Heating with 2 boilers

[Sumboodie]

What's the best way to setup the plumbing for 2 boilers?

Natural gas wall hung "combi" boiler and a cast iron waste oil boiler.

The combi unit is existing, and I'm installing the cast iron boiler.

I'll use the waste oil as "primary" as oil is free to me. If that isn't enough to make the heat or it shuts down, then the natural gas until would heat. Also natural gas unit heats the shower water. It's such low use it's not worth adding in a "boilermate" for DHW to the cast iron boiler.


The cast iron will need a primary/secondary loop and mixing valve.

Thinking to keep it simple and the hot water will just flow through both boilers at all times. I'll valve it so I can take out of the the heating loop if needed. It's in a conditioned space, so not really worried about heat lost from the added water flowing through the boiler that isn't heating.

 

[Sumboodie]

I can't edit, but I changed my post a bit to better explain.

What's the best way to setup the plumbing for 2 boilers?

Reading up and it's not uncommon in large buildings to have a system with 2, or more boilers and more come on line at the heat demand ramps up.

My setup is a little bit different. I would just have one running at a time and they are fueled differently.

One is a natural gas wall hung "combi" boiler, made by Versa (VCT-80_ and a Burnham "old school" cast iron boiler diesel (fuel oil) that I'm converting over to waste oil.

The combi unit is existing, and I'm installing the cast iron boiler.

I'll use the waste oil as "primary" as oil is free to me. If that isn't enough to make the heat or it happens to shut down, then the natural gas until would heat. Also natural gas unit heats the shower water. It's such low use it's not worth adding in a "boilermate" for DHW to the cast iron boiler.

Let's say have the oil boiler thermostat set to 70* and the natural gas one at 65* type thing.


The cast iron will need a primary/secondary loop and mixing valve.

Thinking to keep it simple and the hot water will just flow through both boilers at all times. I'll valve it so I can take either out of the the heating loop if needed. It's in a conditioned space, so not really worried about heat lost from the added water flowing through the boiler that isn't heating.

Other option is zone valves that would only open when a thermostat is calling for heat, but not sure it's really needed.

I tried picking the brains of a few HVAC guys, even asked for quotes and I'm just getting "you have natural gas, why even **** with oil??"


Natural gas for the last owner cost about $2000 a year to heat. I get all the oil I can want for free from work. They PAY to get rid of it right now. I'll have about $2000 into the oil heating system all said and done, so a decent pay off I'd say.

 

[Bondo]

Ayuh,...... I'd plumb it up exactly like I built my ole homemade outdoor wood boiler,......

The house I lived in was already an oil boiler/ hydronic baseboard heat, with a domestic water heatin' coil in it,....

I hooked up my out to the wood boiler line to the manifold at the bottom of the oil boiler, 'n "T"ed in the hot water from the wood boiler to the top piping comin' outa the top of the oil boiler,....

When a zone called for heat, the original circulatin' pumps kicked on,..... all original to the house,....

Out on my wood boiler, I'd built it pressurized, same as in the house,... I had 2 circulatin' pumps on it,....
One would just take the hottest water off the top(190*), 'n push it back into the bottom, mixin' it all up,....
The other pump, would kick on at 180*((aqua-stat was at the top of the wood boiler), 'n pull water from the bottom of the oil boiler, or from the house if the zone pumps were runnin', pushin' it into the bottom of the wood boiler, which in-turn pushes the Hot water into the top of the oil boiler or into the house if the zones were runnin',.....
Best part was, the oil boiler's auto-fill, filled the Whole system,....

Easy Peasy,.... Plumb it like a side-arm,....

 

[yeldogt]

you do it with primary secondary --- twin primary on one secondary.

the secondary is pumping away towards the emitters. close spaced t's OR LLH depending on the radiation head flow needed.

EDIT --- typing error ........ I wanted to say OR ... not "of"

 

[Sumboodie]

Ayuh,...... I'd plumb it up exactly like I built my ole homemade outdoor wood boiler,......

The house I lived in was already an oil boiler/ hydronic baseboard heat, with a domestic water heatin' coil in it,....

I hooked up my out to the wood boiler line to the manifold at the bottom of the oil boiler, 'n "T"ed in the hot water from the wood boiler to the top piping comin' outa the top of the oil boiler,....

When a zone called for heat, the original circulatin' pumps kicked on,..... all original to the house,....

Out on my wood boiler, I'd built it pressurized, same as in the house,... I had 2 circulatin' pumps on it,....
One would just take the hottest water off the top(190*), 'n push it back into the bottom, mixin' it all up,....
The other pump, would kick on at 180*((aqua-stat was at the top of the wood boiler), 'n pull water from the bottom of the oil boiler, or from the house if the zone pumps were runnin', pushin' it into the bottom of the wood boiler, which in-turn pushes the Hot water into the top of the oil boiler or into the house if the zones were runnin',.....
Best part was, the oil boiler's auto-fill, filled the Whole system,....

Easy Peasy,.... Plumb it like a side-arm,....

Huh?

 

[Sumboodie]

you do it with primary secondary --- twin primary on one secondary.

the secondary is pumping away towards the emitters. close spaced t's of LLH depending on the radiation head flow needed

Not needed on the current setup. It heats the water to 90* and shuts off and that's what flows to the floor. What is an emitter? I have a manifold and that's what feeds the floor.

 

[TurnipTruck]

An emitter is anything that emits heat.

This is a screenshot from the free online Caleffi series of workbooks. It shows one boiler, but as many boilers as you have room for can be plumbed in next to it. Your modulating boiler has a bunch more restriction across it than your iron boiler, so hydraulically isolating each boiler with individual heat injection loops will prevent all kinds of short-term and long-term problems.
If you have a spare week or two, I am again recommending everyone designing a system to read everything Caleffi has written.

 

[Sumboodie]

An emitter is anything that emits heat.

This is a screenshot from the free online Caleffi series of workbooks. It shows one boiler, but as many boilers as you have room for can be plumbed in next to it. Your modulating boiler has a bunch more restriction across it than your iron boiler, so hydraulically isolating each boiler with individual heat injection loops will prevent all kinds of short-term and long-term problems.
If you have a spare week or two, I am again recommending everyone designing a system to read everything Caleffi has written.

I have maybe a spare 30 mins or 2. A week... hagahaha. It's been 4 years since I've taken time off.

 

[yeldogt]

Not needed on the current setup. It heats the water to 90* and shuts off and that's what flows to the floor. What is an emitter? I have a manifold and that's what feeds the floor.

What's not needed?

You need what you need to make it work ?

 

[Bondo]

Huh?

Ayuh,..... I'll gladly answer yer questions, but Huh, ain't a question,.....

 

[Sumboodie]

Ayuh,..... I'll gladly answer yer questions, but Huh, ain't a question,.....

No idea what you are trying to day. I'm from Maine, we talk normal.

 

[Sumboodie]

What's not needed?

You need what you need to make it work ?

Primary/secondary loop isn't needed.

 

[yeldogt]

Primary/secondary loop isn't needed.

if you are trying to cascade two boilers and not have one heat the other ..... that's the way you do it

 

[Sumboodie]

if you are trying to cascade two boilers and not have one heat the other ..... that's the way you do it

Coffee not kicked in yet? haha.

I just said that the EXISTING setup does not have a primary/secondary loop.

The primary/secondary is usually used in a cast iron boiler setup where the boiler has to heat to at least ~140*, but you want let's say 90* water to the floor.

One boiler heating the other isn't really an issue though.

 

[yeldogt]

Coffee not kicked in yet? haha.

I just said that the EXISTING setup does not have a primary/secondary loop.

The primary/secondary is usually used in a cast iron boiler setup where the boiler has to heat to at least ~140*, but you want let's say 90* water to the floor.

One boiler heating the other isn't really an issue though.

no -- primary secondly is used with mod cons as well. It all about proper flow. I use primary/ secondary with all low temp radiant. the flows and heads don't match

When you put two boilers together to heat one loop -- they have to be separate or the one operating will heat the one that is off

 

[Sumboodie]

no -- primary secondly is used with mod cons as well. It all about proper flow. I use primary/ secondary with all low temp radiant. the flows and heads don't match

When you put two boilers together to heat one loop -- they have to be separate or the one operating will heat the one that is off

Are you just arguing to hear yourself talk?

That's what I just said!

 

[yeldogt]

Are you just arguing to hear yourself talk?

That's what I just said!

You said --- not needed. ?? I answered

You said you currently don't have primary/ secondary ... I'm saying you may have to do it if you want it to work.

You also gave a reason for P/S as if it is the only one ..... it's not.

So if you have the answers to make it work --- then I guess you don't need to ask.

If you want to know how to do it from someone who has more than once .... I answered.


It's after 12 in the east ... I'm about to have lunch

 

[Balvar24]

Primary secondary is almost always the way to go. Dedicated pump on each boiler. Ensures each boiler get appropriate flow. No muss no fuss.

Set one boiler setpoint couple degrees higher than the other.

 

[PoorUB]

Two primary, or boiler loops and one secondary loop is the only way I would plumb it.

One pump for each boiler and one pump on the secondary loop. Valve the boilers so if one fails you can valve it off and remove it without draining the system. Each boiler should have a low water cut off or flow switch.