First off, nice work on the house. We did stained concrete for the lower level here - kind of a dark leather brown color that came out nice. I think it is a good solution for slabs.
Radiant floor heat - my experiences. I started off thinking an on-demand HW heater was a great solution. I no longer really think so. There are caveats to that, but what i ran into has soured me on them. I have 3 Takagi on-demand heaters. 2 in the house, 1 in the shop. The 2 house ones are very high efficiency (96-97%) and one is dedicated to domestic HW and the other to radiant floor heat. The one in the shop is slightly lower efficiency but still decent (low 90's?) and only covers radiant heat. From the get-go they have some great advantages - small footprint, the ability to do sealed combustion with PVC venting, high efficiency and very simple controls (your T-stat asks for heat and starts the pump, so the heater automatically starts with teh flow from the pump).
HOWEVER.....
I have learned that these are SUPER sensitive to hard water. We moved in Aug 2015 and did not have a softener in yet. It was on the do to list but I didn't consider it urgent. We started having a lot of issues with hard water plugging up the shower valve screens so finally in Nov 2015, I put in a softener. That fixed those issues and i figured we were good. The next year the domestic HW heater started to leak. Tech support was very helpful and they were nice enough to comp me a heat exchanger, which I had to install (THAT was fun...NOT!). They diagnosed it as due to hard water. Now we ran hard water through it for all of 3-4 months at most, then fully soft water, but that was enough to kill it. I had lengthy technical discussion by phone and email on their diagnosis, and it was clear they were right. I bought a good hard water kit and found my incoming well was 24-26 grains hardness. After softening, it was <1. We have not had problems since, for the past 3.5 yrs.
Radiant had the same issue as those heaters got filled with hard water prior to the softener. The one in the shop crapped out this past spring after only ~5 yrs use - same issue as the house DHW. The one in the house is still going (3-4 yrs) and I did flush it with vinegar to see if I could stop the hard water build up. So far it is OK, but who knows if it will die soon? The other wrinkle is antifreeze. Now this depends a lot on where you live. Up here, if the power went out and it got really cold, I would be at risk of freezing the system after sufficient time down. Can't take that risk so I added 50% antifreeze that is designed for radiant systems to both house and shop. I didn't do that right away but when i did things got worse. I start to hear a low grumbling vibration from the heaters when running. This is cavitation that is happening in the heat exchanger due to the antifreeze mix having lower thermal conductivity than straight water. It is bad for the life of the heater. I can improve it by raising the system pressure. Most in-floor radiant runs at 15 psi, but I run 25-30. It still does not quite eliminate it. You also have to up the pressure in the expansion tank and any relief valve to match when you do this. If you are not running antifreeze, you will eliminate part of this issue, but not the hard water issue.
The next time i have to replace one of these, I am going to find a small conventional tank heater and put that in instead. I believe that will improve things but i will lose floor space. Time will tell.
My $0.02
-Dave
