System Efficiency with 2 boilers

Pezdyspensr

Hello!
I am a homeowner of a 1929 2-unit in Maine. Each floor is 1140sq ft with it's own steam system, 7 radiators per. First floor still has the old snowman, and upstairs has a mid-century Peerless - both oil. I am needing to update the snowman, and I have come to a BIG question, which no local heating contractor has been able to sufficiently answer- as none of them seem to really have steam knowledge!

Given that there are many variables which influence the answer, I am asking the following based purely on SYSTEM EFFICIENCY.

Am I better off optimizing 2 small steam systems in a 2280 sq. ft. building, or converting to a single larger boiler to run all 14 radiators? Each floor has an EDR of 78000btus after 1.33 pickup factor. There's a total of 120' of 2 1/2" and 120' of 1 1/2" pipe in the basement.

I know there's lots to consider in the real world of this - ie. cost of conversion, chimney update issues, zoning concerns, etc. Basically, I have an intuitive notion, which may be completed un-based, that steam systems work better on a larger scale than a smaller one? Which is why you tend to still see them in big buildings? Currently, these systems compete with each other. If one is running, the other isn't. Vice versa. Maybe steam is overkill for an 1100sq ft 2 bedroom apartment?
Thanks in advance!

Jamie Hall

From the standpoint of efficiency, if the boilers are properly sized with relation to the connected EDR, it should make little or no difference. The advantage, of course, is that each boiler would be controlled by its own thermostat, so one unit could be nice and toasty and the other nice and cold, depending on the occupants. You could also put run time meters on the boilers and charge accordingly, since these are rentals.

Keep in mind that the boiler EDR rating includes the pickup factor. Be sure that the boilers you get are properly sized.

nicholas bonham-carter

Are the radiators still around for the snowman apartment?
You could remove the snowman, and replace it with a second boiler, piped into a common header with the other one.
With a vaporstat, controlling the lag boiler, the system could fire up on the two boilers, but revert to the one lead boiler, when a few ounces of pressure are attained. Generous main venting would be required, at these low pressures. There are probably some good deals on oil boilers now with so many people having switched rather than fight with the oil company.--NBC

Pezdyspensr

For the sake of cost, I am putting in a used small boiler in the first floor. I have a good heating technician who can do this to manufacturer specs. He was less afraid of steam than the other guys - not that I've found anyone with the real knowledge I get from this forum(any Maine-based techs on this forum?). Anyway, I found a nearly new Weil McLain SGO 3 for $450. It has a burner output of 114,000BTU, and 86,000Btu steam, at 354sqft of steam. The EDR of the radiators after pickup factor was 78,000btus, so I think I'm ok. I couldn't find a smaller make of steam boiler anyway!

But in doing all this, it opened up a theoretical question that has been nagging at me - are small steam systems inherently less efficient?
I need to produce 325 sq ft of steam for each unit of my building. If I have 1 boiler producing 650 sq ft of steam, does it require twice the water, twice the burner, twice the fuel, etc? When I see specs on boilers rated for those steam quantities, the fuel usage actually doesn't quite double. So at the boiler, it takes less fuel to operate the larger steam load. And if this pickup factor is consistent from small to large systems, that implies that it doesn't make much difference if you have a lot of added pipe travel (assuming properly insulated pipes). So wouldn't you see greater overall efficiency operating a bigger system than multiple smaller ones?

Jamie Hall

In theory, of course, it takes exactly the same amount of fuel to produce a pound of steam in a big boiler as it does in a small one. Like so many theories, this one has lots of problems! Most of which have to do with various losses. However, if we make the assumption that both boilers are equally well insulated, and that all the piping is properly done and well insulated, and all the burners are properly adjusted to the loads, and...

Yes, probably one large boiler is slightly more efficient than two or more smaller ones. Is that measurable and consistent? I honestly doubt it.

KC_Jones

Just to be clear when you did your boiler size calculations did you add up the EDR of your radiators then add 33% pickup factor into that? In other words your rads added up to 244 then you added 33% to that and came up with 325? If so you are way beyond what you need. The rating on that boiler subtracts out the pickup factor for you. I only mention this because in your situation (oil) I THINK you could possibly down fire the boiler to get closer to your load. It could also influence the answers you are getting with regards to boiler sizing and efficiency. If you are indeed at around 244 then that boiler (with current firing rate) would be about 30% over sized. Just want to make sure everyone is talking the same language here. Also wanted to add I am no oil burner expert so it would be nice to hear from a pro about that possibility.

Pezdyspensr

Yes, you've got it. Rads were 244, so the total with pickup is 325. Not a huge load. 7 radiators, none of them particularly large. The boiler I mention is the smallest Weil -Mclain makes, which is why I ask if it's even worth running 2 boilers. Currently, there's a MASSIVE coal snowman with an antiquated Wayne burner running those radiators. I was told it could have as much as 30 or more gallons of water, so how's that for oversized? It's original.
My upstairs unit has a Peerless running 444 sq. ft. of steam and 105k btus, and that's for an identical EDR. And it doesn't even heat my unit, which I believe is due to system imbalances such as bad main venting (that's my next project..sigh) Granted, the building is completely uninsulated, with pipes routed through exterior walls, so that was likely the thinking behind the boiler sizing. Don't worry, I'm working on the insulation. I just need these systems to FUNCTION right now.

KC_Jones

The number you need to work from is 244 NOT 325 pretty much every boiler manufacturer includes the pickup factor so you don't have to figure it in. It makes boiler sizing fairly easy, until people complicate it. So like I said that boiler is 30% over sized. Your insulation concerns have nothing to do with boiler sizing on steam. Your output is fixed by the radiator size not the boiler size. I would be curious to hear from a pro about down firing that boiler. I know you can do this with gas to a certain extent, but not sure how it works with oil. It's a question of how low you can go on oil.