He correlated with heat would be expected from atmospheric contamination where (a) both excess heat and He are correlated with time and (b) excess heat could be related to specific physical conditions in the electrolysis cell that promote ingress of atmospheric air
There are no physical conditions in the cell that promote significant ingress, because the background level of helium is the same in blank tests, including deliberate ones with Pd-H and Pd-D that does not work.
The absolute temperature cannot be a factor because the cell is often warmer during a blank run than a run with excess heat.
Virtually no air enters the cell or collection flask. The effluent gas goes through a bubbler to exclude air. If any air entered the flask, it would swamp the background and the helium from cold fusion. The helium level would be totally random, not correlated to heat or anything else.
Miles once illustrated this during a lecture. He was projecting a graph of background helium and helium after a collection period with a cell producing excess heat. The latter was much higher. Quite significant. He moved the laser pointer to the ceiling and said something like, "if this helium were leaking in from the atmosphere, the level would be up there at the sixth floor." In other words, there is no mechanism that would allow you leak in such minute quantities of helium. You could not do it with any sort of needle valve, for example. The only method would be to let it permeate through glass for a few years. How minute is the amount? If you touch the inside rim of the metal flask, the helium from your fingerprint will swamp the background helium and the helium from the reaction. Miles always wore gloves when handling the flasks.
I copied the configuration schematic into this paper, p. 5:
Let me add that various methods are used to check for helium leaks from air, such as looking for argon.
In tritium studies, tritium leaks from laboratory air are ruled out for the opposite reason. In order to leak that much tritium into a cell at a lab like Los Alamos, you would have to increase the atmospheric tritium concentration to such a high level the alarms would go off and the building permanently abandoned, according to Storms.
checking atmosphere for He levels does not help (alone) since the nature of many lab environments is that you get sporadic high levels of He which over time average to a level well above the modal value
No such sporadic changes in background helium were observed in the blank tests. If changes in the lab environment occurred and if they could induce significant variations in the helium collected in the flask, this would have been observed, because these tests were conducted many times over several years.