Baseboard BTU performance seems low?

sunlight33

I took some measurements for the emitter capacity of my system. When the boiler turned off, system temperature (measured from the LLH) dropped from 140 to 120 degrees in 8 minutes, considering there are 350 ft of 3/4'' pipe, that's roughly 8 gallons or 67 pounds of water in the system, so the energy loss is about 168 BTU/min or 10k BTU/hr. Now, the system has about 100 ft. of Slant/Fin baseboards, heat transfer at 120F is at least 200 BTU/hr/ft according to the data sheet, so at 100 ft it should be 20k BTU/hr, not 10k, does it mean my baseboards have performance issues?

Gordy

You are measuring temps at the end of the cycle when the rooms have reached setpoint. So output from the baseboard will be less.

You have 250’ of bare 3/4” copper pipe that is probably giving up btus at different rates per foot depending on where the pipe is in relation to areas in the structure that vary in temperature.

When measuring usable baseboard are you measuring only the part with fins?

Question is are rooms/zones reaching set point?

sunlight33

Yes, the rooms have reached the set point and they are comfy. It's about 100 ft for the parts with fins and 300-350 ft for the total copper runs.
My question is if I add a 18-gal buffer tank, this will provide more than twice the amount of water that's currently in the pipes, will this increase the temp drop from 140 to 120 from 8 minutes to roughly 25 minutes? If so the house would cool off at a slower rate with a longer duration, making people inside more comfy and boiler cycle less often, right?

DZoro

Depending on what boiler you have, it maybe more economical to install ODR.

Gordy

sunlight33 said:

Yes, the rooms have reached the set point and they are comfy. It's about 100 ft for the parts with fins and 300-350 ft for the total copper runs.
My question is if I add a 18-gal buffer tank, this will provide more than twice the amount of water that's currently in the pipes, will this increase the temp drop from 140 to 120 from 8 minutes to roughly 25 minutes? If so the house would cool off at a slower rate with a longer duration, making people inside more comfy and boiler cycle less often, right?

What type of boiler are you using? Mod/con? Cast iron?

Weird, your full post didn’t display until I quoted it.........

Also I see my full previous post is not displaying either I wrote more than that.

sunlight33

It's a Viessmann with a ODR and an indoor temp feedback, once the LLH temp reaches about 135-140 degrees, the delta between system supply and return drops to 5-8 degrees or less, I think the issue is that the baseboards (in a 30 yr old house) are not releasing heat fast enough, eventually the LLH reaches about 145 degrees and the boiler shuts off, then in LLH cools off in the fashion I described above, then boiler calls for heat and turns on again, this was before the indoor temp feedback was implemented because the short-cycling was too much. Now with an indoor temp feedback, LLH will hold at 86 degrees or so for a longer period of time, boiler will fire again when the indoor feedback calls for heat.

My question is, in my case am I better off replacing some of the baseboards with newers ones designed for cooler temperature? or add a buffer tank to the system?

Gordy

The emitters are always in control. So you could add more, or change to a higher output using lower water temps. Depends on the size of your boiler verses amount of emitter output verses actual heat loss calculated. Sometimes when a boiler is oversized a buffer is the best solution if short cycling is your concern.

The delta dropping is a matter of the room temperaure closing on setpoint.

If the boiler is over sized it doesn’t help things.

sunlight33

Gordy said:

Weird, your full post didn’t display until I quoted it.........

Also I see my full previous post is not displaying either I wrote more than that.

I had experienced similar glitch lately too.

EBEBRATT-Ed

@sunlight33
You have an older house that was designed for a much hotter water temperature than your running. Baseboard don't degrade unless the fins are loose, removed etc.

The fact that you can heat your house with lo water temp is good. You have a few choices. Running hotter water will increase fuel consumption but may reduce boiler cycling. More baseboard will reduce water temperature, may not reduce cycling unless you add a buffer tank.

if you boiler is oversized a buffer tank will smooth things out

NY_Rob

If you're considering going as far a replacing baseboard emitters... consider going with cast iron baseboard like Burnham Baseray or Gove-Board (same product). Not only does the cast iron heat up more slowly (preventing cycling), it holds .3 gal water per foot. Install 30 or 40 ft of CI baseboard and you have a built in 12 gal buffer tank and wonderful radiant cast iron emitters that are way more comfortable than fin-tube radiators and will 100% help reduce cycling.

Zman

sunlight33 said:

I took some measurements for the emitter capacity of my system. When the boiler turned off, system temperature (measured from the LLH) dropped from 140 to 120 degrees in 8 minutes, considering there are 350 ft of 3/4'' pipe, that's roughly 8 gallons or 67 pounds of water in the system, so the energy loss is about 168 BTU/min or 10k BTU/hr. Now, the system has about 100 ft. of Slant/Fin baseboards, heat transfer at 120F is at least 200 BTU/hr/ft according to the data sheet, so at 100 ft it should be 20k BTU/hr, not 10k, does it mean my baseboards have performance issues?

I am not following your math. BTU/Hr = Delta T x GPM x 500. Where is your flow rate?
From what I can tell, you are just figuring the amount of energy required to bring the system from a cold start to steady state.

FYI, when you look at the low supply water temp ratings for most BB heaters, there is a disclaimer that states that the data has been estimated and not tested.

Gordy

Yes, slant fin extrapolates the calculated 150 degree output for lower water temps.

sunlight33

I've been thinking about cast iron baseboard, but from the spec sheet, the BTU rating is lower compared to copper fin baseboard, at the same temperature, does it mean I must replace with the same length? or will the larger surface area of the cast iron compensate for the difference in BTU output?

Zman

You are trying to reach a delicate balance. When you lower your water temps, your boiler gets more efficient. You also have less heat output at the baseboard which make the boiler short cycle (less efficient). If your boiler is oversized, a buffer tank is probably the best tool to remedy this.

sunlight33

Zman said:

sunlight33 said:

I took some measurements for the emitter capacity of my system. When the boiler turned off, system temperature (measured from the LLH) dropped from 140 to 120 degrees in 8 minutes, considering there are 350 ft of 3/4'' pipe, that's roughly 8 gallons or 67 pounds of water in the system, so the energy loss is about 168 BTU/min or 10k BTU/hr. Now, the system has about 100 ft. of Slant/Fin baseboards, heat transfer at 120F is at least 200 BTU/hr/ft according to the data sheet, so at 100 ft it should be 20k BTU/hr, not 10k, does it mean my baseboards have performance issues?

I am not following your math. BTU/Hr = Delta T x GPM x 500. Where is your flow rate?
From what I can tell, you are just figuring the amount of energy required to bring the system from a cold start to steady state.

FYI, when you look at the low supply water temp ratings for most BB heaters, there is a disclaimer that states that the data has been estimated and not tested.

I kept the flow the same, all zone valves were open when the boiler was firing and when it shut off, so I was simply calculating how much heat is lost from the BB+copper pipes, using 1BTU = to increase 1lb of water by 1 degree F. Also assume that the loss in the bare copper pipes is insignificant compared to the BB section, if there's three times amount of the water in the system, BB cooling off should last three times longer.

Gordy

Basically you are calculating bare copper pipe btu losses. Not output of the emitters. A known flow rate is needed to make that calculation valid.

Gordy

Gordy

Also the emitters output is based on average water temperature. So you would use 130 degrees.

You also need to know if you are flowing 1gpm, or 4 gpm. Not much of a difference but added up over many feet it can have an impact.

NY_Rob

sunlight33 said:

I've been thinking about cast iron baseboard, but from the spec sheet, the BTU rating is lower compared to copper fin baseboard, at the same temperature, does it mean I must replace with the same length? or will the larger surface area of the cast iron compensate for the difference in BTU output?

I replaced 18' of fin-tube with 19' of cast iron BB two winters ago in a "problem" zone that was exposed to the elements on three sides that cooled very quickly causing lots of boiler cycling and uneven room temps. The CI BB completely solved the cycling and up/down room temp issue because once it reaches temp... it stays at/near temp for another 1/2 hr still producing heat vs. the fin-tube cooling to room temp 5 min after the t-stat reaches setpoint.
I've found the CI BB actually "seems to" put out more than the fin-tube because part of it's output is radiant heat (like rays from the sun) that you actually feel on your body vs. most of the heat that's coming from fin-tube is convention that simply heats the room air.

I use the same low-temp supply water (110F-150F) for my CI BB loop that I use for the rest of the fin-tube loops in the house.. no problem getting plenty of heat out of an equivalent length of CI BB compared to even the "Low temp" Heating Edge II BB I've installed in other areas. I really wish my whole house was CI BB as it would probably completely eliminate any problem areas that remain.

Gordy

^mass is your friend. In your case it’s giving output for a longer duration because of the added mass. Perfect for your scenario.

sunlight33

How does Runtal baseboard radiator (UF-2) compare to the CI BB? Price-wise they are very similar.

sunlight33

NY_Rob said:

If you're considering going as far a replacing baseboard emitters... consider going with cast iron baseboard like Burnham Baseray or Gove-Board (same product). Not only does the cast iron heat up more slowly (preventing cycling), it holds .3 gal water per foot. Install 30 or 40 ft of CI baseboard and you have a built in 12 gal buffer tank and wonderful radiant cast iron emitters that are way more comfortable than fin-tube radiators and will 100% help reduce cycling.

I read your thread from last year on the CI BB install, it seems too overwhelming for me.

My master bedroom is colder than the rest of the rooms, so I could replace it with a low-temp BB, pipe work should be the easiest this way, then ask the "supreme leader of my household" for permission to add a buffer tank.

Zman

sunlight33 said:

How does Runtal baseboard radiator (UF-2) compare to the CI BB? Price-wise they are very similar.

Runtals are lightweight and will react very quickly. Sized correctly, they will perform very well with low water temps.
Cast iron will react more slowly and provide a bit of buffer, due to their mass. Putting the buffer in the space is not great as it may overheat that space.

If it were mine, I would be thinking Runtals and a boiler room buffer tank.

NY_Rob

sunlight33 said:

My master bedroom is colder than the rest of the rooms, so I could replace it with a low-temp BB, pipe work should be the easiest this way, then ask the "supreme leader of my household" for permission to add a buffer tank.

Sometimes if you're lucky you can easily add radiation to cold rooms, other times it's impractical or impossible to add radiation so you need to upgrade what's there.

Our upstairs den and MBR are adjacent rooms and it was super easy to literally double the length of radiation in each by drilling one 1" hole in the adjoining wall and using the inexpensive off the shelf fin-tube from HD. Did both rooms for about $100 IIRC.