OK. There are quite a few radiant heating companies here. I will contact one to see what we can do. So increasing flow rates with a variable speed pump is not necessary or advised? I assumed putting more hot water into that thermal mass faster would help the response time?

Sure, it can help. might not be enough though.

I am hoping I can get the temp up and increase the flow so it works a little better. It is not horrible but I was hoping a few tweaks could make it better. I don't think I can get the water much hotter as it is running off the Polaris which is supplying the domestic hot water as well as the heat exchanger loop. I am looking into this Grunflos UPS26-99FC, 3-Speed Circulator Pump, 1/6 HP, 115 volt so I have the ability to dial in the best speed.

Here is the place for a second stage. We often add more responsive radiant ceilings and walls to radiant floors that wont' keep up. Of course we build these into the designs using purpose-made radiant software model functional heating systems before the first roll of PEX is ordered.

In the worst case a fan coil can be used, bad form, but better than nothing.

> I am hoping I can get the temp up and increase the flow so it works a little better.

Higher source temp and higher average temp (increased flow) will help. You are putting ~26,000 btu/hr into the floors and you could increase it quite a bit (say 40%) if you go to a higher temp and then use a mixing valve to keep excessively hot water from the faucets.

If that's not enough, I agree with using lower mass radiators in the problem room.

Yeah, I do have a mixing valve already installed so I think I'll try raising the temp on the supply side to 130 and seeing if that helps. I guess I will hold off on the new pump for now and see how the boost in water temp plays out. Thanks.

Correction: I was wrong about the slab depth. I looked at the plans and it is only 6" with the pex about 2" from the surface. I was measuring from a sunken footing but the flat surface is only 6" deep. Does that change anybody's opinion of a solution? I purged it yesterday again when I added the flow meters and temp gauges. I am still leaning towards a stronger pump and a higher input temp.

Raising the temp and leaving the thermostat on max for two days are easy, low cost things to try.

" I am still leaning towards a stronger pump and a higher input temp."
Like all things radiant, it is the radiator that limit delivery, speeding up water delivery past the capacity of the radiator does nothing. Quickest way to get more BTU's from a radiator is to raise water temp.