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Gas Furnace or Boiler


Hi...I have new gas boiler with the rads in the house being a combination of cast iron and fin type baseboard. In the basement I have a long section of baseboard rad that only gets hot half way down. As the boiler temperature increases I eventually lose most of the heat i already had in that line. In other words, the baseboard rad cools off the longer I run the boiler. All other rads in house work fine and there is no where to bleed boiler in basement. Is this not a very unusual situation?? Comments or suggestions greatly appreciated Wizard.

We heat our bungalow with full basement with hot water heat and have a combination of cast iron rads and fin type baseboard radiators. The boiler is new however we now have a problem in getting heat in a 30 foot section of fin type rad in basement. All other rads in the house work fine. There is no air bleed valves to bleed air from in this basement rad. The first 10-12 feet of rad begin to get hot but as the boiler temperature increases (approx 140-150 degrees) the pipe grows cooler. Seems very unusual to lose heat in line when boiler temperature is increasing. Never had problem before with this extended length of baseboard previously. Our boiler did quit one evening when i had flue problem but i can't say with any certainty that this created the problem with this basement baseboard heater. Would putting in a bleed valve at the end of this baseboard heater solve the problem??? While there may be air in line how can the line begin to get hot and then lose this heat as the boiler temperature increases??? Is this not strange?? Thank you kindly for your help.


Dear Bhrin:

Yes, what you are describing does sound very unusual.

I assume that this section of baseboard radiators are on a separate zone from the rest of the radiators in your bungalow, correct? By this I mean that it has a separate thermostat, and the piping for circulating water from through the boiler runs separate from the other piping lines that would serve each of the other zones in your bungalow. And further, I assume that your boiler is likely in your basement, right?

A couple of things that could be causing your problem.

First, yes, I'm wondering if you could have air trapped in your system, but I would think that you would have mentioned something about hearing "gurgling" or rushing water sounds in the lines.

Then what I'm wondering is if there is a problem with the circulation pump (or thermostat) for the zone that serves your basement. Then, when your boiler turns on to serve OTHER zones in your bungalow, the hot water warms up the pipe and part of the baseboard closest to the boiler in the zone that serves your basement, but because the circulation pump is not running properly, the rest of the baseboard further down the line doesn't get heated up.

What you can do to test this is turn down the thermostats on all of your other zones, and then turn up the thermostat in your basement, and then go to your boiler and see if you can hear one of the circulation pumps running on the pipes coming out of your boiler (for example, if you have three zones, i.e, three thermostats, you should see three pipes coming off the line coming out of your boiler, and each of these lines will have a circulation pump on it). If none of the circulation pumps are running, then there is either a problem with the circulation pump or with the thermostat for this zone.

And now, regarding why the pipe seems to get colder when the boiler is running, what I am thinking is that the boiler is running to serve other zones, but your basement zone is not circulating as described above. Therefore as the other zones are running, what I'm wondering is as their circulation pumps are running if they actually then draw the pressure down on the zone for the basement, and since this zone is not running that then somehow begins to pull the hot water back from this pipe and therefore makes the baseboard radiator start to feel cooler.

I'm sorta just guessing here, because it difficult to tell without being able to physically see your system.

But why don't we start by doing the test that I described above, to see if the problem is related to your circulation pump or thermostat for your downstairs zone.

And if you do decide that you need to purge / bleed the air out of the zone that serves your basement, you can find the procedure for how to do this in the Question & Answer section of our Baseboard Heating System webpage in our online Maintenance Library:

Hope this is helpful. And please let us know what you find out, and then we can try to help you further.


I am so impressed with the help you offer when its free...this kind of info is way beyond what i expected from a fellow that is as busy as you appear to be.

I tried to be concise with my question so as not to have you read endless amounts of dialogue but then i guess i forgot to provide crucial info. My apologies. There is only one thermostat and to my knowledge only one zone as the system was put in years ago by people obviously not as sharp as you. I believe it always provided good heat before and perhaps it became problematic when my boiler quit awhile ago due to a minor flue issue. The boiler is a year old and a viesmann (circulator brand new as well) so i think the problem must lie in the lines. This particular baseboard is about 35 feet long and has a 90 degree turn but i think i have it reasonably level. The copper line seems to be an inch or perhaps inch and a quarter so it should give off lots of heat. It gets nice and hot for about 10 feet but once the boiler temp reaches around 150 degrees it starts to cool down. Would putting a bleeder valve at the end of the line do any good?? There are no shut off valves anywhere in basement and essentially what i have is a big copper line running the perimeter of the basement. I dont hear a lot of gurling in line but perhaps there is some air in the end. The pressure is about 20 psi and if i did as you suggest to clear out air i would only be able to raise the pressure about 8 lbs or so and would this really make any difference??

Again I am so impressed that you would take the time to help all us dummies out. Thank you very much.



Glad to try to help you out, and yes, we have some GREAT sponsors who allow us to make this free service available to you ( ) !

I'll need to ask a few more questions:

When you say you have only one zone, you mean for the entire bungalow, not just for the basement, right? And there is only one thermostat for the entire bungalow, and near the boiler you only see one circulation pump for one line coming out of the boiler, right?

Could you please tell me more about how the pipe in your basement gets hot, but then starts to cool down when the boiler reaches 150 degrees. Can you describe for me exactly what is happening. Does the pipe in your basement start to get warm when the boiler comes on, and then when the boiler gets up to it maximum temperature does the boiler turn off and THEN the temperature begins to drop in the line down in your basement?

Is the end of the pipe in your basement that gets hot closer to or further away from where your boiler is located (I'm trying to figure out which way the water is flowing through this pipe)?

Do you know if this section of piping in your basement was added after your bungalow was originally constructed? In other words, was your system designed for standard radiators, and then later this section of baseboard heating in the basement was added?

Do your radiators upstairs all have bleeders?

Regarding your question about adding a bleeder to your baseboard line in your basement, I don't think this would help. If water is flowing through this line, it would move the air with it, and then this air would get trapped in the radiators on your upper floors, where you could bleed the air off. However, not knowing what style radiators that you had upstairs is why I was wondering if it could make sense to purge/bleed you system using the drain valve near your boiler. And regarding your question about purging/bleeding the lines, you actually don't have to raise the pressure to do this, it just helps the process go a little quicker. While purging you would want to keep the pressure from dropping.

But again, it does not sound like you have a trapped air problem. However, if you can let me know about the above questions, we can take it one step at a time, to confirm this, and figure out what is causing your problem.



Hi again...more than happy to answer your questions & apologize for not giving u a better handle on this. The house has only thermostat and only one pump with one line going out. The upstairs has a combination of cast iron rads and fin type rads. The basement has only one cast iron rad and about 45 feet of fin type. All the rads upstairs have been bled and work fine while the cast iron rad downstairs and a seperate 10 ft section of fin work well in basement. It is this one 35 foot section of fin where the problem lies. The end closest to the boiler warms up nicely when boiler starts and gets quite hot for about 10 feet. After that it is only warm and usually will get warm right to the end of the line before it heads up to the floor joists. As the boiler continues to run and reaches about 150 the line starts to grow colder even as the boiler runs. When the boiler temp reaches 160 or so and is still running the pipe is quite cold. The end nearest the boiler is still warm but a point 10 feet away is actually cold. I am afraid i cant tell you when things were added but there was an addition (family room) where 2 cast irons rads reside afer original house built. As i mentioned the basement area has always been warm and perhaps the problem surfaced after our boiler went out a couple of times a month ago. I had a young fellow out to look at it but he is relatively inexperienced and just suggested a bleed at the end of the line in basement. His theory, was that the line has air in it towards the end and is not moving. He had no answer for why the line gets colder as boiler gets hotter and i dont blame him as that is puzzler. I live in Canada where it gets real cold(-40) and there is shortage of people who understand hot water heat. Hope this sheds more light on subject and again many thanks. Incidentally, there is a bleed valve in the floor joists above the end of the line and when bled we just get water.



This is helpful, and I think I'm getting the picture here.

The end of the pipe that gets hot is the end that is closest to the boiler, right?

From what you can tell from the piping you see going into and out of the boiler, can you confirm that the line coming back from your basement is at the end of your heating system? In other words, does hot water come out of your boiler, and then have to go through all of the other radiators in your bungalow before it goes down to the basement and then back to the boiler?

If so, I'm wondering if what is happening here is two things:

1) the boiler was not originally designed for as many radiators and baseboard heating pipes as have been added over time. As a result, by the time the water circulates down to the basement, the water is cold since it has already given up all of its heat in the other radiators.

2) the reason the end of the pipe in the basement gets hot is because it is connected to the boiler, and it conducts heat along the pipe. But then as the boiler runs longer, and the water circulates through the pipe in the opposite direction, the cold water from #1 above starts to cool this pipe down.

As I mentioned before, its difficult to diagnose without physically seeing your system operating. But does this sound like what could be happening?


Hi...i just noticed that the cast iron rad upstairs directly above where the fin rad starts in the basement is much hotter on top of the rad than the bottom of the rad. I did bleed it and got a tiny amount of air but discrepancy in temp between top and bottom remained. Could this be the problem??



I'm wondering if what you are seeing with this radiator is as a result of sludge build-up in it. Here's a drawing showing what I'm referring to:

And this is usually indicated by a radiator that is hotter on the top than it is on the bottom (because the sludge acts as an insulator).

Do you think this is what you are seeing?

Gas Furnace or Boiler