Truely slow slow cooker?

I've been cooking 'slow cook' recipes in my oven for decades, but have decided to get a slow cooker which costs 5-10% as much to run.

The first two (new ones) I've tried seem to simmer when on Low, when they should be below boiling.

Has anyone come across any that are properly slow? I hear the old ones from the 80s are... Is it because I've got a Compact (1.5 litre) one?

Thanks for any info...