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radiant through a heat exchanger
Brad White_198
Member Posts: 72
Hi Tom,
I do not know your heat loss nor many of your variables so will go with some assumptions.
I will start with a floor output density of not more than 30 BTUH per SF and a floor temperature of about 83 degrees on average, this being a limit or nearly so of a comfortable floor. I also am assuming a 68 degree space temperature, All of this is a place to start.
By these assumptions I get a heat loss of about 81 MBH, also assuming you are running about 20-23 degree delta-T on your floor side, (which sounds a little high to my taste FWIW). Floor circuits at between 0.4 and 0.5 GPM each. How does that all sound? Just to get the basics down.
Just for giggles and without any real calculation nor reference on my part, I assume your average water temperature will be in the mid-90's under bare or nearly bare concrete to achieve this at that density. How am I doing?
A first concern is that your 20 to 23 degree delta-T will, by back-calculating from a floor temperature of 83F need to be supplied at 103 to 106F. Maybe you need it that high, I do not know off-hand, but would think it would be lower. IF lower, your return temperature might be lower than your floor temperature.. again, conjecture on my part, just trying to nail down your variables of heat loss, flow rate and temperatures.
Forgetting all this, here is the fun part:
If you have a heating load of 81,000 BTUH and your boiler side has a flow rate of 2 GPM (assuming that the radiant load is the only load on the boiler), your boiler delta-T would be....81 degrees!
Assuming your boiler return water temperature came back from the exchanger at say 80 degrees, you would have to heat that water to 161 degrees.
It will work but it is above condensing temperatures starting out, even if it will be below condensing temperatures coming back. It also may not square with the boiler's minimum flow rate if this is all you have connected to it.
Regardless, even if the heat exchanger were but one consumer, your boiler would have to fire to that higher temperature to make that work if at that low flow rate.
For myself, I prefer higher flow rates through my condensing boilers, such that my hottest water starts at 140F maximum and much lower if I can.
It takes more pumping to do this but it makes sense when you consider that lower flows mean higher delta-T's on the radiation side, desirable to "wring out the heat" and get you to low return temperatures. On the boiler side, you really want narrower delta-T's so that you can possibly start -but always finish-, below the condensing curve.
To me, that 2 GPM boiler side would only make sense if you had a high temperature boiler source to begin with, starting with say 180 degrees and returning, with blending before the boiler of course, at about 99 degrees, to get what you need to your floors. Does that make sense?
I would want to size my boiler-side of the plate exchanger to use the lowest water temperature my boiler can put out and still get the job done (of course). This usually means though that the water flow rates want to be a bit more equal.
Sorry to ramble and to be too generic, but I wanted to seek out your actual parameters to see what the effect on your boiler would be. I suspect with the flows you get that you will lose boiler efficiency at the very least.
I do not know your heat loss nor many of your variables so will go with some assumptions.
I will start with a floor output density of not more than 30 BTUH per SF and a floor temperature of about 83 degrees on average, this being a limit or nearly so of a comfortable floor. I also am assuming a 68 degree space temperature, All of this is a place to start.
By these assumptions I get a heat loss of about 81 MBH, also assuming you are running about 20-23 degree delta-T on your floor side, (which sounds a little high to my taste FWIW). Floor circuits at between 0.4 and 0.5 GPM each. How does that all sound? Just to get the basics down.
Just for giggles and without any real calculation nor reference on my part, I assume your average water temperature will be in the mid-90's under bare or nearly bare concrete to achieve this at that density. How am I doing?
A first concern is that your 20 to 23 degree delta-T will, by back-calculating from a floor temperature of 83F need to be supplied at 103 to 106F. Maybe you need it that high, I do not know off-hand, but would think it would be lower. IF lower, your return temperature might be lower than your floor temperature.. again, conjecture on my part, just trying to nail down your variables of heat loss, flow rate and temperatures.
Forgetting all this, here is the fun part:
If you have a heating load of 81,000 BTUH and your boiler side has a flow rate of 2 GPM (assuming that the radiant load is the only load on the boiler), your boiler delta-T would be....81 degrees!
Assuming your boiler return water temperature came back from the exchanger at say 80 degrees, you would have to heat that water to 161 degrees.
It will work but it is above condensing temperatures starting out, even if it will be below condensing temperatures coming back. It also may not square with the boiler's minimum flow rate if this is all you have connected to it.
Regardless, even if the heat exchanger were but one consumer, your boiler would have to fire to that higher temperature to make that work if at that low flow rate.
For myself, I prefer higher flow rates through my condensing boilers, such that my hottest water starts at 140F maximum and much lower if I can.
It takes more pumping to do this but it makes sense when you consider that lower flows mean higher delta-T's on the radiation side, desirable to "wring out the heat" and get you to low return temperatures. On the boiler side, you really want narrower delta-T's so that you can possibly start -but always finish-, below the condensing curve.
To me, that 2 GPM boiler side would only make sense if you had a high temperature boiler source to begin with, starting with say 180 degrees and returning, with blending before the boiler of course, at about 99 degrees, to get what you need to your floors. Does that make sense?
I would want to size my boiler-side of the plate exchanger to use the lowest water temperature my boiler can put out and still get the job done (of course). This usually means though that the water flow rates want to be a bit more equal.
Sorry to ramble and to be too generic, but I wanted to seek out your actual parameters to see what the effect on your boiler would be. I suspect with the flows you get that you will lose boiler efficiency at the very least.
0
Comments
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radiant through a heat exchanger
I have a new 2700 sq. ft. shop with 18 - 200' loops and I'm considering using a Baxi condensing boiler through 5X12X24 plate Taco heat exchanger to heat this shop. My thinking is that by only running approx 2- gallons through the boiler side of the exchanger at about 7-8 gpm. and a 009 on the radiant side that this modulating boiler would be more efficent. Any thoughts on this? we are in Western Montana, Thanks, Tom0
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