Thinking about flow rate with hydronic heating.

Jean-David Beyer

I have a boiler that is piped with primary-secondary piping. In the boiler loop is a constant speed Taco 007 circulator provided by the boiler manufacturer. Let us imagine that there is just one loop in the secondary side.



I have a Taco 007-IFC circulator in the secondary (IFC, because there are two heating loops on the secondary side). The flow rate calculates to be under 4 feet per second, but it must be pretty high because the temperature drop is usually only a few degrees (like 2F or a little more).



I see suggestions for using a smaller circulator, or an ECM delta-T circulator in order to get more temperature drop in the loop. Now from the point of view of heating that zone (radiant slab), I see no benefit in reducing the flow rate (other than lowering my electricity consumption). The arguement seems to be that I will get lower return temperature to the boiler and get more condensing.



But will I get lower return temperatures or not? It seems to me that if I lower the flow in the secondary loop, the return water approaching the closely-spaced Ts will be less, but with a lower flow rate, more of the hot water from the boiler will bypass the secondary loop and just go from the Supply T to the Return T, so the return temperature seen by the boiler will not go down from this exercise. I will just see a less uniform heating of that zone due to the lower flow rate.



Am I missing something?

Bill Clinton_6

lower return temperatures

Yes, you will see a lower temperature at the boiler.  As the return from the heating loop mixes with the water racing around the primary loop, it will reduce the temperature in the primary loop. The rate of flow around the primary loop should change very little with varying flows in the secondary. So, you will indeed get some amount of efficiency gain, probably very small.  You will also use a bit less electricity at the pump as lower flow rates cause the pump to use a bit less power.  You could install an ECM/Delta t pump and save more electric.  Whether you should depends entirely on your motive for doing so.  If you are trying to save money, forget it.  The payback for replacing a functioning 007 is non-existent.



If you indeed have a single zone and you leave the heating on most of the season, outdoor reset can be a good thing.  Outdoor/indoor reset could be even better.  Problems arise though with multiple zones and systems that operate only part of the day.



To me, a two degree delta tee is a good thing for uniform output.  In the end, "it depends."



Bill Clinton

NYplumber

t vs p

Not to go off topic, however I have been a big fan of the delta-p pumps, especially in systems with trvs or zone valves.



Knowing that your home is run mostly off odr, delta t may be a better bet for you.

As mentioned, payback alone doesn't justify the replacement of a working 007.



My question is, when should one consider a delta-t over a delta-p in a multiple zone system? Single zone gets my delta-t vote.





I recently heard of a 15% savings in fuel usage when changing just the pump from a conventional large motor pump to a taco delta t.

Jean-David Beyer

I guess i do not fully understand this.

"Yes, you will see a

lower temperature at the boiler.  As the return from the heating loop

mixes with the water racing around the primary loop, it will reduce the

temperature in the primary loop. "



The water running through the heating loop has two components. The first is the water that goes the long way from the supply T to the heating elements and back to the return T. The second is the water that goes the short way from the supply T direct to the return T. Now if everything were sized just right, and some other assumptions to make the discussion easier, let us assume that all the water at some stage of operation went the long way and none went the short way. If the flow in the long way is reduced to lower the temperature return water, the flow of water in the short way would increase to make up for the flow reduction in the long way. In the extreme case no water would flow the long way and the entire flow would be direct from the supply to the return. If anything, would not the boiler loop see an increase in the return temperature?

Jean-David Beyer

I am still confused.

"Knowing that your home is run mostly off odr, delta t may be a better bet for you."



What happens if I set up delta T to be, say, 10F and on design day I put 112F water into the heating elements? I assume flow is reduced until return water is 102F, right? Now let us say it gets warmer outside  and the ODR reduces the supply to 75F. It tries to reduce the flow so the return is 65F, but since the thermostats are calling for 69F, the return temperature will never get that low. In my system, with a constant speed pump if it gets up to 52F outside, a supply temperature of 75 is just what the ODR calls for. If the flow were to be cut off by a delta T pump trying to get a return temperature of 65F, it would get cold in the house. Even if it did not stop the flow entirely, I assume the heat would be very un-uniform. Setting the delta T to 6F is probably not the answer. With a supply of 75F, the delta T is 1F or less right now.



"As mentioned, payback alone doesn't justify the replacement of a working 007."



If I were designing my 2-zone system, I believe I would use an ECM delta-P circulator and zone valves instead of two constant speed circulators with IFC. But mine is presently zoned with circulators. You can get a measure of the efficiency of 007 circulators when you see they are rated at 1/25 horsepower, but they draw about 87 watts (by memory).

Bill Clinton_6

JDB, my apologies

you have caused me to think, which is always painful. Lowering the temperature of water entering the return of a boiler while maintaining the same flow rate will indeed improve the boiler efficiency.  However, I begin to see that you are correct in your examples: Lowering the flow rate in a secondary loop to return a lower temperature from that loop, all else being the same, will not lower the temperature of the return water the boiler sees.  In fact, since the water returning from the heating loop has had a lower average temperature through the heat delivery part of the system, it will have given up less BTUs and therefore have less effect on diluting the temperature going through the primary loop, so the boiler may in fact see a  higher temperature in the return and therefore operate at a slightly lower efficiency.  Dang!



To have the desired effect, adding heat delivery capacity to the distribution so that it could deliver the full amount of heat at the lower average temperature would be the way to go.  It would be, that is, unless you prove me wrong again.



Bill

Jean-David Beyer

unless you prove me wrong again...

No need to apologize. I have no need to prove you wrong.



"adding heat delivery capacity to the distribution so that it could

deliver the full amount of heat at the lower average temperature would

be the way to go. "



In the zone where I get the least delta-T, I have two baseboards in series. Each was 3 feet long and might have been enough if I were to run 180F water through there. But I wanted condensing, so I replaced those with 14 feet long ones -- the entire width of the rooms underneath the windows. In that zone I run 110F to 135F water depending on the outside temperature. I would even run lower temperature through there on warm days, but then the loss is so low as to cause rapid cycling of the boiler because it will not modulate down far enough. If you bother to look up the charts for baseboard heating units (without blowers) you will see that you get very little heat from them at 110F if the return air is 65F.  The only way to get more heat out of the water is not to slow it down any more, but to increase the amount of baseboard. For me, cost of the baseboard was not very important, since I was replacing the boiler, removing the oil tank, changing from one zone to two zones, etc. But it was not practical to put in more baseboard unless I was willing to have the floors ripped up a little, and neither my contractor nor I wanted to do that. So I put up with one degree delta T on warm days in that zone. For my big radiant slab zone, I get very lo delta T as well.