Return Temp

[George Webster]

Seems the standard for minimum return temperature is stated as 140F by most folks. The manufacturer of my boiler requires 149 be maintained, but based on advice from others I've always used 140F - I have a Danfoss valve with that value thermostat on my return. Wondering if this is a bad thing and what value most folks recommend. What is the science behind this number?

 

[maple1]

Have you measured your return water temps to see what they actually are? And what boiler do you have?

All I know about the number is that it seems to have been one that is generally accepted to be high enough to avoid firebox condensation. Don't know the science side of how that number was derived.

 

[Bob Rohr]

Seems the standard for minimum return temperature is stated as 140F by most folks. The manufacturer of my boiler requires 149 be maintained, but based on advice from others I've always used 140F - I have a Danfoss valve with that value thermostat on my return. Wondering if this is a bad thing and what value most folks recommend. What is the science behind this number?

The goal is to assure the boiler does not run in condensing conditions for extended periods. The valve assures the boiler warms to the temperature you mentioned, above the dewpoint of the flue gas, to prevent creasote build up.

With solid fueled devices that number is a bit of a moving target as it depends on the moisture content of the fuel, and the fuel air mix adjusted at the combustion chamber.

Remember also those valves have an operating differential. Typically a 140F valve has an 18° add. so the temperature at the valve would need to be 158F to close the bypass 100%.

I suspect that manufacturer wants a little "wiggle" room opting on the high side, to protect the boiler. that valve will bypass a small amount up to 158F

Page 24 of this journal explains the science of flue gas condensation and has a dewpoint chart based on the air/ fuel ratio.

http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_10_us.pdf

 

[jebatty]

Bob's comment is good. One mfr manual I've seen says 160F but in talking with the mfr, info was 140F was OK; another manual says 130F minimum; but 140F seems to be the "standard," as you say. Apart from that, my experience is that my Tarm seems to operate better, maybe more efficiently, if I keep the return around 150F or somewhat higher rather than closer to 140F.

I have the Termovar and a balancing valve that I use to manage minimum return water temp. For example, I was gone for a week over Christmas and my shop had cooled down to 40F (outside air temps had been into the -20'sF while I was gone, and highs on some days temps were not breaking 0F). Return water from storage was about 80F, I managed return water to the boiler to a little above 150F by opening the balancing valve quite a bit, which I then closed down most of the way as system return water rose into the 130F range.

I have a temp controller on my storage tank that shuts down draw from the tank at 80F just to make sure that if I'm gone for an extended time, there is no chance that my tank will ever cool to a freezing temp. Supplemental electric heat from a 5000w space heater will keep the shop and all plumbing above freezing. The electric heat turned "on" at the 6th day away point, and the storage supplied heat up to that time keeping the floor at my 62F set point.

 

[George Webster]

Have you measured your return water temps to see what they actually are? And what boiler do you have?

All I know about the number is that it seems to have been one that is generally accepted to be high enough to avoid firebox condensation. Don't know the science side of how that number was derived.

I have an Attack DP35 Profi. The return temperature is measured with an active thermistor device attached to the exterior of the return manifold, immediately before the inlet to the boiler and it is heavily insulated to provide good thermal conduction. It also agrees with an analog gauge which has a well directly in the return flow.

 

[George Webster]

The goal is to assure the boiler does not run in condensing conditions for extended periods. The valve assures the boiler warms to the temperature you mentioned, above the dewpoint of the flue gas, to prevent creasote build up.

With solid fueled devices that number is a bit of a moving target as it depends on the moisture content of the fuel, and the fuel air mix adjusted at the combustion chamber.

Remember also those valves have an operating differential. Typically a 140F valve has an 18° add. so the temperature at the valve would need to be 158F to close the bypass 100%.

I suspect that manufacturer wants a little "wiggle" room opting on the high side, to protect the boiler. that valve will bypass a small amount up to 158F

Page 24 of this journal explains the science of flue gas condensation and has a dewpoint chart based on the air/ fuel ratio.

http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_10_us.pdf

Yes, and I have had issues with the 140, so I am thinking of going to a 149. Certainly, the 9 degree differential has little effect on boiler efficiency and is better for the boiler. I have seen return temps as low as 120 for sustained periods with the current element, even though the boiler output is in the 160+ range. The valve is supposed to just begin opening at 140, so you should never see that condition. Most posts I've seen indicate just the opposite, that the valve opens late, causing the return to be higher than 140 pretty consistently. The 18 degree add is the reason I went with the 140, since that seemed safe. Not what I'm experiencing. I do agree with what you say about return temp as well. My boiler seems to perform far better when I maintain the return above 150. Besides, it' hard to get much transfer to storage when the return is that cold, as it has a direct effect on outlet temp. When I maintain inlet to a min of 150, I get a delta T from inlet to outlet about 7 degrees higher than when the inlet is at 140, so I know the thermal transfer is better.

 

[George Webster]

Bob's comment is good. One mfr manual I've seen says 160F but in talking with the mfr, info was 140F was OK; another manual says 130F minimum; but 140F seems to be the "standard," as you say. Apart from that, my experience is that my Tarm seems to operate better, maybe more efficiently, if I keep the return around 150F or somewhat higher rather than closer to 140F.

I have the Termovar and a balancing valve that I use to manage minimum return water temp. For example, I was gone for a week over Christmas and my shop had cooled down to 40F (outside air temps had been into the -20'sF while I was gone, and highs on some days temps were not breaking 0F). Return water from storage was about 80F, I managed return water to the boiler to a little above 150F by opening the balancing valve quite a bit, which I then closed down most of the way as system return water rose into the 130F range.

I have a temp controller on my storage tank that shuts down draw from the tank at 80F just to make sure that if I'm gone for an extended time, there is no chance that my tank will ever cool to a freezing temp. Supplemental electric heat from a 5000w space heater will keep the shop and all plumbing above freezing. The electric heat turned "on" at the 6th day away point, and the storage supplied heat up to that time keeping the floor at my 62F set point.

My storage arrangement has 2 zone valves in parallel with a circulator, one 8cV the other a standard 3, providing the ability to regulate flow to the boiler side of the FPX during charging. If I use these to help provide more return water, without loading the tank side of the FPX, I can regulate my return well above the 150 point. I'd still prefer to have the valve do its job, just in case. That's what it's there for, so I guess I will try a 149 element. I'm still working with my combustion tuning. Wish I had an exhaust gas analyzer, since it is virtually impossible to view the flame color on my boiler, since the combustion blower is at the flue. You really don't want to open the secondary chamber door when it's running.

 

[Bob Rohr]

Yes, and I have had issues with the 140, so I am thinking of going to a 149. Certainly, the 9 degree differential has little effect on boiler efficiency and is better for the boiler. I have seen return temps as low as 120 for sustained periods with the current element, even though the boiler output is in the 160+ range. The valve is supposed to just begin opening at 140, so you should never see that condition. Most posts I've seen indicate just the opposite, that the valve opens late, causing the return to be higher than 140 pretty consistently. The 18 degree add is the reason I went with the 140, since that seemed safe. Not what I'm experiencing. I do agree with what you say about return temp as well. My boiler seems to perform far better when I maintain the return above 150. Besides, it' hard to get much transfer to storage when the return is that cold, as it has a direct effect on outlet temp. When I maintain inlet to a min of 150, I get a delta T from inlet to outlet about 7 degrees higher than when the inlet is at 140, so I know the thermal transfer is better.

I started with a 140 but found my boiler fan would start to ramp down before the valve closed the bypass 100% I went down one notch and it seems to work well.

I'm not that happy with the temperature controller on my early model EKO 40, which is all metric and Euro voltage, it doesn't like to run up to 180F, with the dial cranked. I have been moving the sensor around and just off the jacket trying to fool that high temperature. I think is my controller operated to a higher setpoint before ramping down the fan speed the higher temperature sensor would work fine.

Maybe and adjustable 3 way would be nice for those that like to fine tune to peak performance. Every system has it's own idiosyncrasies.

 

[George Webster]

I started with a 140 but found my boiler fan would start to ramp down before the valve closed the bypass 100% I went down one notch and it seems to work well.

I'm not that happy with the temperature controller on my early model EKO 40, which is all metric and Euro voltage, it doesn't like to run up to 180F, with the dial cranked. I have been moving the sensor around and just off the jacket trying to fool that high temperature. I think is my controller operated to a higher setpoint before ramping down the fan speed the higher temperature sensor would work fine.

Maybe and adjustable 3 way would be nice for those that like to fine tune to peak performance. Every system has it's own idiosyncrasies.

I think that's the same controller as the Attack uses, an RK2001UA? Piece of crap if you ask me. Much of the way it works makes no sense to me. For example, it has a parameter you set that tells the boiler how long to wait until you reach the setpoint, after which iot will shut down the boiler. I run my output at 190 peak, so if it doesn't reach that, which is often the case - it's just set that high so that the fan doesn't start to back off around 187. My control system will compensate for that and the temp stays down. Anyway, I can't tell you how many times on a long burn, I have a full fire going and the controller goes into fuel out mode. Very annoying. I am going to add a variable control to my combustion fan and another to the circulator so that I can control the combustion process from my Vesta control system. Once there, I will disable the RK.

 

[Fred61]

I started with a 140 but found my boiler fan would start to ramp down before the valve closed the bypass 100% I went down one notch and it seems to work well.

I'm not that happy with the temperature controller on my early model EKO 40, which is all metric and Euro voltage, it doesn't like to run up to 180F, with the dial cranked. I have been moving the sensor around and just off the jacket trying to fool that high temperature. I think is my controller operated to a higher setpoint before ramping down the fan speed the higher temperature sensor would work fine.

Maybe and adjustable 3 way would be nice for those that like to fine tune to peak performance. Every system has it's own idiosyncrasies.

I have the Eccoster2 on my boiler which is euro as you describe. I t only allows the boiler to achieve 80C.

What I did was to move the sensor up into the insulation (it took a few tries to find a location that allowed the boiler to achieve 195) and in it's place I placed the source probe from my Tekmar 156 and I'm now controlling the circulators with it. The controller basically is my on/off switch and high limit switch so with the probe buried in the insulation, the boiler temp is 195F when the Eccoster2 shuts off at 80C. Since I batch burn only to storage it has given a lot of flexibility with the storage probe inserted in the upper well on the tank giving me the option to use the differential setting and other high and low limits on the Tekmar.

 

[George Webster]

I have the Eccoster2 on my boiler which is euro as you describe. I t only allows the boiler to achieve 80C.

What I did was to move the sensor up into the insulation (it took a few tries to find a location that allowed the boiler to achieve 195) and in it's place I placed the source probe from my Tekmar 156 and I'm now controlling the circulators with it. The controller basically is my on/off switch and high limit switch so with the probe buried in the insulation, the boiler temp is 195F when the Eccoster2 shuts off at 80C. Since I batch burn only to storage it has given a lot of flexibility with the storage probe inserted in the upper well on the tank giving me the option to use the differential setting and other high and low limits on the Tekmar.

At least the RK allows an output up to 195. Higher than I ever want mine, but it does provide a lot of flexibility.

 

[BoilerMan]

My storage arrangement has 2 zone valves in parallel with a circulator, one 8cV the other a standard 3, providing the ability to regulate flow to the boiler side of the FPX during charging. If I use these to help provide more return water, without loading the tank side of the FPX, I can regulate my return well above the 150 point. I'd still prefer to have the valve do its job, just in case. That's what it's there for, so I guess I will try a 149 element. I'm still working with my combustion tuning. Wish I had an exhaust gas analyzer, since it is virtually impossible to view the flame color on my boiler, since the combustion blower is at the flue. You really don't want to open the secondary chamber door when it's running.

George, I have an Attack DPX45. Not being able to see the flame in the secondary chaimber drove me crazy too! I met Dean Zook at a boiler fair and his Varms have a little round window in the secondary chaimber and he said they were not factory. He had them put on at the factory for his display boilers (they now have them from the factory) and I had him order one for me. It's a 1" pipe thread you need to cut a hole and tap the cast iron door is the only drawback.

TS

 

[maple1]

I started with a 140 but found my boiler fan would start to ramp down before the valve closed the bypass 100% I went down one notch and it seems to work well.

I'm not that happy with the temperature controller on my early model EKO 40, which is all metric and Euro voltage, it doesn't like to run up to 180F, with the dial cranked. I have been moving the sensor around and just off the jacket trying to fool that high temperature. I think is my controller operated to a higher setpoint before ramping down the fan speed the higher temperature sensor would work fine.

Maybe and adjustable 3 way would be nice for those that like to fine tune to peak performance. Every system has it's own idiosyncrasies.

Sounds like you & George have the opposite thing happening with your Danfoss's - one acts higher than it is rated, and the other lower? Wonder if they're the same model? Different ages?

 

[George Webster]

George, I have an Attack DPX45. Not being able to see the flame in the secondary chaimber drove me crazy too! I met Dean Zook at a boiler fair and his Varms have a little round window in the secondary chaimber and he said they were not factory. He had them put on at the factory for his display boilers (they now have them from the factory) and I had him order one for me. It's a 1" pipe thread you need to cut a hole and tap the cast iron door is the only drawback.

TS

That is a pain - cast iron tapping that is. If you could get me contact info, that would be awesome. Do you recall what it cost?

 

[George Webster]

Sounds like you & George have the opposite thing happening with your Danfoss's - one acts higher than it is rated, and the other lower? Wonder if they're the same model? Different ages?

Mine was new last year and it seems to have behaved this way from day one. It's a 511 series.

 

[BoilerMan]

That is a pain - cast iron tapping that is. If you could get me contact info, that would be awesome. Do you recall what it cost?

It was under $40........ PM'd you info.

TS