast radiators being oversize and I could size the new panel radiators to the space. The floors would be on a second ODR from the boiler control using a 4 way valve and another pump.

Those old boilers are NLA. That is why I'm now using Mod cons

When using a Mod Con It was my understanding that the whole point of putting in a low loss header was to "fix" that mismatch in the flows and eliminate the problems possible with "T" systems. With a LLH the boiler has it flow. LLH allows the boiler pump to give the boiler what it needs through the heat exchanger. Same with the secondary. he secondary pump provides the flow out to the system to provide what it wants. Am I missing something ? The LLH provides the separation.

With the Viessmann having a sensor on the LLH … I'm assuming this gives the boiler some additional temp point to work off of outside of the curve set and the outdoor temp. But — the boiler is always adjusting it's output based on need. This project was spray foamed and it's using the smallest Viessmann boiler they made a few years ago … think around 60k.

Anyway … I'm still confused about the proper flow of the primary through the boiler. Maybe I'm wrong about the variable flow ? It seems that the primary through the boiler is commonly set up with a static "speed" based pump rather that some form of smart pump using flow or an Alpha that can adjust. Since the boiler is adjusting it's output ..it seems just a static flow is what is needed. But — what flow? Do you want a higher or lower boiler temp .. lowering the flow will make the boiler hotter since it needs hotter water water w/ lower flow going out to the LLH

LRCCBJ

@TAG

Anyway … I'm still confused about the proper flow of the primary through the boiler. Maybe I'm wrong about the variable flow ? It seems that the primary through the boiler is commonly set up with a static "speed" based pump rather that some form of smart pump using flow or an Alpha that can adjust. Since the boiler is adjusting it's output ..it seems just a static flow is what is needed. But — what flow? Do you want a higher or lower boiler temp .. lowering the flow will make the boiler hotter since it needs hotter water water w/ lower flow going out to the LLH

The primary is typically setup with a flow rate that will allow a specific DT at full output. Ideally that flow rate would result in a DT of 20°F. However, certain manufacturers cannot get that flow rate through the boiler. If they have a maximum output of 150K, they would presumably need 15 GPM to do it. But, the boiler would need a massive pump to manage that due to the resistance within the boiler. So, they allow a smaller pump that cannot do 15 GPM and live with a higher DT. Let's say that the pump can do 10 GPM and the DT at maximum output is 30.

In theory none of this matters since the closely spaced tees allow the secondary to flow at a much lower flow rate…………..typically 5 GPM for most zones that need less than 50K at design.

Now, let's see what happens in the tees with 10 GPM from the boiler and 5 GPM from the system. Since everytihing that goes into a Tee must come out of the tee…………….5 GPM from the boiler turns right back around and goes into the return to the boiler. 5 GPM from the system goes into the return to the boiler. Right back to the original 10 GPM that was supplied by the boiler.

The boiler receives 5 gallons at the original boiler supply temperature and five gallons at the secondary return temperature. What happened to the DT on the primary? It dropped from the original 30 down to 15. Nothing wrong with that because you are looking at the system at maximum output. It all works.

But, now look at the system flowing the same 10 GPM on the primary with the mod-con running at 50K. Now, the DT is down to 10. What happens in the T's……………….the return to the boiler receives the same 5 GPM from the boiler supply and the same 5 GPM from the system. The DT is now 5. We're starting to get in problems with most mod-cons at this point as 5 is difficult to manage and the boiler will likely modulate down further and might even shutdown on limit because it doesn't see the load..

Now look at the shoulder season when the mod-con is delivering 25K. Primary is still flowing 10 GPM. DT is now down to 5. What happens in the T's…………….the return to the boiler receives the same 5 GPM from the boiler supply and the same 5 GPM from the system. The DT is now 2.5. The mod-con won't run at that level.

So, in conclusion, the flow rate for the primary pump should not be more than whatever the secondary pump can deliver.

Even in this situation, the DT drops quite a bit in the shoulder seasons when the mod-con might have an output of 20% of the radiation requirements at design.

Of course, the above is a bit simplistic since the secondary pump is frequently a DT or DP pump and has varying flow rates depending on the DT or pressure that the pump sees. In such a situation is would only be possible to match flow rates at one specific operating point.

Additionally, the above assumes the secondary DT is constant. Of course this is not the case as the boiler SWT drops with increasing outdoor temperature.