I am researching on if it is possible to build a hydronic underfloor heating system from common materials. I was thinking about purchasing a medium size water heater, water pump, and about 1000 feet of tubing, maybe pvc, or even your higher end common garden hoses as they dont have much insulation to contain the heat in the hose. and it isnt like the lines will be under any serious pressure. I am on a 4' crawlspace now, and I am getting ready to install insulation. The joists have 16" spacing. I have central air but the floors are soo cold at nite. the commercial stuff is way over priced if you have researched you can pick up a water heater for under 150 and a commercial smaller boiler will run 2000. also the pex hosing runs about 10 dollars a foot, your most expensive hose isnt 30 dollars for 50 feet. Any help is appreciated.. Thanks

You to have done some research but here are my comments:

1. You need an anti-backflow valve as the water entering your water heater is contaminated, recirculated and black. A costlier one is a better option and one that would throw the backflow water out the system is the best.
2. A low-pressure regulator as you do not need to circulate water as fast as expected. I don't know the friction of garden hose (which I would never use) but you have to take that into considerations too.
3. Garden hose are not make for high temperature heating. Doing so can burst the tubes (hot water needs expansion) but also wear and tear as water travel constantly.
4. PVC also are not best. Pex tubes might work out for you, specially the ones made for hot water.
5. You will need a circulator because it would have to recirculate in the loop.
6. An expansion tank would be a good idea, but that's up to you. It can and might explode on you as it needs to build volume in the system.
7. How you regulate all this system is one mystery to me. I guess you would turn on and off the heater?
8. Depending on the type of materials above your pipe, wood is an insulator, so you might consider that it will take a while before it reaches your feet.
9. These are a few considerations you have to think of. I would not just jump into this without knowing the components of a hydronic system.

If you have more questions as them here, as it can be useful for other DIYers.

Oh and the heaters, there is a difference in price but you will save on energy cost in the long term. As for 150$ I wonder what you are getting for that price.