Would you replace all this fin-tube with bypass?

dgkula

I bought my house in 2010. It was built in the 70's and in 2009 the owner had a mod-con boiler installed. It looks like they replaced a CI boiler as I can see the closed chimney vent in the maintenance room next to the new mod con.

Prior to the install of the mod con it looks like each radiator was independently and locally controlled as each run of fin tube has a bypass and a local valve. Some of the valves are gone, some are still there but the wires are buried in a basement ceiling or something. So in effect the whole house was run as a single loop with each run of fin tube locally controlled. When the mod con went in, they chopped the house up into zones and put zone valves at the returns in the basement and tossed the controllers/thermostats for the local zones.

Locally on each run of fin tube, the finned element does get hot and radiate but there is a non-zero amount of water running through the bypass as the bypass gets hot as well.

One effect of this is that water that hasnt lost much of any heat gets mixed with the water that has run through the fin tube and returns to the massively oversized boiler and the boiler short cycles as the return temp climbs into the "too-high" zone.

Another effect is a loss in heating efficiency I think. In each run of fin tube some amount (hard to measure, maybe half or a quarter) of the heated water runs through the bypass and isnt used to heat the room! As a result my living room gets cold and we have to run a space heater to supplement the fin-tube.

So the company that installed the mod con, chopped the house into zones and threw awy the valve controllers really screwed this system up IMHO.

My question is if I should replace the "fin tube with bypass" with standard fin tube. I think my family room would be warmer. I would waste less heat. Or is there another option?

Also is there any good way to "close" these valves so that no water runs through the bypass? I think they are mostly magnetic with a spring and would return to the "max heat" position if not energized to avoid pipes freezing in a power outage, so they may be at max open now but here's still a lot of hot water running through the bypass.

Thanks,
David

Jamie Hall

That's an ingenious arrangement... or was. I've not seen it before. Hopefully someone else has.

I doubt that it is actually costing you much in terms of efficiency -- the heat from the bypass pipe is, after all, going into the space as well, just not as much heat as would be if the water went through the fin tube instead.

I expect that the original idea was that the original boiler -- which may have been smaller -- would run based on a return water temperature, or possibly a temperature differential, and that the default (power off) setting of the valves was open -- otherwise you would get no heat with the connections to the valves broken. Then individual thermostats would power the valves closed if the space go too hot. It may have been the reverse, however -- default valves closed and power to open the valves.

Studying the pictures there doesn't appear to have been a way in the original system to direct all the flow through the fin tube, which is very like the way a monoflo system runs.

I think you may need to do some experimentation on the system and determine whether the valves are even operational still, and if so, whether they power open or power close.

Further, you need to find out whether the system was originally a series system, which I suspect it may have been, and if so if when it was zoned if the individual zones are still series connected.

Now -- having found all that out -- the question is with the valves open is there sufficient radiation in each of the spaces at the temperature your new boiler runs at to adequately heat the spaces? I gather from your comment on space heaters that the answer is that there isn't, probably because the new boiler runs at a much lower temperature than the old one did. So then the next question is what is the minimum temperature, with the bypass closed, that there will be enough heat?

It's critically important to understand that whether the fin tube in a given space can heat the space or not is not determined by the relative flow through it or through the bypass, but by how hot the water is glowing through it. Pushing more water through the fin tube won't help a bit if the water is too cool.

Having determined that the fin tube as installed is adequate, and provided that temperature is in a range that your new boiler can reach, and provided that your new boiler has enough total power to heat the whole structure at once (neither may be the case, unfortunately), the first thing I would try would be to simply raise the circulating water temperature of the new boiler to whatever is needed to provide adequate heat.

If it can, the effect of the bypasses will have been to raise the return temperature. If this is higher than desired, you can figure out a way to close the bypasses -- but that will take cutting them and installing caps, so I'd not suggest doing that until warm weather.

EBEBRATT-Ed

Automag is still in business. I was surprised as I never herd of them.https://www.automagzonevalves.com/

Jamie Hall

EBEBRATT-Ed said:

Automag is still in business. I was surprised as I never herd of them.https://www.automagzonevalves.com/

Dang. Learn something every day. Thanks, @EBEBRATT-Ed !

rick in Alaska

I guess I didn't notice it said Automag on it. There are a couple of houses that I still service that use Automag zone valves. They are a dc controlled valve that are powered open, which means they also use an air conditioner thermostat. That is something you really need to know when servicing one.
Rick

Jamie Hall

Thank you both, @rick in Alaska and @EBEBRATT-Ed ! It's a rare day I don't learn something hanging out here...

So. Now the next question for @dgkula is are those valves open? The literature says that they power close ( https://www.automagzonevalves.com/_files/ugd/39dc32_96995d5ad58143fb825453fa249aa5e7.pdf )? Or stuck partway open? Or what?

If they are in fact open -- which would be seen by the fin tube getting warm at the same time as the bypass -- then I'll go back to what I said earlier: look at the heat requirement oof the space and determine what water temperature is needed to meet that, given the amount of radiation present. Then adjust the boiler supply temperature to at least that and see if that helps.

psb75

As stated in their literature those valves are powered closed. If they don't have power...they open. And I sure HOPE so...because I have one installed on a heat-dump zone for my wood boiler. If my power goes out (on a rural electric grid) the valve opens and my wood boiler can thermosiphon.
I suspect the original poster's valves are not powered and are open. Unless they are stuck closed.

Jamie Hall

I kind of agree, @psb75 -- and I also still think that the fundamental problem is that the boiler supply simply isn't hot enough for the fin tube to meet the load from the space.

To go back to the title question -- no, I wouldn't replace it all until I had looked into the low temperature problem. And, quite likely, not even then -- a lot cheaper to run up the control on the boiler, and it won't use much, if any, more fuel.

dgkula

My boiler temp is controlled by a separate unit that has an outdoor thermometer. I provide top (A) and bottom (B) of the heat curves and it calculates the set temp (C) using that curve and the outside temp. The max temp is 194F. Top (A) was set to 180F and I bumped it to 190F. Right now it is 44F outside and that results in a boiler set temp (C) of 158. So I can raise the boiler set temp but within limitations - maybe I can raise the lower point (B) on the curve (now at 130) to 180F and the curve will run from 180 - 190 only. I can experiment with that to continue raising the boiler set temp.

The other question about heating the family room is related to the fact that the zone valves are open as I feel hot water in both the bypass and the fin tube. However, they seem to be about the same temp so my concern is that a sizeable amount of hot water is running through the bypass. I agree this water will radiate heat out of the bare copper tube but at about 1/10 the rate of the fin tube. This results in a substantial loss in heat output compared to fin tube alone - maybe I am only getting 60% of the heat output compared to removing the bypass and only having the fin tube.

The other challenge with the bypass is that since it only emits 1/10 the heat of the fin tube, and the water that goes through the bypass + the water that goes through the fin tube mix at the end of the run and return to the boiler. The boiler expects a temperature difference between the outgoing water (just heated) and the returning water (which has emitted heat to a room). In this case, I believe the high heat content of the water that travels through the bypass is resulting in a returning water temp that is too high and the boiler short cycles.

When it short cycles, the boiler stops heating and the circ pump runs and the return temp drops over time as more heat is radiated. Then the boiler fires up again and the return water temp rises fairly quickly and then the boiler shuts down, say in less than 5 minutes. So even if I raise the boiler set temp to 180F it will only run for a short period until it shuts down owing to too high water return temp. Then the circ pump runs until the temp drops to 140. So given this issue, raising the set temp can only do so much.

So I am getting heat, but the short cycling means that the water temp is far from optimal - it has to drop to 140 or so to start running again. I get a temp of 180F for 5 minutes, then it shuts down and circulates until 140 which takes say 15 minutes and then the cycle repeats.

Jamie Hall

I have no doubt that your perception that you are not getting the heat out of that family room fintube that you would like is correct.

However. You will not get any more heat out of it by putting more flow through it. Not One Single Solitary BTUh. The heat output of any radiation is determined entirely by the geometry -- basically surface area -- of the radiation itself and the temperature of the water (or other medium) supplied to it. Flow rate has nothing to do with it.

This is a common enough misconception -- you are not alone in this -- but a ten second thought experiment will illustrate: take that fin tube. Now supply it with a trickle of cold water. Not much heat? Now supply it with as much cold water as you can force through it. Um... still not much heat, eh?

Basically, the previous owner goofed. There was a nice cast iron boiler, probably supplying nice not water -- probably around 180 to 190. The various bits of radiation got nice and hot and heated the space. The thermostats in each space turned the untis off or on as needed, and the boiler started and stopped as needed. Now you have a nice highly efficient mod/con, no doubt at great expense, and it is set to provide much cooler water. Greater efficiency, you see. But it doesn't, and at those temperatures, cannot heat the spaces such as your family room with water at that temperature.

You have two choices, really: first, install more radiation in the family room so you can get the heat you need out of the low temperature water. Second, crank up that fancy boiler so it supplies hot enough water to keep the space comfortable.

dgkula

Jamie,

Thank you for your reply. I'm sure I'm wrong because you are clear that "heat output of any radiation is determined entirely by the geometry -- basically surface area -- of the radiation itself and the temperature of the water (or other medium) supplied to it."

I was thinking that if 1 unit of water arrives at the bypass Y, 1/2 of that unit of water will travel through the fin tube and generate 1 unit of heat; the other 1/2 of that unit of water will travel through the bypass generating 1/10 unit of the heat.

So I do simple math with (1/2 unit water x 1 unit heat) + (1/2 unit water x 1/10 unit heat) and get 6/10 unit heat output. I compare that to the whole unit of water moving through the fin tube and get 1 unit heat output. That's where I'm saying that I'm only getting 60% the heat.

But really I'm getting additional radiation from the heat of the water running through the bypass compared to the fin tube alone! So I think I'm getting 11/10 heat or 110%

So temperature is all I've got to work with, or add fin tube as you state - very helpful thx!

Do you think that the bypass water returning to the boiler without losing much heat could be a big cause of my short cycling as I wrote above? The boiler expects to maintain a delta T between supply and return and my thinking is that the bypass water emits hardly any heat which puts operation quickly inside of delta T and causes the short cycling.

Jamie Hall

"Do you think that the bypass water returning to the boiler without losing much heat could be a big cause of my short cycling as I wrote above? The boiler expects to maintain a delta T between supply and return and my thinking is that the bypass water emits hardly any heat which puts operation quickly inside of delta T and causes the short cycling."

Quite possible, although usually the boiler is more interested in the temperature being sent out than in the delta T.

psb75

I believe the mod con boiler is very "interested in the btus being sent "out" and then "thinking about" how to adjust its activity by "counting" how many btus are returning. Isn't this the boiler being "interested" in the delta T?

psb75

The boiler doesn't care how you are "handling" the btus, once they are sent "out". It is just keeping track of what it is sending out and how many are returning back to the boiler. It is also being informed by the outside air temperature so that it can keep up with the task it is charged with--that being... to supply adequate amount of btus according to the amount that are LEAVING the house.

EBEBRATT-Ed

A mod con control will look at the supply and return temps and adjust the input accordingly.

All fin tube baseboard is rated at two different water flows. 1 gpm and 4 gpm.

For example Slant fin 2000 with 170 average water temp puts out 500 btu/foot at 1 gpm and 530 btu/foot at 4 gpm not a big difference

Jamie Hall

@psb75 : don't lose sight of the bottom line in your concern about what's going out vs. what's coming back.

The problem you started this off with is that your family room isn't getting enough heat. The reason your family room isn't getting enough heat is that the fin tube radiation isn't giving off enough heat. The reason the fin tube radiation isn't giving off enough heat is that it isn't being fed hot enough water.

You enquired "My question is if I should replace the "fin tube with bypass" with standard fin tube. I think my family room would be warmer.".

The answer to that is no, it wouldn't.

If you allow your fancy boiler to create hotter water -- and you may need to go up to 180 or so -- that would solve your problem of the family room not being warm enough.

Delta T is very interesting, but it tells you nothing -- exactly nothing -- about how much heat your fin tube will produce.

Now you also mention possible short cycling of your boiler. This is quite possible, but a higher target water temperature may make it a bit less of a problem. Why? We have to go back to that dang fin tube. At some given temperature it will put out a certain power in BTUh. No more, no less. When your boiler is firing, it also puts out a certain power in BTUh; yours can modulate, so if the settings are correct it can reduce its firing rate without having to turn off -- up to a point. Let us consider, though, on firing rate -- perhaps the minimum. If it produces more power at that firing rate than the radiation can absorb, the water temperature will rise and the boiler will shut off and circulation will continue until the temperature drops again. Therefore, if you raise the water temperature the radiation will put out more heat and the boiler will fire longer at a time to keep up.

I'm really not sure how to explain things better for you. If you want that room warmer, raise the temperature of the boiler water. Period. End of story.

kcopp

That arrangement was a way to zone each room. I saw this a former fraternity. They had a main circulator that was on in heating season and each room had a valve that would open or close based on the need of each room.

Jamie Hall

kcopp said:

That arrangement was a way to zone each room. I saw this a former fraternity. They had a main circulator that was on in heating season and each room had a valve that would open or close based on the need of each room.

It's a remarkably simple, reliable system. The contractor who tore out the old boiler and the controls for the valves and put in a mod/con really hadn't done his or her homework to figure it out. Unfotunately, our OP is left holding the bag, which is too bad.