HOWS doesn't produce that much light, so when you tune it to the infrared spectrum it doesn't produce much heat either. The infrared HOWS that Hazō just created wouldn't even be sufficient as a handwarmer, let alone to heat a room.
If I may suggest a number and possible modification to this description?
Jiraiya's Awesome Daybright seals are "about as bright as a modern LED bulb that you would use in a table lamp" - which I'm interpreting to be a standard 60W (800 lumens) bulb.
HOWS are "about a third the brightness" - which I'm therefore interpreting as approximately 20Ws (20W incandescent bulbs actually produce closer to a fourth rather than a third of the number of lumens, but I'm having a hard time finding a good information source for lumens per Watt, it's too variable based on the bulb type and housing and most (especially LEDs) have a ton of waste heat with only a small percentage of the input going to illumination (5-15%)).
Infrared heaters use about 25-30W per cubic meter (based on various commercial sites for that technology).
Which means it should warm the hands well, perhaps even be a bit uncomfortably hot immediately adjacent to the seal, more comfortable about a foot away.
However, if we're going with only 15% of the energy goes into the lighting of a LED bulb, and only that energy exists from the seal which is converted into infrared, then it wouldn't likely be enough for the heat to even be noticeable - it would be like the heat emitted from a 3W LED. If the lighting from seals using chakra to energy works in ways we're familiar with and can compare to our modern technology (e.g. a modern desk lamp) then I suggest we use the math and conclusions from above this paragraph.
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Further math and considerations (assuming it's producing noteworthy heat like an equivalent Watt of a light bulb in infrared):
Rooms in Leaf are almost certainly smaller than modern rooms, especially in terms of height, especially civilian rooms, assuming a 2x2x2m room (which may actually be on the bigger end for the average Leaf civilian), that's 8 cubic meter, and would require 200-240W to keep warm, so 10-12 of these seals.
Noteworthy, that's to modern expectations of warmth, i.e. high 60s to low 70s, fewer seals could heat the same space at a lower temperature. It also assumes modern insulation, so that makes them a bit less effective in Leaf (probably?).
So 12ish seals for "weeks" (3-4?) of heating a room and not having to pay for firewood or be worried about the smoke might very well be economically worthwhile, assuming we can't make it better, which we should be able to once Hazou determines that was the seal working properly and not a seal failure.
Putting that aside, while not warming the room entirely, one pointed directly at a person laying in bed from a foot away should definitely warm them through a cold night, not entirely, but definitely in a way likely to help save lives in a pre-industrial society if firewood isn't readily or cheaply available. 2-3, properly placed, should keep a bed fully warm at a safe temperature.
If nothing else, it frees up the labor currently used to gather firewood though it makes those laborers unemployed. Even if just used in the Clan, with no improvements to the seal, that would probably be a substantial benefit - and may be advantageous with the Dragons since heat emissions are likely less detectable than light by the Dragons, especially since the light is sure to make some heat anyway.