Design Loss and sizing cast iron baseboards

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Old 11-08-12, 08:52 AM
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Design Loss and sizing cast iron baseboards

Hi all.


I've got a rancher with Baseray all through the house. I know there's way too much of it. For instance, one zone is the kitchen, living room and dining room. It's about 800 sf and I've got just under 60' of 9A. It's a monoflow system and the last branch barely gets warm.


I've got an assembly tool and nipples and I'm ready to go. The only problem is figuring how much I need. I've done a rough heat loss calc for that zone and my design loss is around 10,000 BTU/hr.


My question is, looking at this chart for Baseray below, do I just match up my design loss with the column under 180 degree water and figure 17'? I figured I'd add some fudge factor and go with 25'.


Am I way off?


Thanks for any guidance.
 
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Old 11-08-12, 10:45 AM
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You say it is a monoflow system and one section of radiator does not get warm? Sounds like you have air stuck in the section. Baseray should be pretty easy to get the air out of. There should be a vent somewhere on that baseboard. Getting the air out should solve your problem.
 
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Old 11-08-12, 11:09 AM
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Thanks, but the radiators are fine. There's just too much of them.

There's four branches in this zone - the first with about 40' of cast iron. Each branch down the line takes longer and longer to heat up. I know that's normal to a point, but the last section is usually luke warm by the time the boiler kicks off.

Just by my rough calculations there's way too much baseboard. I'm just trying to get closer to what's right.
 
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Old 11-08-12, 04:26 PM
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Remember calculations are done for worst case scenario. 99.9% of the time your baseboard will be oversized. Heat loss calcs are done for the coldest day of the year so if you ended up with 10k btuh's then go with 17 ft.
 
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Old 11-08-12, 04:31 PM
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If this is a monoflo system, all branches should be heating evenly. That is the whole point of that design.
 
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Old 11-08-12, 04:39 PM
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I dont think he's worried so much about the evenness of the heating. He has overheating in one room
 
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Old 11-08-12, 05:23 PM
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I dont think he's worried so much about the evenness of the heating. He has overheating in one room
Wouldn't overheating in one room be uneven heating?
 
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Old 11-09-12, 09:57 PM
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Sounds like it to me... but I don't think OP ever mentioned overheating in one room.

Adam, how do you know this is monoflo system?

Air in monoflo is VERY common and would be my first suspicion, but you seem so awful sure that it's not. Why are you so sure?

What you are describing can be quite common with a series piped system, but not so much with monoflo. As long as the flow in the main is up to snuff, each branch of a monoflo will be served with nearly the same temperature water.

I would like to save you from doing a lot of extra work and incurring a lot of expense if it's something as simple as an air blockage.
 
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Old 11-10-12, 06:48 AM
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Because each zone has a 1" main around the ceiling of the basement and each room has a tee feeding the baseboard and a diverter tee on the return. The only zone that is in series is the basement.

What's happening is that since there's so much cast iron to heat, the returns from each branch stay very cold for a while. The first branch has 40' of 9" baseboard for only 500sf. By the time the room heats up to temp. only half the of the radiator is warm.

So, on the main after each diverter tee, the water is cooler and cooler. Hence, the supply to each branch down the line is getting cooler water. When there's a call for heat, it takes a quite a while for main the even get warm at the return to the boiler. And yes, my Taco 007 is working fine.

I think we're getting off track here though. My only real question was about sizing my radiators, not diagnosing them.

Thanks!
 
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Old 11-10-12, 08:49 AM
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the returns from each branch stay very cold for a while.
This symptom is so typical of an air blockage in a monoflo (or ANY system), and is the reason I'm dubious of your diagnosis.

And yes, my Taco 007 is working fine.
OK... but, is it up to the task of pumping say 6-7 GPM through all the monoflo tees?

I believe that you should be focusing more on assuring that the pump is proper for the application and that the flow rate in the loop is correct, and that there is no air in the branches.

Understand that in a monoflo system one tee on the return might divert say 25% of the flow in the main. If that amount of diversion is enough to cool the main enough so that the last branch on the circuit stays too cool then I would STRONGLY suspect that you need a bit more pump behind it. Increasing the flow rate will lower the delta T throughout the system and possibly solve the problem without hacking at your baseboards.

Is the 007 the original pump, or has it been replaced?

I asked, and you didn't answer:

Air in monoflo is VERY common and would be my first suspicion, but you seem so awful sure that it's not. Why are you so sure?
Do each of the branches have an air bleeder at the DOWNSTREAM END of the section? Are you bleeding them with the pump RUNNING?

I think we're getting off track here though. My only real question was about sizing my radiators, not diagnosing them.
I understand that. Since you are so certain of your diagnosis, then have at it. All we can do is offer suggestions that would save you time and effort and money.
 
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Old 11-10-12, 01:21 PM
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I have a monoflo system in a 3500 sq ft house (plus basement), with four zones of cast-iron baseboard. Each zone typically has about 40' or more of baseboard, similar to yours. Yes, it takes a few minutes for the heat to reach the end of each zone, but it eventually does. The zone mains are black steel, 1" and the risers are mostly 3/4" and some 1/2". My calculated design heat loss is about 105,000 Btu/hr.

My system was originally provided with a Bell & Gossett HV-series pump. I ultimately replaced it with a Taco 0012, which has similar characteristics as the HV. I'm sure that a Taco 007 wouldn't do the job for my system. Monoflo systems require a generous flow velocity for the venturi tees to develop the required differential pressure. (By the way, make sure your tees aren't installed backwards. The ones on the returns must be installed with opposite orientation of of the supplies.)

Note: I said I have four zones, but the zone valves are manual plug valves for balancing. I leave the plug valves wide open, so when the pump runs, the whole system gets heat. My bedroom baseboards have local adjustment valves which I throttle for comfortable sleeping.
 
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Old 11-10-12, 05:36 PM
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To answer your question, you need to do a room by room manual J calculation. You can size your radiators to the design heat loss using 180 degree water. However, since your baseboards were probably installed before manual J calculations were done, you more than likely have way more radiation than you would need. This is a good thing. It will allow you to run cooler supply temperatures to heat your home.

In this situation this is what you should do. Figure out the maximum output of your baseboard will be based on 180F temps for each room. Compare that output with the room's heat loss. Make sure each room has roughly the same ratio of radiator output to heat loss. That will give you even heating.

How low your temps can be will be dependant on the type of boiler you have and all your other numbers.
 

Last edited by drooplug; 11-10-12 at 06:10 PM.
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Old 11-10-12, 06:14 PM
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To answer your question is divide the heat loss by the design water temperature you will be operating at which is usually 180f. So use 590 if you are designing at 1 gpm per your chart. If you increase flow you get more heat, slow the flow gets you less heat. At 4gpm you would use 620 btu's per ft.
I guess my question is why would you want to reduce the size of the radiation? Excess radiation is a plus not a negative affect. I would not reduce the size of the radiation. Your problem is flow or air. I would assume more a flow problem then an air problem.
Flow does not always mean gpm but near boiler piping. Does the boiler heat up well when running? What kind of water temperature do you see out to the radiation?
 
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