Uneven heat with loop baseboard system

241comp

TL;DR
The last room on each of 2 baseboard loop zones is colder by 3-5 degrees, possibly due to insufficient baseboard radiator length. Attempts to balance the temperature in these rooms with the rest of the zone have helped reduce the differential from 6-7 degrees down to 3-5 degrees. Suggestions on next steps would be appreciated!

Background:
I have a valve-zoned radiant heating system with 4 zones that are baseboard loop and 1 zone that is a low-temperature radiant floor (slab). The boiler is a Burnham Revolution RV5 and the circulator is a Taco 007e. We use setback both day and night, throughout the unoccupied zones (bedrooms set back by day, living spaces set back by night).

The problem:
Two of the baseboard loops are relatively large and have an uneven heat situation. One covers 3 beds and 2 baths (~800sf) on the 1st story. The other covers the entire 2nd story (~900sf). Both loops have 180-deg supply and about 160-deg return temps. The problem that we have is that the last room in both of these looped zones is significantly colder than the rest of the zone. Each loop has a total of ~25ft of baseboard. The last room in the bedroom loop (1st story) is a ~175sf bedroom with ~3ft of baseboard radiator. The last room on the 2nd story is a 180sf office, also with ~3ft of baseboard radiator. Both of these rooms run 4-5deg cooler than the rest of the zone, depending on the outside temps (greater differential with colder temps). My guess is that these two rooms are simply under-radiated as they have less baseboard per square foot than other rooms in the zone. However, they both have less outside wall length, which may be why it was designed/installed this way.

Solutions already attempted:
1. Increased the insulation where possible in both the bedroom (below the floor, above unconditioned basement) and in the office (knee walls). This closed the gap by about 1 degree.
2. Partially or mostly closed the baseboard radiator vents in other rooms in the zone. This helps close the gap by about 1-2 degrees but leaves these rooms slightly cool on single-digit days (such as today) so we end up opening the vents again.
3. Insulated the (copper) supply pipe before and between the baseboard radiators, in the basement ceiling. These seems to have evened out the temps by 1 degree or so.
4. Added remote sensors in these rooms and scheduled the thermostat to use these sensors when we want it to. This allows us to warm the rooms to the target temperature, but results in the rest of the zone being too warm by 3-5 degrees.

Possible solutions:
2. Larger circulator for higher flow rate, resulting in higher temps at the last baseboard. This might not work, however... typically only 1-2 zones are calling for heat simultaneously and the 007 should be sufficient.
3. Increase the length of existing baseboards or add secondary baseboards. This would be difficult/expensive to accomplish on the 2nd floor, but would be relatively straight-forward to do on the 1st floor because the basement ceiling is unfinished.
4. Replace existing 3ft baseboards with Haydon Hi-output 958 baseboards for ~50% increase in BTU output.
5. ????

Any suggestions on what my next step should be?

hot_rod

Small gains by increasing flow, you can go up to 4 gpm in 3/4 fin tube, maybe 5. But if you start hearing a hissing sound you are flowing too high, above 4 feet per second.

This graph shows the small output difference from increasing flow rate from 2- 4 gpm. But it may be just enough? Increasing SWT is a fairly linear increase, but flow changes, not so much.

You may be down to replacing with high output board.

Or electric space heaters

george_42

do not go electric, ad high output to cold rooms