I need a little guidance sizing radiators...

TimCAD

Hey everyone, I'm hoping someone with more know-how might be able to help me figure out where I'm going wrong here.

We're refinishing the lower of our split-level; half below-grade on one side. The lower level was previously heated on its own zone, which consisted of one, long, series loop of slantfin running round half the perimeter (that which was below-grade).

I would like to replace that zone with a series of radiators or wall panels (Runtal or Buderus are my top choices at the moment), because I hate having the entire baseboard of a room occupied by radiators that complicate furniture placement.

So I'm trying to size radiators for each room, using various online calculators to help me out with a ballpark, and I'm getting a variety of different answers. For example, my office: 15'x13', 7.5' ceilings. 2x4 Framing, slab on grade underneath, with a heated room above, and a single 3'x3' double-glazed window. 2543btu, 4472btu and 6268btu are the varying answers I've gotten trying to figure out how big a radiator this particular room will need. Obviously, they can't all be right

Any tips on sizing the radiators per room would be appreciated!

Ironman

You need to size the new radiation according to the old or it will be out of proportion to the rest of the system.

Standard size BBs have an output of about 500 btus per lineal foot of element at 170* average water temp.

Use 170* AVERAGE water temp to size the new as well, not 180*.

TimCAD

Hey, thanks for the reply.

When you say it will be out of proportion to the rest of the system, you mean out of proportion to the other two zones?

I can't size off the old BB radiators for two reasons, 1) they're in the garbage and 2) We've re-framed the entire lower level. The room sizes are no longer equal to what they were for those original runs.

This zone is disconnected all the way back to the zone valve, from which I just want to run a new series loop using a different type of radiator.

The boiler is a Slantfin CHS 155, with three zones off it: the downstairs, the upstairs and the master suite. If I can accurately size the radiators to heat the rooms downstairs, doesn't it stand to reason it would be roughly in line with what was there previously? I mean, the total cubic footage didn't change, it's just chopped up differently now.

And I'm still feeling lost in regards to determining a radiator size for a room.

TimCAD

Well I can't size off the old for two reasons. A) The old bb runs are in the garbage. And We've reframed the entire lower level. So even if I had the old runs, they'd no longer be accurate to the new room layout.

However, I have to imagine when they installed that zone, they calculated the heat needs of the rooms. If I do the same (properly size my radiators), doesn't it stand to reason that it would fall into the same ballpark? The cubic footage of the lower level hasn't changed, it's just divided up differently now.

If it helps, my boiler is a Slantfin CHS 155, with three zones (upstairs, downstairs and the master suite). Since one of those zones heated the lower level, and I'm looking to use that same zone to again heat the lower level, how would I end up out of proportion?

And I'm still not clear how to determine what size radiator each room requires.

For instance, in the example above, entering all my dimensions for my office into a calculator spits out a requirement of 2543btu to heat that room (assuming that's an accurate calculation). Could I then mount this radiator in that room and expect to be all set? http://www.supplyhouse.com/Runtal-UHX-6-36-17-25-W-x-36-H-Vertical-Flat-Panel-Radiator-3360-BTU

newagedawn

what program are you using for the heat load calc?, 2543 sounds close, under grade spaces need less btu's, due to the fact that its below grade, and you have heated spaces above,your program should be able to do this calc, if its a reputable program trust the numbers, as long as youve put the right info into the program,..ive done whole houses that are 1600ft and only have 50,000 btu's

ratio

I think what Ironman means is to measure the length of the old bb (only the parts with fins) & cypher up what the output of the old baseboard was, & distribute that the throughout the new rooms in proportion to their size. The reasoning is thus: the system as a whole was (presumably) sized for x BTUh output on that zone, & a significant variance will throw the whole system out of balance. Also, unless you've done improvements to the exterior walls or slab, the total heat lots of the space as a whole hasn't changed much.

I would consider a series system with TRVs for the new rooms, in an attempt to keep the head loss about the same as before & not throw off the system-wide balance. You will need to cypher up the ΔT of each panel/radiator to size them properly, as the AWT will drop through each one and the later panels will need to be bigger.

Maybe someone else will have a better idea.

Ironman

The amount of radiation needed is determined by doing an accurate heat loss calculation. Most, if not all, of the small online calculators that only look at square footage are useless and very inaccurate.

SlantFin has a free load calc that you can download that works well - if you enter the correct info.

If that area of your house has been substantially tightened up, then it's gonna require less heat than was originally installed. However, as Ratio made clear, balance is an issue. The fact that the system is zoned helps in that area, but a lot more info is needed.

How is the zoning accomplished? With zone valves or multiple circulators? If ZVs, what circulator do you have?

Some pics of the boiler, its near piping, circulator(s), controls, would be helpful.

TimCAD

Absolutely. I'll admit, I was working with those basic online calculators, which I knew based on the bare minimum of info they were asking for, couldn't be accurate. But I hoped they'd put me in the ballpark to start with.

I've downloaded Slantfin's app, and will see what that tells me. In the meantime, here are a few pics of the boiler setup.

Here's most of the region. You can see the CHS 155 boiler, the Bock tank being used as hot water storage, and the four Grundfos circulators. Three (on the left) for the three heating zones and one on the right (above) for the hot water.



Closer look at the three heating zones on circulators (the one on the right is off, that's the zone for the lower level we've disconnected).


And the current settings on the boiler.

Ironman

That's a very large boiler - especially for a mod/con. What's the square footage and age of your house?

I suspect that someone replaced a cast iron boiler with a mod/con of the same btu rating?

If that's true, the boiler could be short cycling which will use more fuel and result in an early death. Zoning will compound that problem much more.

Over-sizing your radiation would actually be a good thing as it would allow lower water temps which would increase the efficiency of the boiler and aid somewhat in longer run times.

I would suggest that you use the SlantFin app to do a calc of your entire house to see how closely the boiler is sized to your actual heat loss.

Bigger is NOT better with boilers, it's worse. If the boiler is over-sized by much, then you should look at combining zones and/or adding a buffer tank.

TimCAD

House was built in '79. Roughly 2600 Sqft served by hydronic heating. We've put an 800sqft addition on, but that's heated/cooled by a separate forced air system.

When we bought the house, the heat was supplied by oil. We had the oil tank and boiler removed, and the new natgas system installed. I'm not privy to the thought process of the heating contractor on that job in terms of choosing this particular boiler.

I'm playing with the SlantFin Hydronic Explorer app. For the room I mentioned above, it's suggesting 3793 BTU/HR. Though where it's asking for Doors I'm not sure if wants interior doors or just exterior. For the door factor, it's asking about storm sashes and glazing, which makes me think it only cares about exterior doors...

TimCAD

I'm getting a total BTU/HR requirement of 14,931 for the lower level rooms I'm looking to heat, via the SlantFin app.

Ironman

Exterior. Everything that loses heat is to the exterior of the envelope.

Your boiler's twice the size it needs to be...on the coldest night of the year. At 35* outside, it's 4 times too large! That's with all zones calling at the same time. If only the 4K btu zone calls, then it almost 40 times the size it needs to be.

I would highly recommend that you install a buffer tank of at least 30 gal. An electric water heat tank can be used as a less expensive, yet reliable alternative. Don't connect the elements: you're just wanting the buffer of the water's mass to prevent short cycling.

TimCAD

Isn't that the function the Bock water tank is serving?

Ironman

No, that heats your domestic water.

Installing a 30 -40 gal buffer tank in your SPACE HEATING circuit would cause the boiler to run for a minimum of at least 10 minutes preventing short cycling and increasing efficiency. Short cycling drastically reduces the life of the boiler and its components. 10 minutes is the MINIMUM run time that a manufacture recommends. If the boiler and system are designed and sized properly, that is easily achieved. Over-size the boiler and/or zone up the system (like yours) and you have created the issue.

Your mod/con boiler is low mass and highly efficient in transferring heat. Zoning it will create issues that the old high mass cast iron boiler didn't have. Adding the buffer tank replaces the mass that the cast iron gave.

TimCAD

Ok, fair enough. I can discuss a buffer tank with a heating contractor. But if you're saying my system is over-sized (very possible) doesn't that mean I have some leeway with the radiators I install in the lower level?

Help me out with this basic principle: I do the heat loss calculation for a room (in this case, my office) and it tells me 3793 btu/hr... could I stick this radiator (listed at 4000btu) into that room and call it a day? http://www.supplyhouse.com/Buderus-3-22024-Model-22-20-x-24-Hydronic-Panel-Radiator

Or am I missing something else?

ratio

Well, yes and no. Assuming the heat loss is correct, that means that, at design conditions, that room will need 3793 BTU every hour to not drop in temp. Design conditions around here are IIRC 70* indoor, 0* outdoor. Any change in those conditions changes the heat loss, eg at 65* indoor, or 45* outdoor the heat loss will be less. That radiator will emit 4000 BTU every hour when fed with "X" gal/min at "Y" average water temp, when the room temp is 70*. Since you're unlikely to find a radiator that emits exactly 3793 BTUh, it's fine—however, we want to insure that the GPM & AWT we​ need match what's available from the system. If you don't check these numbers, you're taking a gamble. It will probably heat. It might make you comfortable. It won't be right, but it might be close enough.

Remember, it's a heating system, any thing you change will effect the whole system. That's why this sort of thing is easy to do poorly.

Ironman

TimCAD said:

Ok, fair enough. I can discuss a buffer tank with a heating contractor. But if you're saying my system is over-sized (very possible) doesn't that mean I have some leeway with the radiators I install in the lower level?

Help me out with this basic principle: I do the heat loss calculation for a room (in this case, my office) and it tells me 3793 btu/hr... could I stick this radiator (listed at 4000btu) into that room and call it a day? http://www.supplyhouse.com/Buderus-3-22024-Model-22-20-x-24-Hydronic-Panel-Radiator

Or am I missing something else?

Not necessarily. The boiler only has to be large enough to supply enough btus to the radiation. Over-sizing the boiler will not cause the radiation to increase its output, but under-sizing it would cause it to decrease.

Look at the spec sheet for radiator on the site. It shows the output of that rad being 4000 btus at a 108* delta T. That means a temperature difference of 108* between the Supply Water Temp (180*) and the room temp (72*). Basic law of physics: heat moves toward cold; the greater the temperature difference (delta T), the faster it moves. So if you supplied 152* water to the rad, the delta T would be 80*and its output would be 4000 X .677 = 2708 btus.

The principle that your mod/con boiler operates on is this: the lower the return water temp (RWT), the higher the efficiency of the boiler. The boiler will began condensing its flue gasses at 140* RWT and below. When that happens, the gasses give up their latent heat to the water and efficiency encreases. Therefore, we want to design the system, and set up the Outdoor Reset curve (ODR) to keep the water temp as low as possible. This means that over-sizing the radiation is highly desirable.

Here's your situation: you have a 38 year old house that has baseboard rads that were designed for 170* average water temp (AWT). That what's needed at design temp (coldest night of the year). But at 35* outside, your heat loss is half of what it is at 0* outside. That means your water temp could be lowered to where your radiation's output is reduced to 50% at 35* outside. Which would be about 140* AWT. If your house has had the envelope tightened up (more insulation, better Windows, etc), then the heat loss is even less and the water temp can be adjusted even lower resulting in greater boiler efficiency.

This is why I recommend calculating the entire house. Then measure the amount of lineal feet of BB element (not the enclosure length) that you have. From those two things, your necessary SWT can be determined. From that (SWT), you would size the new radiators to match the heat loss for that area.

I know this is probably more detailed than what you wanted to hear, but it's the only way to do it right and get maximum comfort and efficiency. There's no short cut where you can do that and "just call it a day". Considering the little amount of time and effort required to do it right vs. the long term benefits, it's well worth it.

TimCAD

Actually Bob, that was a perfect amount of detail.That's a huge help for getting me closer to understanding the way the system is working and why. It's a clearer explanation than I've found anywhere else.

And with that, I am off to start measuring linear footage of baseboard for the other two zones

TimCAD

So I went around and calculated heat loss per room using SlantFin's app. I also measured the linear footage of finned radiator in each room. Zone 3 is the blank slate re:heating.

This is not to scale, but I drew up a quick layout, since it's an easier way to present information.




At 500btus per linear foot of standard radiator, these are oversized. But as you explained, that's at 170*, and my system is not outputting at 170*. I believe "Outlet Temp" is 150*. I can't find the BTU/linear foot for standard BB at that temperature.

I have, as you mentioned, done a lot of work on the house. I've added a ton of insulation to the attic, replaced all the windows, etc, and due to an addition we put on, some formerly exterior walls are now abut another heated space. So all of that changed the heat loss from whatever it was when they built the home.

Bob Bona_4

Just keep in mind, boiler is supposed to be sized for the heat loss of house. Your radiation count just gives you an idea if it's enough or oversized for the rooms. You may find more linear feet in one room and just enough in another. That will come into play when you start resetting the boiler temps.

TimCAD

And to determine the HL of the house, I just add up all the rooms I calculated?

On my floorplan above, in zone 3 there is a utility space, and a bathroom next to it that are surrounded on all sides and above by finished, heated areas. I'm assuming those don't even need an active heater, but do they contribute to HL?

Ironman

So, your boiler is 3 times the size it should be. This is what happens when contractors won't, or can't, do heat loss calcs. No doubt it was sized based on the previous boiler which wasn't correctly sized or was over-sized because it had a tankless coil.

That being said, your total heat loss is 46.6k btus, of which 31.6k btus is being served by 95.5 ft. of BB. You would need about 145* - 150* SWT. Average Water Temp is the difference between supply and return which is generally sized for a 20* delta T. So, you would want to calculate your rad's based upon a 150* SWT.

As mentioned, if you connect them in series, each rad farther down the line receives a lower SWT and may need up-sizing to compensate.

This means that a 20 x 36 x4 or a 24 X 36 x4 Buderus panel rad would be the correct choice for the office.

If TRVs are used, you'll have better control over each rad, but you'll need a by pass on each one or a delta P circulator such as the Grundfos Alpha.

TimCAD

So then, knowing that my SWT is 150* (which is where it's currently set, based on the photo I posted?) I could also use the 20x48x2 Buderus for the same output (3900btu)?

Likewise, for a larger room with 6k heat loss, rather than doing one huge radiator, I could two smaller ones at 3k each?

As far as tackling the too-large boiler problem... you suggested a buffer tank would be healthy for the boiler itself. Is there anything else I should do? Look for more ways to use that extra power, such as joist-tracking some radiant floor heating etc? Or does that not accomplish what I'm imagining it does?

Also, TRVs seem like a positive addition to a radiator system. Do you recommend them, or is it "just one more thing that can go wrong" in the system?

TimCAD

Oh, and another general question about the placement of a radiator wall panel like the Buderus:

It's my understanding that while standard baseboards work mostly by convection, and thus are almost required to be placed along outside walls/under windows, these panel rads also heat through radiation; that is warming the objects in the room.

Considering that the rooms in the lower level do not have a lot of window area, and that those windows have been replaced with brand new, double-glazed windows (so there won't be as much of a draft that needs a thermal barrier)... do I have some flexibility in determining where to put these panels? Can I get away without necessarily putting them under a window, and still expect them to heat the room they're in?

Ironman

Yes on using multiple rads in a room. But remember, use the output multiplier in the spec sheet to get the corrected output.

No as far as adding radiant to help short cycling. It won't change the heat loss of the house.

newagedawn

wow, getting a good edumacation on heat load calcs,please post pics of finished job