Two boilers of different output

Shahrdad

If it's decided to use two smaller cast iron boilers instead of one larger one, do you ever combine two boilers with different outputs? For example, if the calculated heat loss on a design day is around 186,000 BTU/hr, is it advisable to use two Buderus CI boilers one of which has an output of 112,000 BTU/hr and the other 87,000 BTU/hr?
And if this combination is installed, what system would you use to control the firing of the boilers? For a Buderus boilers, do you all tend to go with Buderus' own Logamatic control, or do you prefer something like Tekmar or Taco?
Also, will this system still be able to alternate the boilers firing to avoid premature wear on just one boiler? For example, will the controller be smart enough to use the boiler that comes closest to the required heat output?

Jamie Hall

Well, no theoretical reason not to. You would have to take the usual piping precautions to ensure that they could, in fact, operate independently if wanted, or together. But that would be true whether they were of equal size or not.

Control is another matter. Two differently sized boilers -- and I'm assuming hot water here -- would not be set up to alternate. Rather, your control system would want to fire the smaller boiler for small loads, the larger for medium loads, and both together for full power. If they are not modulating, this can be done based on outdoor temperature without too much trouble. If they are modulating, you wind up with a fantastically wide range of modulation -- but to take advantage of it, you will need a very sophisticated control outboard of, but integrated with, the boilers' own controls. Interesting project...

Shahrdad

Jamie Hall said:

If they are modulating, you wind up with a fantastically wide range of modulation -- but to take advantage of it, you will need a very sophisticated control outboard of, but integrated with, the boilers' own controls. Interesting project...

I am thinking of the Buderus GC-144 series of boilers, which to the best of my knowledge, are not modulating. Are there modulating cast iron boilers out there?

pecmsg

How did you come up with 186K ?

Shahrdad

pecmsg said:

How did you come up with 186K ?

One of the installers went around the house and did a ton of measurements. That's the number he came up with. The number I came up with by watching my gas consumption on a zero-degree day is a little lower than that.

pecmsg

Shahrdad said:

How did you come up with 186K ?

One of the installers went around the house and did a ton of measurements. That's the number he came up with. The number I came up with by watching my gas consumption on a zero-degree day is a little lower than that.

Where is this house located?  Is Zero the design conditions or extreme low?

Hot_water_fan

Ha this is fun.
Maybe two of the smallest and then the 87k? Gives you longer runtimes at almost every outdoor temp, especially the more common, mild days. Plus if even if the 87k goes down, you'll have 124k Btu, whereas best case scenario you'd have 112k Btu in the original scenario if one was down.

Shahrdad

pecmsg said:

Where is this house located?  Is Zero the design conditions or extreme low?

St. Louis. Zero degrees is the design temp. We had a week of it a few months back.

Shahrdad

Hot_water_fan said:

Ha this is fun.
Maybe two of the smallest and then the 87k? Gives you longer runtimes at almost every outdoor temp, especially the more common, mild days. Plus if even if the 87k goes down, you'll have 124k Btu, whereas best case scenario you'd have 112k Btu in the original scenario if one was down.

That is fun! But I think having three boilers is a bit of an overkill for me, and I think two can be safely vented through a lined 7" chimney. I have also seriously considered Vitocrossal 300, though I wonder whatever gas savings I have would be more than offset by the cost of annual maintenance. The idea of some redundancy is also quite appealing.

Hot_water_fan

That is fun! But I think having three boilers is a bit of an overkill for me, and I think two can be safely vented through a lined 7" chimney. I have also seriously considered Vitocrossal 300, though I wonder whatever gas savings I have would be more than offset by the cost of annual maintenance. The idea of some redundancy is also quite appealing.

The Vitocrossal seems perfect for this application, you could have runtimes in the months and just have a system pump. No idea what annual maintenance costs, but savings with this load and climate are significant. No redundancy though.

Shahrdad

Hot_water_fan said:

The Vitocrossal seems perfect for this application, you could have runtimes in the months and just have a system pump. No idea what annual maintenance costs, but savings with this load and climate are significant. No redundancy though.

My situation is a bit complicated in that Hydronics is not big in my area, and it's hard to find installers who are familiar with Viessmann. For condensing boilers, they're always pushing Lochinvar, which doesn't seem to be as beautifully designed as the Vitocrossal 300. And I want it vented through the top of the chimney, as I really don't want any holes drilled into the side of my old masonry house, and I despise the look of steam coming out the side of the house.

pecmsg

When it was 0 outside did the old boiler run continually or did it cycle on and off?

Hot_water_fan

My situation is a bit complicated in that Hydronics is not big in my area, and it's hard to find installers who are familiar with Viessmann. For condensing boilers, they're always pushing Lochinvar, which doesn't seem to be as beautifully designed as the Vitocrossal 300.

Oh reality . The Lochinvar does have low head loss through the boiler at least, which would pair well with your high operating hours scenario. What's the natural gas $/unit cost in your area?

Shahrdad

Effective November 16, 2020

Customer charge $22.00
Winter billing (Nov. - Apr.) $0.23330 per therm
Summer billing (May - Oct.) - 1st 50 therms $0.20994 per therm
Summer billing (May - Oct.) - over 50 therms $0.25435 per therm

Robert O'Brien

Shahrdad said:

If it's decided to use two smaller cast iron boilers instead of one larger one, do you ever combine two boilers with different outputs? For example, if the calculated heat loss on a design day is around 186,000 BTU/hr, is it advisable to use two Buderus CI boilers one of which has an output of 112,000 BTU/hr and the other 87,000 BTU/hr?
And if this combination is installed, what system would you use to control the firing of the boilers? For a Buderus boilers, do you all tend to go with Buderus' own Logamatic control, or do you prefer something like Tekmar or Taco?
Also, will this system still be able to alternate the boilers firing to avoid premature wear on just one boiler? For example, will the controller be smart enough to use the boiler that comes closest to the required heat output?

There's so much fudge in the factors used to calculate, you could easily use two smaller ones. As an experiment, leave one off the first winter and see when you need to turn it on. Probably never.

Shahrdad

Robert O'Brien said:

There's so much fudge in the factors used to calculate, you could easily use two smaller ones. As an experiment, leave one off the first winter and see when you need to turn it on. Probably never.

Well, when it was around 0 degrees this past winter, my boiler with a rating of 196K ran continuously, though I think it's actually putting out closer to 168K going by my gas meters and assuming 80% efficiency. So I think I will need two boilers for the super cold days. But most of the winter, I'm sure one will do the trick.

Dave T_2

Why not install a single modulating boiler. It is the most cost effective. Likely least expensive to install, run and maintain. I never liked multiple boilers unless the load was over 300k.

The Steam Whisperer

If your old boiler is still in reasonable operating order, Just add a small boiler big enough to handle typical winter days( usually about 60% of your peak load) It will provide about 90% of the heating needs and the old boiler will only need to run a few days a year or as a backup. Stage firing single input boiler is highly efficient, if plumbed and controlled properly. You will probably get about 2/3 of the savings of a mod/con boiler without the added complexity, much shorter life, and increased service expense.

Shahrdad

The Steam Whisperer said:

If your old boiler is still in reasonable operating order, Just add a small boiler big enough to handle typical winter days( usually about 60% of your peak load) It will provide about 90% of the heating needs and the old boiler will only need to run a few days a year or as a backup. Stage firing single input boiler is highly efficient, if plumbed and controlled properly. You will probably get about 2/3 of the savings of a mod/con boiler without the added complexity, much shorter life, and increased service expense.

It still works fine, but is 50 years old and was never piped properly in the first place. There is no boiler loop, and it has run as subcondensing temps all these years. Every year, I clean a lot of rust from the bottom, so it's just a matter of time before it springs a leak.