Fake decoction mashing

I did a wheat last week, and the mash profile called for 10 minutes at 120F and then an hour at 150F. So I figured out water amounts, and a plain infusion would have required too much boiling water in the mash to get to 150F.

Instead I tried doing a sort of a decoction mash; taking off about half the wort, boiling it and returning it to the mash. It brought the temperature up to 150F, but efficiencies ended up very low, around 50%.

In retrospect I probably denatured all the amylase in the decocted wort, and there wasn't enough diastatic power in the remaining grain to get good conversion. Are there any better ways to do this? Continuous recirculation/gradual heating would probably work but I don't have the plumbing for that. Does this normally happen in decoction mashes, and if so how do breweries that do that get around it?

You should have pulled mostly grain which should leave behind a lot of the enzymes in the thinner mash. Then you bring the decoction up to a sach rest before boiling.

Going from 120 to 150 is a big jump and may have been better handled pulling a third of the mash and using boiling water to get there. You could have also done a double decoction, resting at 145 and then to 156.