heating hydraulic oil to heat floor

Hi all; Been thinking for a while now about burying a tank of water in the mass and pump it thru pex's to heat a concrete slab. Big concern is (boom squish ) I don't think that my mass could reach boiling temps but... New idea is hydraulic oil, flash point of apx. 430 degrees F , i know my mass can't get that high! Easily pumped with a chevy engine oil pump modified to be driven with an external source. Heat holding property's similar. readily available. Am I missing something here ? Or does this seem like a valid idea ? Much safer than any water based fluid. Thanks for any input Tom

It's more expensive than water... and a lot harder cleanup if you spill it. Not the sort of stuff I want in my living space.

Tom; Building this in my shop , i also have access to free used hyd oil .

If you are recycling used oil in a shop, that seems like a win-win.

Make sure your materials are oil compatible. I don't know what hydraulic oil does to pex, if anything.

Polyethylene is inert to hydrocarbon solvents. The only thing that makes it break down is sunlight (from the UV damage). I say go for it -- and report back on the results.

interesting idea!


is there a way to get the liquid away from heat source, in case of pump failure? like maybe a reservoir lower than everything else.

i have seen systems, on the net, running the liquid through car radiators with a fan too.

Hi Thomas, I've not tried it but from the experiments I've done and have discussed with folks, my take on this kind of system is that your limiting factor is going to be the thermal transfer rate of the cob. It's so slow compared to the fluid that you end up with a cool section of cob all around the exchange lines. In water heating experiments done this way I've found I can get a bit of hot water at first, then it pretty quickly stabilizes at luke warm at best. I think you would do better trying to use a different coupling medium, like a large water barrel, to run the coil through for transfer.

Can you plan for the eventual failure of the system? Maybe like a shower/bathroom floor pan/liner filled with absorbent clay, such as Oil-Dri?

Hi Matt; I'm very happy to hear your input on this. My original thoughts on this were simply to have a rmh in the shop. I then came up with my idea / post about a "milk can in my mass". This thought of heating a small portion of shop floor was kind of like whipped cream on my cake. All along i have planned on valves just outside the mass and if my experiment failed well ... drain the tank , fill with mass and move on to the next idea. So That said, I get regular temps on the outside of the mass of 130-140 if I were to have a say a 2'x16" metal tank buried just outside of my transition area directly over the start of my horizontal pipe with possibly 4-6" of cob from pipe to bottom of the tank I'm thinking that temps there are much higher than the external temps i have been reading. The tank would be plumbed out thru the side of the mass and the valves and pump connecting to pexs. No internal coils, just heat the whole tank. I have access to salvage parts for equipment and used oil, hydraulic or motor,build a pump from spair parts .Temps could be checked at the tank itself or at the valve connections just outside. I have so many projects and never enough time that if this will perform poorly it may not be worth the extra time. Your mention of a water barrel and coils ,are you thinking a barrel on top of the mass, or buried in the mass ? I am going to pour a section of the floor this year. The pexs is being given to me I just have to insulate (dig) 4" more,if i want to use it. What do you think, talk me into it or talk me out of it ! Thanks for your help. Tom

Ohh, geez man, I was just talking about using cob as part of the transfer equation. As for the whole big question of radiant floor and how, man, that's a big one. I think I'll dodge the question a bit and just say that in my opinion RMHs are fairly low power heaters by nature, and radiant flooring takes some BTUs, so either a dedicated rocket water heater system, or a mass heater. I don't think you'll make much of a dent in either heating need by having a combo system, unless the space is tiny.