Heat loss calculation

Is that radiator on its own thermostat?

If not, the calculus isn't whether that radiator is right-sized, it's how it compares to the other radiators that are on the same zone. That radiator could be perfectly sized, but if all the other radiators on that zone are double the size they need to be, the thermostat is going to keep the other rooms comfortable and that room chronically underheated.

So tell us about where the thermostat is and what other radiators are controlled by it and how they're connected.

A quick check is to look at the other rooms and see how many feet of exterior wall they have and how many feet of radiator. They should all be roughly the same.

You have a room that is 99 sq feet. At 30 BTUs per square foot x 99 = 2970 BTUs required. Average baseboard is 570 BTUs per foot.x 8= 4560. The problem is not a shortage of baseboard. If this room is the last one on a zone, it could be a water temperature issue. Not knowing what piping system you have ( series loop, mono flow or paralell flow ) Series loop systems where they put in too much baseboard on the loop. ( I use 60' of element as a cut off ) is a common problem.

Good luck

Steve

Check the supply and return temp at both sides of radiator on the copper pipe at each end, see what the temp difference is, then compare it to other radiators where rooms tend to heat well. Best if you had a digital thermometer with a bare end thermocouple wire. Strap tightly to the copper pipe with a zip tie. Make sure boiler has been running a while and mostly up to temp. the in and out temp difference should be close to each other if getting decent flow.

You can control output of radiators on a series loop by closing the air flow thru the baseboard with the damper that is included with the baseboard. This however will reduce the total output of the baseboard that is closed off. The result of that adjustment is to get the lower temperature room warmer. But if that reduced flow thru the baseboard on that series loop causes the ability of that loop to maintain temperature on design temp days (Coldest days) then the fix is not really a fix.

I would ask @Tonerlow how many feet of element is actually on the problem loop? i agree with @Mustangman with the maximum amount of actual element being 60' with zone valves, and maybe 75' without zone valves.

Regarding Outdoor reset, I believe that the boiler is a Mod/Con that already has ODR in the control system. But that was a good suggestion (before knowing what boiler was there).

When you mentioned "dampening" the radiators, did you mean turning down a balancing knob, or just closing the louvers on the baseboard?

When the radiators are in a loop like that it's common to use monoflo tees, which allow you to use a single loop, which saves piping, but still allows for balancing valves on each radiator. If you look in the basement you can see if there is a tee at each end of each section of baseboard, with a pipe that bypasses the radiator.

If you have tees like that you can do some sort of balancing, either manual or automatic. If you don't the only way to change the balance is to change out the radiators.

If you were to read this booklet I used in my Hydronics Seminar, Zoning Made Easy
https://s3.amazonaws.com/s3.supplyhouse.com/product_files/108119-Reference%20Guide.pdf You would be able to answer all the questions in this workbook. https://documentlibrary.xylemappliedwater.com/wp-content/blogs.dir/22/files/2012/12/FH-Z300_BG-Zoning-Workbook.pdf So If you were to look at this question on page 6 of the workbook

Then you would see that your 75 feet of element at the maximum based on rule of thumb 170° average water temperature with a 20° temperature difference (∆T) between the inlet and outlet of the boiler.

Now consider this, with the ModCon boiler reset curve set to offer lower water temperature What is the possibility that the ∆T is somewhat different and since the starting temperature is much lower, will the resulting ending temperature be low enough to cause the radiator in the last room to be insufficient?   This may be a result of incorrect near boiler piping, If the new circulator on the system loop is moving the heated water thru the loop slower, then the heat can be released to the other rooms and by the time it gets to the last radiator the temperature just ain’t there   If you are not using the proper piping design from the I/O manual, then your flow rates may be way off from the flow rates you had with the old boiler.

I know that you can set the ODR to operate like the old boiler with higher operating temperature all the time.

If it uses monoflos it can't be reversed.

The Monoflo® tee idea is not probable.  Since you can see that the first floor is a series loop, it is highly unlikely that there is anything other than a series loop upstairs.  A second clue is that you can purge all the air from a series loop system at the boiler room (basement), but you can NOT purge a Monoflo® tee system.  Each pair of tee branches must have their own vent at the high point of that branch loop.  Purging will not force water from the One Pipe Main up a branch tee across the radiator or convector and back down the return to the One Main Pipe.  Gravity don’t work that way.

I'd say it's worth opening up one of those access panels just to be certain how the circuit is plumbed.

The installer of the new Viessman made a mistake in using the wrong zone valve for job. If the installer used the existing zone valves or replaced the zone valves with these

, either way it is the wrong one. I believe that configuration of the V8043 valve has a Cv rating of 3.5 or maybe 4.0. If the installer replaced the zone valves with V8043 that looked like this

Has a Cv over 7.0 or even as high as 8.0. That valve you have is restricting the flow of water to that zone/loop. Get better zone valves for that 70+ ft of element upstairs. If you look at the Workbook illustration from page 6 posted above, and use the basic rule of thumb on the Zoning Made Easy Textbook on page 6 you can clearly see that the poor design will cause the problem you are experiencing. Get the near boiler piping correct and get a less restrictive zone valve, and your temperature difference will be reduced because your pipe can carry more BTUs of heat.

At this point you have to decide whether it was once working and has stopped, which means it needs to be fixed, or whether it was never working, which means it needs to be redesigned.

In terms of fixes, if it is a string of radiators in series, your options are pretty limited. You can increase the heat output of the string by increasing the water temperature or the flow rate. You can decrease the drop in output from first to last by increasing the flow. How you get there is a separate question, but those are your two options.

There are ways to change the balance between rooms, but they are so invasive I wouldn't just do them based on guesswork. My inclination at this point would be to do at least a back-of-the-envelope check of the engineering. The standard is to do a room-by-room Manual J, I know there is free software that can help with it. Once you have the room-by-room heating loads you can compare them to what's there and see if that's ever going to give good results, or if it's just too far removed from what's needed and you need to start making changes.

The most difficult part about doing Manual J is in an existing house you have to make educated guesses about what the existing insulation is, and often that's not apparent. What I like to do is follow the process laid out in this article:

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

to calculate the whole house heating load based upon your actual fuel usage and weather history. Then do the room by room, add up the rooms and compare to the whole-house. Keep adjusting your assumptions until the room-by-room is reasonably close to the whole-house.