Delta-T thoughts

[twitch]

From what I've read here the higher the Delta-T the better the efficiency. I can understand the thought, but what about the return temp protection?

For instance, I just went to stoke the boiler because it was starting to cool off and after a few minutes these were the temps:

Boiler source: 169
Boiler return (after termovar): 155
Boiler return (before termovar): 110

I'm thinking that's a Delta-T of 59, sounds good but the termovar is barely open not letting much actual return water back to the boiler. So the boiler is basically feeding itself and not much is going to the air handler to heat the house.

Now this is when the house was cold, about 64, so the return air was cold and was pulling quite a bit of heat out of the water, but I think a smaller Delta-T in this situation would be better.

My 3 speed grundfos was on high and the air handler blower was on high. My system is a Tarm Solo 30 and an air handler, no storage.

It seems to me that sometimes, too large a Delta-T from source to prior to return protection is not that good a thing.

I could be completely wrong here and am looking for input

Any thoughts??

 

[Nofossil]

So you're getting a rise of 14 degrees through the boiler and a drop of 59 degrees across the load (HX?). Sounds like the boiler isn't putting out much heat, and the Termovar is doing a good job of making sure that you have a good inlet temp, and that results in a usable outlet temp. When the boiler isn't putting out a lot of heat, you could use a much slower speed on the Grundfos - no point in circulating tons of water through the bypass.

 

[twitch]

My thought for having the pump on high was that the air exiting the HX would be hotter than if the the pump was on a lower speed, trying to heat a cold house up faster. I'm thinking the boiler isn't putting out much heat because the before termovar return is too cold, causing it to close and having more of the boiler source going directly to the return...does that make sense?

 

[kabbott]

As long as the boiler is not idling it really does not matter what the delta is(within reason at boiler safe temps), all the heat is being moved.

Seems to me if you want the delta lower you would need to slow down the blower speed but I don't think that is necessary

As long as the delta never becomes to LOW, you could probably lower the circulator speed as well.

 

[Nofossil]

No, the amount of heat that the boiler puts out has nothing to do with how hot or cold the water is coming into it. If you have a good fire and you're putting out the rated BTU/hr, you'll get a good temperature rise at a decent flow rate. A 14 degree rise through the boiler is a bit low, but not terribly low. What's surprising is the enormous drop through the heat load. Do you see those numbers for a sustained period?

 

[twitch]

No, the amount of heat that the boiler puts out has nothing to do with how hot or cold the water is coming into it. If you have a good fire and you're putting out the rated BTU/hr, you'll get a good temperature rise at a decent flow rate. A 14 degree rise through the boiler is a bit low, but not terribly low. What's surprising is the enormous drop through the heat load. Do you see those numbers for a sustained period?

The only time I see those numbers is when the house / system is cold and I'm starting a new fire to warm it all up. When the house is up to temp, things run pretty well. In my warped mind, the rise through the boiler depends on the return protection. My termovar at minimum is usually around 155 independent of the temp of the return before the termovar. So even when the boiler source is fairly good (170+) and the return is low (110 or so) the return temp causes the termovar to close and the source goes more to the return than the load.

I really need to get my monitor system logging so I can experiment with different pump / blower speeds.

 

[twitch]

The way I'm looking at it, and it may be wrong, is that if the pump is running faster, the Delta-T at the water to air HX will be lower (hotter return temp). My HX is plumbed with a reverse flow so the source is at the exit and the return is at the entrance.

 

[kabbott]

I was thinking that 14 degrees is a bit low. I see around 17-22 on my boiler with a taco 00R on high and a very short loop of 1 inch copper and 1.25 inch
iron near boiler piping.

Does the deltaT drop much when the boiler is really cooking?
Now that I am getting used to burning the new boiler I have come to rely on the stack temp, I know where it normally runs with good dry wood and when it drops a few dozen
degrees the boiler delta goes down a bunch. Usually means the nozzle is not covered( I have been burning alot of "junk" and it tends to bridge some)

Nofo, I think the large drop at the fan coil is because the flow is so low because most is just being re-circulated through the boiler, which should gradually come up if the boiler
is large enough.(I think :red: )

Now that I have 2 variable speed pumps(boiler and bypass loop) wacky things happen to the temps as the speed changes... It gets complicated at that point.

 

[kabbott]

The way I'm looking at it, and it may be wrong, is that if the pump is running faster, the Delta-T at the water to air HX will be lower (hotter return temp). My HX is plumbed with a reverse flow so the source is at the exit and the return is at the entrance.

That would be true... However with the termovar it will re-circulate more water to the return.
I think as long as the water is hot enough for your fan coil there is nothing to worry about. If you want a higher air temp then slow down the blower a bit.

 

[willworkforwood]

I usually don't feel qualified to comment on other folks systems, but in following this thread I'm feeling like Nofo's original low boiler yield comment might be the key. In my setup, with the current outside temp of 38, I'm maintaining an already-warm house with 165* water (very small fires). But it would take quite a while if my house was 64* to warm it with 165*. In that situation, I would load more, get a bigger fire going, in order to get the boiler output up to 180-190+*. Now, I believe forced air doesn't need the temps that my BB rads do, but I'm still wondering if your 170 output is enough to heat your cold house up quickly. Do you have a heat loss calc telling you what you need? Have you seen boiler output temps over 170, or is that the max so far? Do you have a big fire going with well-seasoned wood (20%). If yes, maybe a brand new boiler refractory hasn't dried out yet?

 

[jebatty]

Boiler source: 169
Boiler return (after termovar): 155
Boiler return (before termovar): 110

I’m thinking that’s a Delta-T of 59, sounds good but the termovar is barely open not letting much actual return water back to the boiler. So the boiler is basically feeding itself and not much is going to the air handler to heat the house.

I see nothing unusual here. I will get nearly the exact same result with my Tarm Solo Plus 40. It takes substantial boiler output to preheat the return before Termovar to provide the 155 supply water. Only the excess btu's will be output to the system.

With an air handler system that may require 180 water and with a deltaT of 20, to get consistent, good heat, and assuming no storage, you are going to have to maintain hot boiler water. System recovery after a cool down will take quite a bit of time, depending on the amount of cool water in your system.

If I assume 50 gallons of water in your system (no storage), it will take about 30,000 btu's just to heat that water from 110 to 180, and that's with your air handler "off." Your Tarm 30 is rated at 100,000 btu's, but that would be maximum, so assume 80,000 btu's normal output. So it would take nearly 1/2 hour, and longer on a cold start firing, just to heat the system water before you can start to get "good" heat from the air handler.

If not, I would suggest a control system that circulates water but does not turn the air handler "on" until system water temp reaches the temperature that the air handler needs to provide its rated output, or at least some temperature close to that. That way, at least, you will get good heat as soon as the air handler turns "on." Similarly, the air handler should be "off" when some minimum system water temp is not available. If you don't have that control, your handler will be outputting cool/warm/drafty air until the whole system can catch up, will obviously takes an undesirable amount of time.

What is the rating of your air handler at what temperature input and gpm's? What is the heat load of your house? Is your boiler, air handler, heat load, and circulator matched to provide the heat you need? All of those are critical.

I also suggest changing the circ speed to low. You might find that the low setting is adequate for continuous use. Moving from low to medium probably will result in a pretty good increase in gpm's, depending on system head. But moving from medium to high likely results in very little increase in gpm's. And it would only make sense to turn the speed up, if needed at all, after system water is up to minimum temperature needed by the air handler to produce the warm air you want.

With my Tarm Solo 40 and system return of 110, like you have, I have my 15-58 set at low and I leave it there until system return gets into the 150-160 range. Only then will I consider increasing the speed, as it then is likely that boiler output may exceed demand and idling will begin to occur. For you, I suggest increasing the speed from low only if your boiler starts idling. If it doesn't idle, there is no need to increase the speed because you can't get anymore btu's than the boiler is producing. Idling can only occur when boiler output exceeds demand and boiler water temp reaches the 185-190 limit.

 

[bigburner]

"i see nothing wrong here" could not agree more. It's being operated out of the system parameters. Do an "air to air split" or "water to water split". estimate GPM then you got BTU'S. Cycling the blower motor keeps the temp up in the HX but you gain nothing. hotter air/less cfm or more cfm/cooler air = same BTU's If the boiler ides then an adjustment is needed if demand isn't meant.

 

[mwk1000]

My system will pull water from the outlet side of the HX before it is returned to the tank. This is 110 at it's coldest point and enough to heat my house. 110 will give me 90 at the registers the same as the propane furnace does. ( Hate it -- never feels WARM)

I have put the tank circulator on low before to raise it a bit but it stays on medium now. Came home to a 90 degree tank top to bottom. furnace running nonstop trying to keep house at 68 so the tank was fully mixed.

Here are my HX readings as you progress from cold to hot, they seem similar to twitch's

Just started circulator boiler at 140 thermovar does a good job of keeping the returns at 140 after two days the tank was very cold yesterday. The number after the : is the delta T of that side of the HX



SourceFeed higher than LoadFeed - set relay flag to HIGH/ON 143/96
Setting Relay pin HIGH Run Sec: 131858 ( been off for this long, 86400 = 24 hours ) This is the line that actually turn on the tank circulator
log:SourceFeed:F:143.60 ( into HX from boiler )
log:SourceReturn:F:113.00:30.60 ( HX return to boiler)
log:LoadFeed:F:91.40 ( into HX from tank bottom )
log:LoadReturn:F:134.60:43.20 ( out to tank top )
log:BoilerFeed:F:145.40 ( Output right at boiler, calibrated with inline thermometer )
log:BoilerReturn:F:140.90 ( inlet right at boiler after thermovar )
Requesting temperatures... DONE
Not setting relay - cycle interval not cleared: 181/300
log:SourceFeed:F:145.40
log:SourceReturn:F:95.90:49.50
log:LoadFeed:F:91.40
log:LoadReturn:F:111.20:19.80
log:BoilerFeed:F:147.20
log:BoilerReturn:F:141.80
Requesting temperatures... DONE
Not setting relay - cycle interval not cleared: 241/300
log:SourceFeed:F:145.40
log:SourceReturn:F:95.90:49.50
log:LoadFeed:F:91.40
log:LoadReturn:F:111.20:19.80
log:BoilerFeed:F:148.10
log:BoilerReturn:F:141.80

30-40 min later still cold but 20 degrees is going into the tank with almost 60 stripped from the boiler due to still low flow rate. ( The source flow is almost double the load flow when fully open )
equesting temperatures... DONE
SourceFeed higher than LoadFeed - set relay flag to HIGH/ON 156/97
log:SourceFeed:F:156.20
log:SourceReturn:F:96.80:59.40
log:LoadFeed:F:92.30
log:LoadReturn:F:114.80:22.50
log:BoilerFeed:F:158.00
log:BoilerReturn:F:149.90


Starting to open and the flow is now higher at the source ( Thermovar half open )

Requesting temperatures... DONE
SourceFeed higher than LoadFeed - set relay flag to HIGH/ON 172/102
log:SourceFeed:F:172.40
log:SourceReturn:F:122.90:49.50
log:LoadFeed:F:97.70
log:LoadReturn:F:152.60:54.90
log:BoilerFeed:F:173.30
log:BoilerReturn:F:158.00

Full speed with a very hot boiler ( the source return is at 136 so the thermovar has little to do and is almost wide open. The delta T is now much lower on the source side due to the bigger pump.

SourceFeed higher than LoadFeed - set relay flag to HIGH/ON 169/112
log:SourceFeed:F:169.70
log:SourceReturn:F:136.40:33.30
log:LoadFeed:F:107.60
log:LoadReturn:F:158.00:50.40
log:BoilerFeed:F:170.60
log:BoilerReturn:F:157.10

 

[Nofossil]

Good discussion. As long as the situation described in the original post is a startup condition (first hour or so of operation) the system is doing exactly what it should.

Running the circ fast doesn't result in any more flow through the loads because the Termovar limits that flow to prevent too low an inlet temp. At startup, the flow through the load HX starts at zero and only ramps up as everything in the system comes up to temp.

Most systems are designed with a flow rate through the boiler that will result in about a 20 degree rise at rated output. That's not a hard requirement, and there may be reasons to choose a flow rate that's higher or lower. This system's reported 14 degree rise is a bit low and suggests a higher than normal flow rate through the boiler and/or a boiler that's not up to full output (still using heat to bring the boiler mass up to temp, for instance).

If it were me (and it's not) I'd consider allowing a lower inlet temp, since 155 is *probably* higher than you need to avoid condensation, then adjust the boiler flow rate to get an acceptable outlet temp.

I wouldn't worry at all about the numbers here if they're happening during startup.

 

[markmudd]

I Don't have lots of experience on this matter. On my wood boiler water to air heat exchanger, the return temp and boiler output temp keep dropping as long as that fan is running like it would in a cold house based on thermostat calling for heat. I would think you need a limit switch running to fan relay to make sure fan will not run if return temps are cold to allow boiler to keep up although it depends on fan speed and exchanger size too. Cycling fan with hotter air seems to work better than continuous run of fan pushing cooler air at least for me.

I just got my system on line and am experimenting with flow rates etc. So far I have only used low pump speed and it is working good. It is a 1" loop close to boiler and no other zones on line yet.