Hi amic,
Sit n Sleep doesn’t list the harbor firm but there is a rough description of it here Buying Your New Mattress is Easy at Urban Mattress which indicates it has a comfort layer of talalay latex (at least one layer but possible in combination with polyfoam) over a polyfoam core with an organic cotton quilting.
In general, the upper layers of a mattress along with the ticking and quilting and the mattress pad, sheets, and bedding that you use will have the most effect on how hot a mattress sleeps. The more insulating the materials are and the less they wick moisture away from your body, the warmer they will sleep and the more ventilating/breathable they are and the more they wick moisture away from your body, the cooler they will be.
There are three types of foam that are generally used in the comfort layers of a mattresses which are memory foam (and it’s variant gel foam), polyfoam (which is the most common), and latex foam. Of the three, latex, especially Talalay latex, is the most breathable/ventilating and the coolest which seems to be what you have in your mattress. Cotton also does a good job at wicking moisture away so based on these materials your mattress would be cooler than most.
The support system is much further away from your body so changing it for an innerspring will have much less effect on the temperature of your mattress than the comfort layers (although an innerspring is more breathable than a polyfoam core). You will still be sleeping on thick layers of insulating polyfoam (which is less breathable than latex) with your two other options and unless you go with a mattress that is much more expensive (which uses natural fibers over an innerspring and has no foam at all) you won’t be able to avoid foam of some type in your mattress. It’s just a matter of which type of foam and where it is in the mattress along with the type of quilting/ticking that the mattress has.
This leads me to suspect that the problem is either in the type of foam that is in the top layers of your mattress (ie the talalay is not in the top layer of the mattress or it is just a very thin layer) or that the problem could be in your mattress protector or sheets.
Do you know the layering details of the mattress (this could help to track down the problem)?
Normally I would suspect the mattress protector in these cases (especially if you are using one of the thin waterproof/breathable types) and then the sheets (synthetic polyester can sleep much warmer than cotton, bamboo, or linen) but it seems you’ve already been looking in this direction. Have you tried any sheets that are not polyester?
Your situation is somewhat of a mystery and I think that if you can confirm that you have tried cotton, linen, or bamboo sheets with no mattress protector then tracking down the foam layering is the next step.
I certainly wouldn’t go in the direction of the Beautyrest which will only bring you the additional problem of poor value and low quality polyfoam in the mattress (you can see the layering of the plush firm version here). The Lumina Spirit firm is an innerspring with mostly polyfoam comfort layers as well although the foam quality is probably higher than the beautyrest.
A more breathable wool mattress protector such as this one can increase ventilation and moisture wicking so this may also help quite a bit.
But first I would call Sit n Sleep and find out what is in the upper layers of your mattress (and the order of the layering) so that you aren’t replacing it with another type of foam that is even less ventilating.
Phoenix