Primary Loop - 1 1/4" vs 1"

SteamtoHotWater

I'd really like everyone's thoughts on this.
A Laars Combi Boiler (MFTCW140) calls for 1 1/4" pipes on the primary loop. The manual reads:

Use at least the MINIMUM pipe size for the entire boiler loop piping (connecting boiler to and from the primary/secondary connection). Use only primary/secondary piping as shown. Failure to follow these guidelines could result in system problems.
CH pipe minimum size : 1 1/4 ̋

And the diagram shows:

The inlet and outlet on the unit are 1". And I'm planning on using 1" pipe on the secondary.
In a separate thread hot_rod wrote

hot_rod said:

The 1-1/4 between boiler and hydro sep is overkill and the 1-1/4 sep is more $$

I'd much prefer to use 1" pipe. Not just for the cost savings, but also because I could use a manual press tool instead of sweating. I'm a big believer in following a manufacturer's instructions. But, I'm a bigger believer in real world experience - of which I have none. Is the manual wrong and hot_rod right?
Dunno if it's relevant, but I'm running eight loops, each ≈250', to two zones. Is there some sort of calculation I should be doing to determine if 1" pipe on the primary is sufficient. I want hot_rod to be correct. Anyone else care to chime in?

pecmsg

I’m sure there’s a reason they call for 1 1/4” primary. My guess is to maintain minimum GPM through the heat exchanger. 

Ironman

You’ve got a scenario which is not ideal: namely, your primary loop requires more flow than the total of your secondaries. A buffer tank is the best solution, but a hydro-sep will help.

A 1-1/4” line is good for about 14-16 gpm; a 1” is good for 8-10.
The connection size at the unit is not relevant to pipe sizing.

A 140k btu boiler needs a 1-1/4” line to maintain 14 gpm at a 20* delta T. However, your secondary is only gonna carry about 6 gpm if your loops are 1/2”.

Your boiler is more than twice the size needed for space heating because it’s sized for domestic output.

You could get by with 1” if you limit the firing rate for space heating to 80k or less.

SteamtoHotWater

pecmsg said:

I’m sure there’s a reason they call for 1 1/4” primary. My guess is to maintain minimum GPM through the heat exchanger. 

But if that were the case, would the inlet and outlet on the boiler itself also be 1 1/4"?

Boiler 1" <-> Primary 1 1/4" <--> Secondary 1"
To my layman's eye, that seems like you're restricting flow before and after the primary.

dko

er

SteamtoHotWater

Ironman said:

The connection size at the unit is not relevant to pipe sizing.

Got it.

Ironman said:

A 140k btu boiler needs a 1-1/4” line to maintain 14 gpm at a 20* delta T. However, your secondary is only gonna carry about 6 gpm if your loops are 1/2”. Your boiler is more than twice the size needed for space heating because it’s sized for domestic output. You could get by with 1” if you limit the firing rate for space heating to 80k or less.

Interesting…
I think I'd rarely go above 80K (never?) for space heating. Before I decided on a combi, I was looking at 80K boilers. Out of curiosity, would something catastrophic happen if I went, say, 100k on very cold day with 1" on the primary.
Given what you wrote, it seems better stick with 1" on primary to avoid the mismatch with the secondary.

pecmsg

How did you size this?

SteamtoHotWater

pecmsg said:

How did you size this?

That's a long story. Succinctly, consensus between a couple different HVAC pros. Supposedly, I need a boiler of around 54K BTUs (at most). Eighty was essentially the next size up, with turn down expected to keep it around what I need. Then I decided to go combi, so 140K.

hot_rod

Required flow thru the boiler is based on the delta t also. The manual may show an example at different delta t

Many if the boilers allow you to downrate the firing rate for heating loads also. That fire tube boiler probably has a low flow requirement if you plan on locking it down to 80 k or so

SteamtoHotWater

hot_rod said:

That fire tube boiler probably has a low flow requirement if you plan on locking it down to 80 k or so

Do does that mean stick with manual specified 1 1/4" or take a chance on the 1"?

MikeAmann

SteamtoHotWater said:

pecmsg said:

How did you size this?

That's a long story. Succinctly, consensus between a couple different HVAC pros. Supposedly, I need a boiler of around 54K BTUs (at most). Eighty was essentially the next size up, with turn down expected to keep it around what I need. Then I decided to go combi, so 140K.

To me, this sounds like you made a poor decision. I think your heat and hot water needs to be separate systems. You have a very large mismatch.

SteamtoHotWater

MikeAmann said:

To me, this sounds like you made a poor decision.

It would not be the first time.

MikeAmann said:

I think your heat and hot water needs to be separate systems. You have a very large mismatch.

Isn't that the whole point of Combi unit with a 10:1 turndown ratio - to deal with the mismatch?
Somebody's taking a shower, 140K BTUs to the DHW. But most of the time, the unit is churning out 30K BTUs for CH. Am I not understanding correctly?

Ironman

First, as I mentioned, you can run 1” if you limit the firing rate to 80k btus in space heating.

Second, the primary purpose of modulation is to match the firing as closely as possible to the actual load as outdoor temperature changes. It’s not to compensate for oversizing.

Your situation is the major drawback with a combi: in order to heat domestic instantaneously, the boiler is almost always oversized for space heating.

Unless it’s just recently been changed, the 140k btu model has a 5 to 1 turndown; the 199k is 10 to 1.

If you haven’t installed the boiler yet, I’d highly recommend that you go with a heating only boiler and an indirect tank. The combi may be cheaper up front, but it requires a lot more maintenance and service and has a shorter life expectancy.

Have you done a heat loss calculation?

Do you know what your domestic demand is?

SteamtoHotWater

Ironman said:

First, as I mentioned, you can run 1” if you limit the firing rate to 80k btus in space heating.

Good. Because I really want to use 1".

Ironman said:

Unless it’s just recently been changed, the 140k btu model has a 5 to 1 turndown; the 199k is 10 to 1.

You had me really worried. I've looked at so many boilers that I could have easily gotten confused. I double checked. Both are 10:1

Ironman said:

Second, the primary purpose of modulation is to match the firing as closely as possible to the actual load as outdoor temperature changes. It’s not to compensate for oversizing.

Sure. But in practice, potato/potatoh? As long as the boiler can turndown to adequately match the various heating loads and then fire up to match DHW draw I'm not seeing the conflict. At 10:1, that means 14K, 28K, 42K and 56K are my lowest settings. It seems highly unlikely that my domestic heating loads will fall outside that range.

Ironman said:

If you haven’t installed the boiler yet, I’d highly recommend that you go with a heating only boiler and an indirect tank. The combi may be cheaper up front, but it requires a lot more maintenance and service and has a shorter life expectancy.

I have not installed the boiler. But when I first started researching, the math on a combi vs condensing + indirect didn't really work in the latter's favor. Off the top of my head, the heat-only condensing boiler plus indirect tank route was 1.75x more expensive. And it seems doubtful I'll get 1.75x more life out of it. But on your advice, I'll revisit.

Ironman said:

Have you done a heat loss calculation?

I have. Twice. Once from a guy trying to sell me a boiler and another from a guy who was supposedly RESNET and BPI certified. They differed, so I split the difference.

Ironman said:

Do you know what your domestic demand is?

I do. The Laars 140 is more than sufficient.

Ironman

It’s not just life expectancy: the combi has a plate heat exchanger for the domestic; they scale up very quickly - especially with hard water. That requires descaling on regular basis. There’s a 3way diverter which switches between space and domestic heat. There’s also a modulating valve which tempers the domestic. Both of these have very high failure rates. Then there’s the repeated starting and stopping for domestic calls. It all takes its toll.

I’m in the field every day and see all this on a regular basis. We’ve reached the point where we don’t offer combi’s even if it means losing the job.

SummitMechanic

I would just pipe it in 1 1/4 for the boiler loop. Sweating the small amount of fitting vs pro pressing them as far as time goes is negligible, and the fittings themselves are cheaper. More btus= more better in this case. Your secondary piping wants to pull as many btus as it needs at any given time anyway, might as well make it happy and remove problems down the road.

hot_rod

I would install any/ all tankless with valves like these. It makes the delime process a quick easy task

The amount of scale build up is related to the hardness of your water, any idea of the hardness?

The indirect will see the same  scaling, just spread over a larger surface so it is less noticeable. Minerals come out if the water when it is heated, regardless of the heater design

with good flow through a plate hx they do self scour, but low faucet gpm does  them no good

SummitMechanic

Ironman said:

It’s not just life expectancy: the combi has a plate heat exchanger for the domestic; they scale up very quickly - especially with hard water. That requires descaling on regular basis. There’s a 3way diverter which switches between space and domestic heat. There’s also a modulating valve which tempers the domestic. Both of these have very high failure rates. Then there’s the repeated starting and stopping for domestic calls. It all takes its toll.

I’m in the field every day and see all this on a regular basis. We’ve reached the point where we don’t offer combi’s even if it means losing the job.

This is like buying a hummer because a blazer is a little bumpy on dirt roads. Scale and hard water should be reduced before they even make it to the boiler. Space for parts is the main reason why you go with a combi and they 100% can be installed correctly requiring the same amount of maintenance a traditional Boiler/side arm does. I mainly install the later, but there are situations where a combi is absolutely the best option.

SteamtoHotWater

SummitMechanic said:

Your secondary piping wants to pull as many btus as it needs at any given time anyway, might as well make it happy and remove problems down the road.

Could you elaborate on this? I just don't see any situation where I'll need more BTUs than a 1" primary can provide. I get that Laars specs the primary at 1 1/4" in case CH demand really is 140K, but that's definitely not my use case.

SummitMechanic said:

I would just pipe it in 1 1/4 for the boiler loop.

So far we have SummitMechanic and the Laars manual on the 1 1/4" side.
Ironman and hot_rod on the 1" side.

SummitMechanic

SteamtoHotWater said:

SummitMechanic said:

Your secondary piping wants to pull as many btus as it needs at any given time anyway, might as well make it happy and remove problems down the road.

Could you elaborate on this? I just don't see any situation where I'll need more BTUs than a 1" primary can provide. I get that Laars specs the primary at 1 1/4" in case CH demand really is 140K, but that's definitely not my use case.

My only elaboration is that 1: it will not hurt anything and 2: If for some unknown reason Laars has to get involved at some point for warranty reasons it could be a sticking point for them.

I guess i can also add the that the time/cost dynamic for 1" pro press and 1" 1/4 sweat might even land on the side of sweat for such a small section of your piping. Boiler efficiency and gas cost to heat the piping will not increase running costs with a 1" secondary piping because the amount of btu's carried vs the amount being produced will balance themselves out by the boilers actual fire rate.

And on an opinion basis, i would rather have my boiler loop make a bit more than i need instead of making the exact amount or in worse case scenario, less then i need.

The other upside would be the added available BTU's for additions to the central heat at some point or addition of snowmelt down the line. I would rather overbuild for a slight cost increase than save a bit of time and money for a headache down the road.

SteamtoHotWater

SummitMechanic said:

My only elaboration is that 1: it will not hurt anything

The way I'm reading what Ironman wrote, a 1 1/4", instead of a 1", primary actually could cause a problem. Am I misunderstanding?

Ironman said:

You’ve got a scenario which is not ideal: namely, your primary loop requires more flow than the total of your secondaries.
A 1-1/4” line is good for about 14-16 gpm; a 1” is good for 8-10.
A 140k btu boiler needs a 1-1/4” line to maintain 14 gpm at a 20* delta T. However, your secondary is only gonna carry about 6 gpm if your loops are 1/2”.

SummitMechanic said:

and 2: If for some unknown reason Laars has to get involved at some point for warranty reasons it could be a sticking point for them.

This is a very fair point.

EdTheHeaterMan

The SEP will negate any of the oversizing that @Ironman is referring to. Also a pipe that is larger than the minimum required has rarely been a problem in this industry. Just because you CAN pump 14 GPM thru a 1-1/4" pipe, does not mean that you WILL pump 14 GPM thru that pipe. A 1-1/4" pipe can allow less than 1 GPM flow with no problem. Ask anyone that has designed a gravity hot water boiler system.... Oops, they are probably all dead by now.

SummitMechanic

SteamtoHotWater said:

SummitMechanic said:

My only elaboration is that 1: it will not hurt anything

The way I'm reading what Ironman wrote, a 1 1/4", instead of a 1", primary actually could cause a problem. Am I misunderstanding?

Ironman said:

You’ve got a scenario which is not ideal: namely, your primary loop requires more flow than the total of your secondaries.
A 1-1/4” line is good for about 14-16 gpm; a 1” is good for 8-10.
A 140k btu boiler needs a 1-1/4” line to maintain 14 gpm at a 20* delta T. However, your secondary is only gonna carry about 6 gpm if your loops are 1/2”.

Your primary is going to flow based off of its own pump. The secondary is going to pull off of that a percentage of the btus depending on pipe size and pump flow rate. The boiler loop will maintain its set heat point and the secondary will pill off of that accordingly. Primary Flow rate will not effect secondary flow rate. Especially if it is piped 1 size larger as the primary piping will have the least amount of resistance and that is where the water is going to want to go anyway.

SteamtoHotWater

EdTheHeaterMan said:

The SEP will negate any of the oversizing that @Ironman is referring to. Also a pipe that is larger than the minimum required has rarely been a problem in this industry. Just because you CAN pump 14 GPM thru a 1-1/4" pipe, does not mean that you WILL pump 14 GPM thru that pipe.

Got it.
To be clear, you would favor the installing the 1 1/4" over the 1"?

EdTheHeaterMan

SteamtoHotWater said:

EdTheHeaterMan said:

The SEP will negate any of the oversizing that @Ironman is referring to. Also a pipe that is larger than the minimum required has rarely been a problem in this industry. Just because you CAN pump 14 GPM thru a 1-1/4" pipe, does not mean that you WILL pump 14 GPM thru that pipe.

Got it.
To be clear, you would favor the installing the 1 1/4" over the 1"?

I would use 1-1/4 to be sure on warranty issues.

I would use 1" in my own home because I know it will work.

So the score is 2.5 to 1-1/4" and 2.5 to 1"

SteamtoHotWater

EdTheHeaterMan said:

So the score is 2.5 to 1-1/4" and 2.5 to 1"

Well that makes my decision really easy ; )

hot_rod

you could also size the boiler pump so it cannot flow more than 10 gpm. Either with a pump selection or adding a balance valve

Ill bet the boiler has a way to lock it in to a max modulation rate, most do now It basically limits the fan inducer to a not to exceed rpm

so lower flow and  lower fire rate, protects the hx

yet in dhw the boiler can still high fire to provide maximum domestic production 

Any number if ways to make this a safe installation 

Or the additional $$ to go 1-1/4