Mixing valve in a close loop

[tankd0g]

Hi all, I am finally at the stage of my garage to hook up the in floor heat and I'm getting conflicting advice regarding water temperature. Some people report that 120F water cracked their floor and others say they run it as high as 160F. The one I have actually been in int he middle of winter says he runs the water at the same temperature he wants in the air, which is 60F-70F and it works great. I'm not sure my oil fired hot water heater will go that low, it's a John Woods, perhaps I can get a different aquastat for it, I believe it is set at 120F from the factory.

I have 900ft of 6" spaced, 1/2" pex in the floor and it will be a a closed loop system. I have a three speed pump, expansion tank and 30PSI blow off valve for the tank. If I wated to integrate a mixing valve, obviously I don't have a cold water feed, so can you draw water off the return feed for cooling? I'm hoping to get away without doing that but it's not much extra money at this point.

 

[koditten]

My input temp to the floor is around 110*f.

Are you using a boiler? The boiler will want a somewhat constant temp coming into the boiler. You don't want to shock the boiler with a real low inlet temp. Hence the mixing/bypass valve.

 

[tankd0g]

My input temp to the floor is around 110*f.

Are you using a boiler? The boiler will want a somewhat constant temp coming into the boiler. You don't want to shock the boiler with a real low inlet temp. Hence the mixing/bypass valve.

It's a domestic hotwater tank with an oil burner, so I assume it's designed for cold water intake exclusively, so I don't think I need to worry about going the other way and keeping the intake temperature up.

 

[yeldogt]

Actually it is not going to like the return temp.. I have one and they will quickly condense with cold return. Mine is in a weekend house and they don't like to be cold .. and 110 is cold for an oil burner.


Do some more research on setting it up .. and also do a load calculation .. you can figure out what the temps will need to be if you used standard pipe layouts and insulation

 

[tankd0g]

Actually it is not going to like the return temp.. I have one and they will quickly condense with cold return. Mine is in a weekend house and they don't like to be cold .. and 110 is cold for an oil burner.


Do some more research on setting it up .. and also do a load calculation .. you can figure out what the temps will need to be if you used standard pipe layouts and insulation

when I layed out the loop, the calculator said 140F but I don't think it cares if your floor cracks or not... By condensation do you mean condensation in the flue because the burner will not run long enough per cycle?

 

[pseudorealityx]

A domestic water heater isn't designed to get a constant flow of cold water. It's meant to get a small inrush of cold water, and then sit and fire to heat it up for the next 'use'. You try dumping cold water in constantly and it won't be happy. Floors are about THE perfect application for a condensing boiler, since it's a place you can use the big efficiency gains when you're giving them cold return. They get more efficient as you lower the intake temps. That said, they come with a big first cost increase too. But giving cold water to a traditional boiler is a bad idea.

To whoever you talked to who said that you should run the water temp the same as you want your air.... he's dumb. Don't listen to him.

Heat transfer works like this. You make [something] warmer than you want it to be in your space. Then this warm [something] transfers heat to things that are colder than it, thereby lowering the [something]'s temperature. In the meantime, the space is being 'cooled' by the surroundings, walls, windows, ceiling, etc. So it's losing energy. You need to keep inputting energy into the system to get to an equilibrium.

 

[tankd0g]

A domestic water heater isn't designed to get a constant flow of cold water. It's meant to get a small inrush of cold water, and then sit and fire to heat it up for the next 'use'. You try dumping cold water in constantly and it won't be happy. Floors are about THE perfect application for a condensing boiler, since it's a place you can use the big efficiency gains when you're giving them cold return. They get more efficient as you lower the intake temps. That said, they come with a big first cost increase too. But giving cold water to a traditional boiler is a bad idea.

To whoever you talked to who said that you should run the water temp the same as you want your air.... he's dumb. Don't listen to him.

Heat transfer works like this. You make [something] warmer than you want it to be in your space. Then this warm [something] transfers heat to things that are colder than it, thereby lowering the [something]'s temperature. In the meantime, the space is being 'cooled' by the surroundings, walls, windows, ceiling, etc. So it's losing energy. You need to keep inputting energy into the system to get to an equilibrium.

Well as I said in my post, I have been in the shop that is running 60-70F water from an electric hot water heater and it's perfectly comfortable to work in, in the middle of February (-20C). The overall air temp is not actually 70F, that would be too hot to work in, but that is the temerature of the surface of the slab where the thermostat pickup is. The hot water tank holds enough water to fill the loop several times over when the thermostat calls for it, so I'm not sure how that would differ from someone taking a long hot shower as far as the tank is concerned. It being a garage I'm hardly ever going to be in, I'm not spending money on a high effeciency boiler, I certainly would go that route for my house if I had radiant heating, I just want to keep the garage warm enough to prevent condensation on my tools. I plan to plum in evacuated solar collectors next year, so I need a storage tank anyway, and it was free.

 

[yeldogt]

Over the years I have seen a few spaces that were successfully heated with a water heater -- all small. One was a well insulated cabin that used a heat exchanger and the other was an extremely well insulated one car garage that used a 50 gall unit with the internal secondary coil for radiant heat.

The only way they work is if the heat load is low enough. Another problem is that oil water heaters are not very efficient ... mine is around 55%.

I keep the oil for my weekend house because I can turn it off when I leave and in 15min when I show up for the weekend .. it is hot ... and I never run out. With electric or propane I would have to leave them on for weeks without use and would need a very large unit.

What is your electric rate -- you may be able to run an small electric boiler for less money.

 

[tankd0g]

Over the years I have seen a few spaces that were successfully heated with a water heater -- all small. One was a well insulated cabin that used a heat exchanger and the other was an extremely well insulated one car garage that used a 50 gall unit with the internal secondary coil for radiant heat.

The only way they work is if the heat load is low enough. Another problem is that oil water heaters are not very efficient ... mine is around 55%.

I keep the oil for my weekend house because I can turn it off when I leave and in 15min when I show up for the weekend .. it is hot ... and I never run out. With electric or propane I would have to leave them on for weeks without use and would need a very large unit.

What is your electric rate -- you may be able to run an small electric boiler for less money.

I only have 240v/40A availabe in the garage, otherwise I would have gone electric and not had the expense of a chimney and oil tank. Even though electricity is quite expensive here at $0.1465/kWh, most hydronic setups here are done with electric hot water tanks or tankless domestic "boilers", and I imagine it gets as cold here as anywhere in the US. Of course everyhting is insulated a minimum of R20 these days, I put R20 in the garage walls and R30 in the cieling.

 

[tankd0g]

Thank you for your concerns but the heater is more than capable of doing the job, it's rated for use as a radiant heat source and it even tells you how to hook up a hydronic loop in the manual. My only question relates to the fact that I do not have an external cold water source. The schematic in the manual shows a mixing valve with cold water coming in, I do not have that available, so I either have to make 100% sure I never exceed a floor damaging temperature or I need to install a valve that draws water from the return line to cool the feed line, but that doesn't seem like it's possible since the pressure will be equal on both sides of the valve. Would you need two pumps to make that work?

 

[mygarageone]

Thank you for your concerns but the heater is more than capable of doing the job, it's rated for use as a radiant heat source and it even tells you how to hook up a hydronic loop in the manual. My only question relates to the fact that I do not have an external cold water source. The schematic in the manual shows a mixing valve with cold water coming in, I do not have that available, so I either have to make 100% sure I never exceed a floor damaging temperature or I need to install a valve that draws water from the return line to cool the feed line, but that doesn't seem like it's possible since the pressure will be equal on both sides of the valve. Would you need two pumps to make that work?

The cold side of the mixing valve would be the return side of the radiant system . Your return line should go straight to the return manifold with a tee branch to the mixing valve cold side.
You valve should have come with a diagram showing this type of installation
And by the way , run your water temp to the tubing no lower than 100 degrees.

 

[tankd0g]

The cold side of the mixing valve would be the return side of the radiant system . Your return line should go straight to the return manifold with a tee branch to the mixing valve cold side.
You valve should have come with a diagram showing this type of installation
And by the way , run your water temp to the tubing no lower than 100 degrees.

Thanks, I have not purchased a valve yet because I wasn't sure it would work this way. Do you have to put your circulation pump before the cold side T in order to push water into the valve?

 

[mygarageone]

Thanks, I have not purchased a valve yet because I wasn't sure it would work this way. Do you have to put your circulation pump before the cold side T in order to push water into the valve?

Circ pump pumps away from temp valve to the radiant tubing.

 

[tankd0g]

Circ pump pumps away from temp valve to the radiant tubing.

Makes sense, thanks.