Downsizing Radiators

foresthillsjd

Hi friends,
I tried to post this under my original thread on heat loss calculations, but it doesn't seem to be taking, so I'm trying with a fresh thread:

original post here

So, I faithfully did my heat loss calculations for my three-story, 2700 square foot house. My boiler is Weil-McLain EG/PEG-50 with an output of 145,000 BTUs/hr.

It had about 80,000 BTUs/hr of connected radiators, but based on heat loss calculations, I reduced the total radiators to 50,000 BTUs/hr. (I was replacing the radiators with shorter ones so they could fit better in the built-ins, but since I was at it, I went ahead and sized them according to heat loss.)

Problem is, the radiators got delivered today, and they look so tiny compared to the old ones that it has me second-guessing myself about whether they are properly sized. Also, is it possible that even though the output is 37.5% less, the volume of these things is more than 37.5% smaller? Is there like, less air inside the tubes? They just seem so dinky! I had the 25" three column type and went to the 19" Gov-free ones. I'm trying to picture the order of operations.

1. My big-a$$ boiler will power on.
   
2. The properly vented system will fill with steam quickly.
   
3. The steam will condense in the radiators, sacrificing their latent heat which gets stored in the cast iron.
   
4. The soul of the steam (water) will slink back to the boiler, to be reincarnated later.
   
5. The radiators will give heat up via convection and radiation to the room.
   
6. Room heats up, thermostat will turn the boiler off. (Is there a universe when the pressure will actually cause it to cut out first?)
   
7. Room continues to heat because of the heat stored in the cast iron, causing the temp to overshoot (by a lot? a little?)
   
8. Room cools down.
   
9. Boiler fires up again.
   

Do I have that right?

But I'm assuming the boiler will cycle more frequently now because the bucket of heat stored in each radiator is smaller. If so, is that a good or bad thing? Will it take longer to heat up? Will it keep the temperature swings in the house within a narrower band? Is the cycling bad for efficiency?

Is there a disadvantage to having a boiler so big relative to the connected radiation other than it cost more, takes more space and needs a bigger vent?

Thanks in advance for listening to the rantings of a mad woman. This dang steam thing is keeping me up more than my 3-month-old daughter.

~Jackie

EBEBRATT-Ed

In a perfect world the boiler size and the radiation would match perfectly.

Smaller radiators will make the boiler short cycle

Have you already changed the radiation??

I would have left the larger ones in.

Are you planning on replacing the boiler? That's probably the only real fix to run with the smaller radiation.

I would re check your #s and make sure the new radiation will heat the house before you install them if you haven't already. Your boiler is 3x larger than the radiation

ethicalpaul

If you got the radiators like the ones in my thread that you posted on recently, then I know exactly what you mean. The new ones are very wimpy. I don't think the concern is less air in the tubes, I think the concern is the tubes are smaller and thinner than old radiators, so there is less surface area AND there is less thermal mass to heat up and then to radiate heat after the burn cycle.

To address your steps you listed, that is how I think of the process, with the exception that step 5 can have varying duration. A properly sized system in my opinion may run for some time (some number of minutes) with a continuous process of steam condensing in the radiator as the radiator gives off heat to the room (especially if it is recovering from a setback).

> (Is there a universe when the pressure will actually cause it to cut out first?)

Yes, and if you haven't already experienced it, I think you will if you swap in all these modern radiators. What happens is that more steam is produced than can be condensed by the radiation (EDR) in the house. Then the only option is for pressure to rise, and the boiler to be cut off.

I don't know how many or what size you ordered, but I would do this: return the smallest ones, the ones that seem totally insanely small. Then pick one of the larger new ones and replace one of your old ones that you want to get rid of, and see how it heats the space. That should give you an idea of how the new ones compare to the old ones.

You could also do other swaps. You could take out an obviously currently oversized old radiator and replace it with a smaller old radiator from your house, then put one of the new ones in place of the old smaller one.

But do these changes once at a time! Good luck!! Feel free to share the number and size of the ones you bought, and pictures of the ones you are looking to replace, I bet people would be happy to offer more opinions

Fred

It sounds like you have taken a very expensive path to potentially get right back to where you were and adding some problems that you probably didn't have, along the way.
Your old radiators, while they may be large, as long as they fit in the built-ins, what, in your opinion makes them fit "better". I mean it's not like you need to be in those cabinets, except for service, which isn't often.

I assume you know that when you change the radiators, you also need to change the valve and spud so that they match, right?
Are the valves/spuds for each smaller radiator the same size AND height as the old ones? If not you will need to figure out how to down size the steam pipe so that the new valve can be threaded onto it and at the same time maintain the height necessary to maintain the needed pitch so that condensate will drain.

You do realize that, with the larger radiators, they can be down-sized by simply putting different vents on them. The slower you vent the radiator, the longer it takes for steam to heat it and you can adjust the venting down so that you don't heat the radiator all the way across, essentially making adjustments to more closely match your heat loss on a room by room basis. All that costs is maybe a few new vents. If you already have adjustable vents, you don't even need to spend money for new ones.

I assume you also realize that by reducing the connected EDR (radiators) that you will most certainly cause the boiler to be over-sized and no matter how much main venting you put on your system, you will cause the boiler to short cycle, on pressure, and shut the boiler down multiple times per heating cycle. Not at all desirable.

You can also control the temperature over shooting by using a programmable thermostat (which you may already have) that allows you to change the number of cycles per hour from "1" up to "5". Typically programming the cycles to "2" or "3" CPH will even out the temperature and virtually eliminate over shooting.

I am really concerned that you have set yourself up for some real issues, at least until you can also reduce the size of the boiler. You also need to remember that while you have 80K BTU of connected radiation, you also need about 33% overhead at the boiler for piping and pick-up. That would mean a boiler around 107K BTU output. With your current boiler you are already significantly over-sized, even with the older larger radiators.
This whole change out is a disaster in the making. Rethink what you are planning to do and return the new radiators.

foresthillsjd

@Fred (I love that you were a member early enough to get "Fred" as a username and not "Fred123")
Thank you for your detailed response. I posted a follow up with more specifics in the original thread that can be found here:
https://forum.heatinghelp.com/discussion/169425/heat-loss-calculations#latest

I'll admit that replacing radiators for aesthetic reasons is not the most practical one, but that ship has sailed. (We've gutted the house and are rearranging the layout. Some radiators have to be relocated, others need to be shorter than the window sill they sit in front of.) Since we are replacing the radiators, I wanted to balance the rooms a little better and also do so with an eye toward the day that I replace the boiler.

But based on the advice on the wall and mulling it over (I do see why having a bigger reservoir in each room would be advantageous), I am going to return a lot of the small ones and get ones that are a closer match to what was there before. (I shudder to think about the restocking fees they may charge...)

Fred

@foresthillsjd , I now understand your reasoning and commend you for keeping the steam rather than ripping the entire system out and installing Scorched air. Still, Going to this much trouble to down-size the radiators and ignoring the boiler will be something you may regret for a long time (until the current boiler dies) If you can't change out the current boiler, at least get a steam Pro in and see if the boiler can be modified to accept a two stage gas valve or, at a minimum, down-fired some.

mikeg2015

Might be able to get away with installing a 2 stage gas valve. But did I read correctly that your total output is now 50,000 and boiler output is 145k? Not much fixing that. IS the number of radiators the same, only the size is smaller? If so you could use 2 stage on a vapor stat. Run high fire until pressure builds then drop of low fire (probably once header heats up).

Still need to add as much venting the main as possible. Then slow the radiator vents way down with Maid or Mist #4’s or Hoffman 40’s and see how that works.

Even with pickup, you still have more than double the boiler you need. What will happen now, if the number of radiators is the same, is they will be heated across more than a larger radiator that might only be rated 20-50% most of the winter. Your challenge is that if you use setbacks, once fully heated, it will cycle off pressure, rapidly.

Again a 2 stage gas valve and a delay timer (maybe 5 minutes) once it reaches pressure limit.