Radiant heat, heat exchanger and pump sizing

[Flail]

I have radiant heat pex in my shop. Thinking of running antifreeze in it so it can be left off if we go on a winter vacation. The heater and associated plumbing is in a seperate small room with backup electric heat so I’m not worried about that freezing. With one water heater I would like to get potable water and heat the floor. Thus thinking of using a plate heat exchanger.

Question one. Do I size the heat exchanger to the same btu/hr as the water heater?

Question two. I’ll need a pump to circulate the pex antifreeze to the floor and another to circulate hot water to the heat exchanger. How are these typically sized? Ie is the floor pump more gpm than the heat exchanger pump or equal? Is there a resource to calculate these online?

 

[MT Mike]

Why would you shut the radiant heat off during a vacation? It costs so little to maintain a temperature once achieved that it makes no sense to lose the heat sink to save a few dollars.

 

[Lonnies Performance]

The BTU rating is a starting point for HX sizing.
BTU = GPM X 500 x differential temperature of water.

Glycol will require more flow & pump capacity as it is harder to pump & has less heat capacity than water. A 30% Propylene glycol mix requires approx 7% more BTU & 10% more pump pressure to provide the same floor heat. (operating temperature changes these values, so I suggest you look up a glycol chart & run the numbers yourself)

Since no HX is 100% efficient in transferring all it's input energy, you will have to either increase flow or temperature on the heater (input) side to get the floor side output you desire.

Also look at the pressure drop of the HX to be sure your pump has the capacity to overcome the flow resistance the HX creates.

 

[86turbodsl]

If you're going to use a heat exchanger anyway, look into using methanol for your antifreeze. Pumps a lot easier than glycol, and you don't need quite as much for low temps. I wouldn't use it in a whole system due to proximity to flame sources.

 

[Flail]

Would methanol be allowed with a potable water system? Are the heat exchangers now considered reliable enough to prevent cross contamination with the potable water?

 

[Flail]

Lonnies Performance: Thanks for that formula. Since you reference gpm I’m guessing temps are F and not C? If the temp differential across the HX (or floor) is high does this imply either inadequate pump flow or too little HX (or floor) surface area?

 

[Lonnies Performance]

Yes Deg F

The difference between the primary & secondary side will normally result in a temperature differences.

For example, at the same flows:
Pri: 100 inlet, 80 Out
Sec: 75 inlet, 95 Out
Same quantity of energy is transferred, but at a lower temp.

Otherwise the HX would have to be unnecessarily large to get near 100% transfer efficiency.

You can increase primary flow to induce more energy potential into the equation or just live with the difference by raising the boiler temp a few degrees.

 

[86turbodsl]

Would methanol be allowed with a potable water system? Are the heat exchangers now considered reliable enough to prevent cross contamination with the potable water?

I probably wouldn't if one side of your heat exchanger is potable. methanol is poison after all.

My system is separated. I don't like potable / combined systems in general, the chance for legionella is too high risk for me.

 

[Flail]

Curious about the legionella risk. My water heater-to-heat exchanger loop would be about 3 feet. In my plan, the water heater would circulate water about 3’ to the heat exchanger and back. This water is constantly diverted and refreshed and is consumed for showers, sink etc and thus is replaced. This water would seem (in my thinking) to have low risk to grow bacteria and have same risk as any home water heating system. The floor-to-heat exchanger loop would have ethylene glycol and be constantly recycled. To my thinking, the floor heat loop has a chance to grow some nasty bugs if they find ethylene glycol a suitable habitat. Are there additives to prevent the ethylene glycol loop from harboring nastiness?

 

[brewchief]

The loop to the heat exchanger is a pretty low risk, where problems start is when someone does a system that has the potable water going through the floor. That water will sit stagnant for long periods and is run at a lower temperature so its a better environment for nasty stuff to grow.
Some circulator relays have an option to bring the pumps on for a minute or so a day, doing that will reduce the risk even more.

Sent from my SM-G965U using Tapatalk

 

[That1Guy]

I know this is an old post but the info is still relevant, especially since we are being dragged, kicking and screaming, into winter now. So I'll throw in my 2 cents....
As far as turning the heat off while on vacation...In most cases, it takes more energy (money) to recover or return the shop to temperature than it does to maintain a low temperature - at least in my experience. I can't afford any long term vacations - lol.

Also, rather than run the risk or hassle of contaminating potable water with your system water (with or without antifreeze) have you looked into a small on-demand (point of use) water heater? I've seen them small enough to be hidden under a sink. How much hot water does a shop require? The occasional hand washing? Maybe a shower? I don't know - but a small on demand point of use water heater should be able to keep up. I mean, by the time you buy a heat exchanger and a circ pump, you could just as easily pay for the point of use water heater.

And if you ever did go on an extended vacation, those heaters are easy to plumb to make it easy to drain them. Just a thought. I'd be interested to know what you ended up deciding on and how it worked out for ya. Everybody's situation is different. That's what I love about this forum. So many ideas and points of view.

 

[Nivekdodge]

I have mine rigged this way and so far it seems to be working.

 

[fitter30]

For what a HX cost you can buy another heater and winterize the domestic water. Propylene has to have a minimum 30% concentration for the inhibitor protection.