How well is my heating working at lower water temps?
Hi. Now that the cold has really hit, I have been trying to really stress my heating system to max efficiency. A little background. I have a pretty leaky 1950's cape renovated about 10 years ago, improved insulation, air sealing etc. During this time changed to forced hot air w/ hydro air handlers. In the beginning it really struggled to keep up on very cold days even with supply water set from 165-180. Eventually the old Navien CH NG combi gave way to new Navien NCB-E two years ago. There are three zones (one up, two down) run with two Ecobees and one Alexa stat.
The other night I experimented w/ a 140 degree supply. I was getting return air of around 110-120 (was aiming for under 130 to get condensing) but it struggled to hit 69-70 set points. I tried dialing in 145 which it's still on, and getting about 125 return. Below is the run graph for the upstairs stat. Just at a glance, how does it look in terms of number of cycles and cycle time?
Thanks for any feedback!
Comments
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it’s hard to tell the time duration of the blocks? Is it on setback thermostats? It looks like a long non stop run starting at 4:00 AM
Knowing the output of the air handlers compared to the heatload might be useful.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
Initial glance on that graph, you can go down on the supply temp if you so desired. Even at the coldest part of the morning (6F?), your system not only maintains but brings up the temp. It doesn't seem to be struggling at all in that 24 hr period. Lowering the supply temp will minimize the cycling frequency but won't necessarily improve your overall energy efficiency as those air handlers can use a good amount of energy for delivery. I've seen many at 500 watts or more. Do you know how much your's pulls?
Do you know your btu/h output during that long 0400 run?
What design temp you are targeting?
This is from ecobee Thermostat? What equipment settings do you have it set for?
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@bob.. the long block at 4am is continuous running of about 2.5 hrs. The short bars are heat runs for about 20 minutes with 10-15 minute breaks. Even weirder the white bar is the fan running… no idea why it's not running in conjunction with the heat call. It does run on setbacks but it's only one degree difference from 69 overnight to 70 at 7am. I really have no answer for this yet, other than it's been pretty cold these last few days just like everywhere else, and I've been messing around w/ lowering the supply temp lol.
The graph is for the upstairs unit which is 2.5 tons cooling, I think it supplies 800-900 CFM of air. I will try to see what more data I can get from the reporting.
@bjohnhy yeah that was my big concern, having it running constantly vs cycling constantly. Not sure if there is a happy medium somewhere. I am however pleased that it's keeping up… in the bad old days I've had supply temp cranked to 180 at 20 degrees outdoor temp and it struggled.
I THINK design temps around here (Southern CT) for heating are like 15-20 degrees…however my installers weren't so good at the design part… I don't think they did a manual J and they were not familiar at all w/ the newer Trane/AS handlers that the builder spec'd, and just went on gut feel. As above I'll see what other specs I can gather from the Ecobee report and equipment manuals.
Thanks for the feedback!
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You might appreciate the "beestat" phone app. It can display the ecobee data in various ways, time to heat, estimates temp loss based on outdoor temp, etc. This all can be informative for fine tuning of those supply temps or outdoor resets curves.
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Hydro coils are hard to dial in. What I have found works is once you get your outdoor rest in the ballpark is try reducing the water flow rate through the coil. Up to a certain delta T, the heat output (ie supply air temp) does not change much but you get much lower return water temperature which is what matters for condensing efficiency.
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Quick heads up, the 15-20F design day temp you mentioned is incorrect. The 99% design day temp for Connecticut is 0F-2F
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Thanks for confirming!
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As was mentioned earlier, it is difficult to dial in the fan coils with modulating input boilers. Especiall;y with the outdoor reset feature. The fact that you have a boiler that will modulate supply water temperature based on the outdoor temperature should eliminate any on off cycling of the boiler once the outdoor temperature drops below the boiler’s turndown ratio minimum input requirement. As the outdoor temperature drops the boiler should increase in temperature to the fan coil that runs constantly. Best if there is an air handler with a variable speed fan to adjust the fan speed as the outdoor temperature changes. Thus it will have a fan that never stops (just changes speed) and a boiler input that constantly adjusts the water temperature to the fan coils based on the outdoor temperature.
Since there are no controls for that particular combination of equipment currently on the market for general use, You will probably never get the efficiency you might expect from a radiant floor system or cast iron radiator system. That said, the combi has resulted in lower fuel usage from the original “on/off” boiler system from years ago. I have a feeling that you are going to keep adjusting for some time, Until you have a real issue to resolve like a daughter getting married or a parent going to the nursing home. That usually takes focus away from heating systems. Even for the engineer types.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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I don't even have to wait THAT long to stop adjusting… plenty of better problems before all that happens :D. But no seriously, the one thing I truly need to do is re-hook up the ODR which has been installed outside and not hooked up since the CH to NCB swap.
Question though…. you mention variable speed, the TAM7 is indeed variable speed (and can be commanded to blow at different speeds via W1/W2/W3 jumpering. However from my understanding that is mostly for staged heat strips w/ a heat pump installation (for emergency heat or aux heat). I am guessing by your next comment that I'm SOL in terms of the handler varying fan speed for anything it's not already programmed to do (ie, after a cooling cycle, it will run for about a minute after the compressor stops to help avoid freezing of the coil and blow out residual humidity… I think)
Have you guys seen any system leverage W1-W3 for a hydro heat setup? (vs a heat pump)
Thanks for the great responses thus far!
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Interesting…. W1-W2-W3 terminals on the air handler can be staged by the water temperature on the hydro coil.
I'm sure that there is a digital control out there somewhere that can be adapted to do something like that however, I'm not a digital kind of guy. The only thing I can think of is some analog temperature switches that are set for 2 different temperatures. When W1 is closed by the thermostat that will activate low speed on the fan. As the outdoor temperature drops, the fan coil temperature will rise. When that coil reaches a set temperature the make on temperature rise switch will jump W1 to W2 and increase the fan speed. Then the second make on temperature rise switch will be set at a higher temperature to jump W1 to W3 and bring on the highest fan speed needed when the outdoor temperature is much colder.The net result will be staged but will give you the benefit of the variable speed fan as the water temperature changes in the fan coil. Selecting the proper temperature switches with a small differential between off and on action is going to be critical. Like a 3° differential switch would be much better than a 10° differential switch
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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The Amazon link below may work for that purpose. I've used a digital temperature switch before similar to this one but was only 1 channel. This one has 4 channels....
Easy Operation,Zigbee WiFi Temperature Switch
https://a.co/d/al8HJmo
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You want a multi stage thermostat running that fan. When you need more heat, run the fan higher. When you need less heat run it slower.
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Thanks All. Ecobees only support two stages of heat tops so I might be limited to medium and high
Last question - is there any wisdom to the idea that the slower the fan speed, the hotter the air? (thinking if it lingers more at the coil it has more time to heat up). Only problem is messing w/ static pressure and ensuring the air gets where it needs to go as some of the supply runs are fairly long.
The TAM7 is a weird beast w/ confusing instructions. It is not really clear what to set fan speed at in terms of a slide in coil (which I have) - it gives you a hydro heat control board with the coil, but the manual doesn't really explain if it contributes to the fan speed setting or just tells the mainboard it's there and the mainboard does it. Either way, my hack installers set it up on maximum blowage using the setting for an add on (not slide in) heat coil, that sits on top of the handler (I do not have this setup). It blows hard for sure but always wondered if it was too hard.
Update as well that I hooked up my ODR sensor to the Navien. It correctly read temp at 28 degrees, and I dialed in the "fan coil" curve preset. It set the supply water as high as 170 which means the water is coming back in the 140-145 range. I guess I could cap a max temp but in that case I might as well just keep setting it manually. Will keep an eye on it - lots that can be tuned!
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"question - is there any wisdom to the idea that the slower the fan speed, the hotter the air? (thinking if it lingers more at the coil it has more time to heat up). Only problem is messing w/ static pressure and ensuring the air gets where it needs to go as some of the supply runs are fairly long."
Yes, the slower the fan speed, the air coming out will be hotter. But the overall btu/hr extracted from fan coil will be lower than a higher fan speed. Also with slower fan speed the return water temp will be higher.
Re the ODR. If your targeting the lower return water temps , you will probably not want to use that FanCoil preset. I believe it is the hottest preset curve . You can try the others (Radiators an option?) or make a custom.
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Along with adjusting the fan speed, if you really want to nerd out, you can install a delta T circulator. This would automatically pick up the when the fan coil is running at low speed and reduce the flow rate to match.
I picked up a Wilo-Star E 21 ECM circ that does delta T while back, great value for the money.
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With two stages you have three possible settings — high, low, and off.
With outdoor reset, set the temperature curve so that on an average day the low speed is about right. On a windy day the high speed will kick on, on a day with lots of sunshine the fan will turn off.
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I'd worry that the fan and circulator would fight each other. The fan speed kicks up, so the circulator kicks up, which causes the heat output to rise, so the fan kicks back down and then the circulator kicks back down and the cycle repeats.
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With FCU water flow rate doesn't effect output all that much. 5F delta or 20F delta, unlike a change in air flow, BTU output is about the same.
Getting good delta does reduce RWT, so that is always a good thing with a modcon.
Most residential thermostats are designed for hot air supply, so they have a decent control parameters for it.
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At least in this setup w/ this handler, the ODR will have no effect on the fan speed. All it really does from what I can tell is vary the supply water temp to the hydro coils, and the handler just knows to switch on and off (at full fan speed). This is where I am hoping to learn more about other configs of the TAM7 to see if there is some ability to tune it more granularly.
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As for the physics behind why the BTU output is more despite the air being less hot, is simply due to a lot of warm air at the registers vs. only a little hot air? (ie, it's not as hot, but it's warm enough and there's a lot more of it delivered in an hour)
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