Hot Topics: Hot Water Baseboard Heaters

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Original Post: How to Install Hot Water Baseboard Heating

sandonn - Member

I converted part of my garage to a living room, and I'm up to the heat.

I have a 5 room ranch(now 6), w/ 1 zone of hot water baseboard heating.

Can you tell me what it takes to add the additional room into the loop.

I could use a general how-to from start to finish. If you know of any websites that would be cool.


johny2050 - Member

First you will need to do a heat loss for the new room to determine how much baseboard you will need (Slant Fin, has a free heat loss program on their web site).

Then you will need to know if your boiler can handle the new heat load. If it can you can tap in to a Supply and Return and plumb your new baseboard . However, there a max lengths you will need to take into account so you don't run out of heat. For example, 1/2" cooper should not exceed 25ft of baseboard and 3/4" copper 67ft of baseboard. If you go to Bell & Gossett they have some good info you might want to read first. On their web site download "How hydronic system components really work" and "Zoning made easy" they are good reading .

sandonn - Thread Starter


I found a good site w/ a good explanation, and it had this listed:

Q: So how much element can I safely use and still stay within the bounds of a 20-degree F temperature drop?

A: As a rule of thumb, you should not exceed these limit on any loop:

1/2" - 25 feet of element

3/4" - 67 feet of element

1" - 104 feet of element

1-1/4" - 177 feet of element

I'm still doing more research and I still need to measure the amount of actual baseboard in my house. Can't do it now, everyone's sleeping.

But here's my question. Looking in the basement, I can see that at the last point there is 1" pipe returning to the boiler. Everywhere else is 3/4" pipe. W/out actually measuring the baseboard, I think I have about 70 feet. I plan on adding another 10-15 feet. The new room is actually at the beginning of the run. If I tap it in w/ 1" pipe, should that help reduce the heat loss?


NJT - Member

"If I tap it in w/ 1" pipe, should that help reduce the heat loss?"

No, that won't help.

Here's the reason for the limitation of 67' of element on a 3/4" circuit:

Most baseboard is rated at 500-600 BTU per FOOT of lineal length.

For several reasons, you don't want to flow more than 4 GPM (Gallons per Minute) through 3/4" pipe.

Four GPM can transport 40,000 BTU of heat to the baseboards and return the water 20° cooler to the boiler.

If you divide 40,000 by 600 (btu / foot of element), you come up with appx 67' ...

The 20° difference isn't 'cast in stone' ... it can be more or less, but if it's too much, you can end up returning the water to the boiler TOO COOL, and cause problems with a condition known as "flue gas condensation" ... flue gases will condense on surfaces inside the boiler that are less than approximately 135° for gas, somewhat cooler for oil fire ... this is the "DEW POINT" ... if the return water is too cool, you can create this problem. This condensate is ACIDIC and will EAT the boiler from the inside out.

Are you planning on adding the new circuit in SERIES with the existing circuit ? I wouldn't do that.

You would be MUCH better off adding the new circuit in PARALLEL with the existing, and possibly adding a zone control with a separate thermostat.

Is the boiler positioned in the home such that you can run the pipes from the new baseboards directly back to the boiler and TEE into the supply and return lines AT the boiler ?

You _might_ get away with adding in series ... keep this in mind though:

When you pump water through a baseboard heating loop, the water will drop BTUs (heat) off in each room. If you add baseboard to the beginning of the loop, and cooling it before it gets to the existing baseboards, it is _possible_ that you can throw off the balance of the system. By the time the water gets to the end of the circuit, it may well be too cool to heat the end rooms properly.

How many BTU is your boiler ?

Have you done a HEAT LOSS CALCULATION on the new room to determine how much new baseboard you will need to heat it ? Just adding "10 to 15 feet" isn't really the right approach.

Take John's advice and go to and download the Hydronic Explorer II software ... install and run the program and determine FIRST how much baseboard you will need to install...

Also, read "Zoning Made Easy" at the Bell & Gossett site, it will explain all of the above much better than I can.

EdinupstateNY - Member

cast iron to base board hot water

How do I figure how many linear feet of base board fin hot water do I need to replace cast iron hot water. Bought an old place with 3000 sq ft and 13 cast iron radiaters - all were frozon and cracked - I want to use the main pipes in the floor and walls and replace the cast iron radiators with base board.

Who - Member

The baseboard is easy to calculate, the MFRs have tables. It generally runs around 600 BTU per lineal foot at 180°F. Calculating the existing standing iron is tougher but there are some books and guides on this.

You should do a room by room heat loss calculation and size off of that.