Using baseboard with radiators

JeffGuy

I have a big old house with original 1900 radiators, and a 1945 Smith coal-fired boiler (which was converted some time after that to an oil burner). Back in the 1980's a large family room was added onto the kitchen, and at that time the radiators in the kitchen were removed and slant-fin baseboard heat was used for the new construction and the kitchen. The rest of the house has its original radiators, which were originally designed to be gravity operated, although a Taco 130 pump was added in 1945 along with the Smith boiler. When the high-output (840 btuH) multi-pak 80 slant-fin was added, it was given its own zone fed by 3/4" copper pipe and a small Taco cartridge pump.

So far so good, but the much larger gravity zone was not given any check valves - and there is none on the Taco 130. So what happens is that when the slant-fin zone calls for heat, it gets the gravity zone running (even though the gravity zone is NOT calling for heat), and in fact the gravity zone is so well engineered that the gravity zone part of the house soon gets much warmer than the slant-fin zone (there ends up with about a ten degree difference between the two zones - even with the gravity zone thermostat turned ALL the way down so it never calls for heat). So we end up turning the slantfin zone thermostat down so it is much too cold in order to make the rest of the house comfortable.

I will need to replace the old boiler this summer - it has a slow leak when it is cold (i.e. in the summer), although it seems to lose very little water when it is firing in the winter. And of course being 54 years old it is probably not the most efficient boiler around. Let alone the fact that we are tired of freezing in the part of the house that we use the most (family room/kitchen).

I have read on this site (which has been enormously helpful) that a Mod-Con boiler may be a good match for this old radiator system. And when I replace the boiler and near-boiler pipes I assume I will need to put on some checkvalves on the two zones to prevent the leakage that I'm getting currently. I will have questions about the Mod-con size and the near boiler piping to use - but I have another planning question which is more urgent since we're doing some kitchen remodelling planning sooner.

I've read on this board about cast-iron baseboard (the base-ray type), which some here seem to like a lot. So my question is whether I should replace the slant-fin copper/aluminum baseboard with cast-iron baseboard as part of the kitchen/boiler work? The base-ray does not have as high a heat output rating as the multi-pak 80 (620 btuH vs. 840 btuH), but would it be a better choice, given the fact that the rest of the house (in a separate zone) uses cast-iron radiators? Would would be the advantages and disadvantages of replacing the slantfin?

Unknown

Mixing F/T with CI radiation

is not recommended because of its ability to hold the heat, and fin-tube`s ability to quickly give-it up. As you well know, its very uncomfortable.
As the CI rads need less water temperature, the only way to counteract this is using a C/V on each flow(in the circ), then a mixing-valve to temper the section feeding your CI rads.
If they are rads-on-feet, TRV`s may be a good choice for each of the CI ones.

Dave

JeffGuy

Let me be clear that these are not mixed within a zone, but are in separate zones - or they're supposed to be. As you say, using check valves should fix most of this. The water temp does not go above 170, so I'm not sure I will need a mixing valve (both zones seem to be about double over-radiated based on a room by room heat loss I worked on last year). And I've looked at TRV's, but don't want to put those in unless I find I have a real room-to-room balance problem once the boiler is replaced.

Assuming that check valves are installed with the new boiler, would your comment about holding heat vs. giving up heat mean that replacing all the F/T would make a big improvement - or is it not needed if the other changes are made?

Unknown

Well,

try all you want Jeff, but you`ll NEVER get comfortability mixing fin-tube with CI rads.
Regardless of CV`s, heat is still creeping where you don`t want-it, right?
If you install C/V`s (at, or in the pumps) for each zone, how are you going to control the water temp for the CI side?, which is overheating.
A M/C is an ideal choice, but that alone wont solve your balance issues.

Dave

tbuck

F/T & CI

Cast iron radiation and fin tube can work well together in a system as long as they are controlled and circulated seperately. It sounds like this part was done correctly.

Heat calls in a cast iron "zone" tend to be long; it takes a while for all that metal and water to get hot enough to satisfy the thermostat. Once the demand ends the radiation continues to "radiate" for a long time. So heat demands are looong and the so is the time between them.
Copper fin heats up and begins to heat the space very quickly. Also when the thermostat is satisfied it doesn't retain much heat. So the calls for heat are relatively shorter and the time between them is shorter too.
They operate differently but are not incompatable...

Your fix:
Two things that can make hot water go where it's not supposed to are gravity flow and induced flow.

Gravity flow takes place because cool water has greater density than hot water. Thats why gravity systems worked. If there is hot water in the boiler and cool water in the radiation the two are just naturally going to "trade places".
Induced flow occurs when a circulator operating in one zone causes flow in another.

You could have either or both. The purpose of a flow control is to prevent these things from happening. If you look at many boiler manufacturers installation instructions they show flow controls on both the supply and returns of each zone. That's not to make the manufacturers more money. It's because in a situation like yours the problem has nothing to do with the piping or installation; it's the damn boilers fault.

In your new installation there should be a flow control in both the supply and return pipes to the cast iron system (you can get gravity flow within 1 pipe) and at least 1 in the copper system. If no other piping oopses are present that should eliminate the hot/cold problem.

I would not change the fin tube unless it's badly beaten.

Enjoy your new boiler - the oil company is going to hate to lose you!!

Unknown

Are you sure?

The boiler runs at 1 temp, F/T radiation takes the higher,, CI rads will be lower,, again, how will lower temps be maintained from 1 boiler to satisfy both requirements?

Dave

tbuck

It is what it is...

Gravity systems were designed with a 165 supply water temperature with a 35 degree drop through the system (it was an open system - if the temp exceeded 211 the result was most disturbing to occupants). The temperature the radiators have to achieve to satisfy the system may be much lower than 165.
Baseboard systems are designed with 180 degree average water temp (190 out, 170 in). With standard residential BB that results in about 600 btu's per foot. That may be why the remodel used the "semi commercial" baseboard - to get more btu's in that zone while the CI zone is calling. If only they had installed flow controls I doubt we would be discussing this

Up until the condensing products came along boilers needed protection from low return temps in CI systems. It was a whole different discussion. Now the mod coms eat up low return temps and their efficiency goes up as return temps go down.

It's possible to provide 2 temperstures without getting too pricey - use primary/secondary with the CI as the primary and pipe the BB off the boiler/secondary piping. That way if both are calling the baseboard would always be getting somewhat higher temperature water.

If I was doing it I would tie in doing as little as possible to disturb the existing piping - it's old. With proper flow checks installed, if both call at once the return water is coming back at 65? The boiler puts 100% firing rate into it and the supply water goes out at ___ (fill in the blank - I'm not wearing my long distance x-ray glasses). Whatever that temperature is it's going to both zones and as the heat demand goes on; it rises...70,75,80,85,etc. The BB doesn't put out any heat to speak of until the supply water gets up to 100 degrees. Then it's only 135 btu's per foot; not much. So maybe the family room/kitchen temp drops a bit. When the water temperature gets to 160 it's putting out about 500 btu's per foot. Eventually the CI zone is satisfied and the BB gets whatever it needs...and the CI is not going to call again for quite a while.

In this scenario the BB zone may drop a degree or two below setpoint...but that is a hell of a lot better problem to have than what he is experiencing now!!