Fiddling with Return Temperature

[velvetfoot]

My buffer tank has a mixing valve on it so the return temperature to the boiler can be modified to anything.
The boiler currently running is oil
I had it at 155, now it's at 165. 165 gives 175-180 supply temp.
I figure with baseboard heating, if the boiler is running, 180 will give more heat.
165 setting is filling the buffer tank the same as 155.
I'll probably have to bump it down for the pellet boiler.

Any ideas/opinions on fiddling with return temperature?
(I know there is one person who bumps it up on a Froling wood boiler to get it to modulate.)

 

[JohnDolz]

My buffer tank has a mixing valve on it so the return temperature to the boiler can be modified to anything.
The boiler currently running is oil
I had it at 155, now it's at 165. 165 gives 175-180 supply temp.
I figure with baseboard heating, if the boiler is running, 180 will give more heat.
165 setting is filling the buffer tank the same as 155.
I'll probably have to bump it down for the pellet boiler.

Any ideas/opinions on fiddling with return temperature?
(I know there is one person who bumps it up on a Froling wood boiler to get it to modulate.)

You've seen my philosophy, I am running 134 Supply right now (tied to Outdoor Reset but just went down to look) and the house is 70+.

 

[velvetfoot]

You've seen my philosophy, I am running 134 Supply right now (tied to Outdoor Reset but just went down to look) and the house is 70+.

Yes I have, and it is intriguing, but probably not for me. I do setbacks, try to get a lot of % of heating from a wood insert, and only have a 120 gal. tank. Many times the boiler seems to be running when returning from setback, so I figure the higher supply temp, the quicker the return.

I was thinking about dual purposing the mixing valve to supply lower temp while on tank, but I don't think it's worth it.

 

[maple1]

IMO the higher the return temp, the lower the boiler efficiency.

But I have no idea the magnitude of the efficiency difference - it may or may not be negligible. It seems pretty evident though when trying to charge storage up to the utmost temps - seems to take more burning to get that last temp rise, and dT seems to be a bit lower thru the boiler at the higher temps. I think that is one of our 'rules of physics' re. heat exchanging - the bigger the temp differences between sides, the more heat moves from one side to the other. In this case, one side is the boiler water temp, the other side is the flue/internal tube temp.

 

[JohnDolz]

Yes I have, and it is intriguing, but probably not for me. I do setbacks, try to get a lot of % of heating from a wood insert, and only have a 120 gal. tank. Many times the boiler seems to be running when returning from setback, so I figure the higher supply temp, the quicker the return.

I was thinking about dual purposing the mixing valve to supply lower temp while on tank, but I don't think it's worth it.

Set back is definitely a challenge and not sure why I do it. I start my recovery at 3:00 AM because I am running such low flow temps and now such low flow rates (don't know the actual rate but the Alpha reads between 0 & 1 GPM). Was thinking that your pellet boiler could modulate similar to a modulating condensing boiler, running constantly but at a much lower temp. Kind of like cruise control vs. stomping on the gas. I think I shared with you that I have an Indoor sensor that overrides the Outdoor rest, basically by setting back every night I spend 5 or 6 hours "recovering" (this means that my actual flow temp is higher than what it would be if only the Outdoor rest were controlling the fow temp - at the end I am not sure if I am saving BTU's by setting back or losing BTU's because my Recovery requires higher flow temp). Maybe I can talk you into calculating that for me?.

 

[velvetfoot]

Maybe I can talk you into calculating that for me?

Yeah, good luck with that!

It's all interesting to think about.

 

[JohnDolz]

Yeah, good luck with that!

It's all interesting to think about.

I'm a Sales guy, always have to ask. My gut tells me I would be better of without using the setback but the other part of me can't help using a feature hat I have.

 

[Bob Rohr]

No question that the lower you run the boiler the more efficient. With condensing boilers, staying below 130 or so increases efficiency a lot as you recover energy that would go up the flue.
Boiler manufacturers have this data if you can find it. I have some data for condensing type boilers, not much on non condensing as far as % of efficiency increase at lower operating temperatures.

If you can, spread the ∆T that the boiler operates on to allow the coldest return. If you really need 180, run a wide delta to return back to the boiler as low as the manufacturer allows, the dew point of the fuel is the determining factor.

Obviously with fossil fueled equipment you need to stay above dew point, around 130F.

Another eye opener is how many days an area is at or below design condition. ASHRAE and others have that data available for most locations.

So the key to outdoor reset is to supply the exact, and lowest supply needed to cover the load at every changing condition.

Here is an example for Boston, with a design of 0F. Notice how many days you are at or below that condition.

Supplying the exact amount of energy provides the best comfort but also lessens or eliminates cycling of the equipment. Think cruise control.

 

[JohnDolz]

No question that the lower you run the boiler the more efficient. With condensing boilers, staying below 130 or so increases efficiency a lot as you recover energy that would go up the flue.
Boiler manufacturers have this data if you can find it. I have some data for condensing type boilers, not much on non condensing as far as % of efficiency increase at lower operating temperatures.

If you can, spread the ∆T that the boiler operates on to allow the coldest return. If you really need 180, run a wide delta to return back to the boiler as low as the manufacturer allows, the dew point of the fuel is the determining factor.

Obviously with fossil fueled equipment you need to stay above dew point, around 130F.

Another eye opener is how many days an area is at or below design condition. ASHRAE and others have that data available for most locations.

So the key to outdoor reset is to supply the exact, and lowest supply needed to cover the load at every changing condition.

Here is an example for Boston, with a design of 0F. Notice how many days you are at or below that condition.

Supplying the exact amount of energy provides the best comfort but also lessens or eliminates cycling of the equipment. Think cruise control.

Hi Bob - I run a Triangle Tube Boiler as well but really never figured out how to control the Delta on the Return.I barely get any Delta between Supply and Return which causes the short cycle. I get around this by setting the minimal time between firing cycles, This basically forces flow temp top drop before it fires again. I hardly ever have this boiler kick in (learned how good of a unit it was after I installed my wood boiler). In talking with Triangle Tubes Engineer we kicked around having it heat my 1000 gallons of storage (basically have it run for long periods of time at a very low rate) but the effort required to sync everything didn't seem worthwhile for how little I use it.

Regarding the diagram you posted, I think I am missing the obvious - how do I read how many days are at or below design conditions?

Working on the cruise control thing (outdoor reset tied to an indoor sensor) but every time I add something (this weekend was an electric zone valve to shutoff my bypass) I find that water finds a new place to go. You may recall our zone valve discussion on a different thread - I may need up there at some point out of necessity.

 

[Bob Rohr]

The flow rate through the boiler will determine the delta across it. In the installation manual they typically show a few options for flow rates.
Here is the pump sizing chart from one of the TT installation manuals, all are available online. Looks like this particular model will accept up to a 40° ∆ T across the HX. Low speed on a Grundfos 15-58 will provide that flow rate, looks like.

The example graph posted above shows a blue dotted line at design condition of 0F, requiring 160F supply. So a heat load calc was run for an example home, heat emitters selected to operate at 160F at design condition 0 outside maintaining 70F inside, for example.

The bottom axis shows the various temperatures throughout the Boston heating season. These are usually generated from 10 or 20 years worth of weather data. That data is also available online for most weather stations in the US and Canada.

Across the top the number of hours and % at that temperature.

So .2% of the heating season below 0° 0 being the design temperature for the Boston area.
2% 124 hours between 0-14°
36.5% 2258 hours between 28-41°
27% 1675 hours between 41- 54°

So in a nut shell 63.5% of the heating season is between 28- 54°

It's rare to find any location at or below design for more than 10% of the heating season. Basically indicating how most heating systems have oversized equipment for most, 90% of the season.

Variable output boilers and variable speed pumps are helping narrow that mismatch greatly.

True that, water will take the path of least resistance, some times balance valves are needed on multi zoned or complicated piping to assure proper flow rates to all loops or zones.

That is why you see those adjustable flow setters on quality radiant manifolds, it fine tunes the flow to each loop.

 

[velvetfoot]

Today, I was home when the oil boiler started, just as a 6 degree recovery from setback started. It took an hour and a quarter, which I thought was good. It was mostly one zone, the upstairs since the wood stove was assuming the downstairs load. It was a pretty cold day today (ambient in graph is about 7F higher than at our place-elevation).

I think that if there was no buffer tank, the oil boiler, which does not modulate, would've had to stop and start several times before the zone was satisfied. The buffer took the excess. If the boiler were turned down somewhat, to the degree that it could be, there'd be less quantity of heat going to the buffer, with the tank at a lower average temperature, but then, maybe there'd have to be another shutdown-tank drawdown-boiler start cycle to satisfy the zone. Right now, the boiler runs maybe once or twice a day.

Return from setback when the boiler isn't running takes longer. The tank draws down until the top is 128 and then the oil boiler kicks on. It takes a little while recircing while getting up to 135 or whatever the Caleffi protection valve starts to open. Admittedly, letting the tank drop down that far adds recovery time. So, the higher the supply temp the better.

Just maintaining a temp, the buffer tank can go a surprising way. Thing is, the wood stove can usually handle the non-transient load.

It'll be different with the pellet boiler though, which modulates and does not like as high a supply temp as the oil boiler.

The pellet boiler does like longer run times, and hence the buffer tank. Longer run times on the oil boiler, compared to the way it would normally operate, can't be bad for it either.

 

[JohnDolz]

The flow rate through the boiler will determine the delta across it. In the installation manual they typically show a few options for flow rates.
Here is the pump sizing chart from one of the TT installation manuals, all are available online. Looks like this particular model will accept up to a 40° ∆ T across the HX. Low speed on a Grundfos 15-58 will provide that flow rate, looks like.

The example graph posted above shows a blue dotted line at design condition of 0F, requiring 160F supply. So a heat load calc was run for an example home, heat emitters selected to operate at 160F at design condition 0 outside maintaining 70F inside, for example.

The bottom axis shows the various temperatures throughout the Boston heating season. These are usually generated from 10 or 20 years worth of weather data. That data is also available online for most weather stations in the US and Canada.

Across the top the number of hours and % at that temperature.

So .2% of the heating season below 0° 0 being the design temperature for the Boston area.
2% 124 hours between 0-14°
36.5% 2258 hours between 28-41°
27% 1675 hours between 41- 54°

So in a nut shell 63.5% of the heating season is between 28- 54°

It's rare to find any location at or below design for more than 10% of the heating season. Basically indicating how most heating systems have oversized equipment for most, 90% of the season.

Variable output boilers and variable speed pumps are helping narrow that mismatch greatly.

True that, water will take the path of least resistance, some times balance valves are needed on multi zoned or complicated piping to assure proper flow rates to all loops or zones.

That is why you see those adjustable flow setters on quality radiant manifolds, it fine tunes the flow to each loop.

Thanks for the explanation on the graph. I was missing the fact that the "design" is 1 set temperature. I am so used to thinking about things in terms of design being on a slope (see attached). I have the ability to move the slope up or down as well as the ability to "bend" the ends of the curve.

My Triangle Tube also runs with a slope (linear) and I have it set to something similar to the purple line in your graph. The result is that I am most often running a design temp that is 120 or lower which results in a relatively small Delta. When I was playing with the Trianlge Tube (early last year) I wasn't really thinking flow rate (I keep learning more every day and flow rate was not on my radar at the time). Since then I have replaced my 007's with Alpha's so I have the ability to slow things way down (I need to look at which Grundfos I have serving as my main Circulator in front of the Triangle). It sounds silly but being so mentally invested in the thermal storage heating I never want to shut it down and play with the Triangle. One of these days I am going to be away on a business trip, something will happen with my wood boiler and I will get the call from my wife and I will learn if I have my Triangle setup on the right slope.

My current "issue" is that I put a zone valve into my bypass so that all supply water must go into my zones. Each zone has an alpha (that I set on speed 1) and the main alpha set on Auto. The problem that I ran into is that the main alpha was pushing out more water that the calling zones needed. I realize the GPM reading on the alpha's is not accurate but figured it was somewhat relative. This made me wonder where the "extra" water was going. When I started looking at the tank temp for my Indirect DHW tank I found the answer. Basically water was getting shoved through the non-calling zones. I realied that this was a Supply question so I gave President Trump a call (he is a Business man) and he said if you have too much supply you need to increase demand (drop prices, increase pump speed at the zones, etc.). I opted to increase the zone pumps to speed #2 and this seems to have worked. I am waiting on a call from Grundfos to get their thoughts on ideal pump setup (this situation wil eliminate itself when I go to propane as the zone valve opens when the Triangle fires so if I ever want to use it as my primary heat-source I can knock he aplha's back to #1 and see if I can generate that Delta).

 

[velvetfoot]

I had unwanted flow through non calling zone pumps, fooled around with flow rates, and wound up adding zone valves (2). A little redundant, duh.

 

[JohnDolz]

I had unwanted flow through non calling zone pumps, fooled around with flow rates, and wound up adding zone valves (2). A little redundant, duh.

One day I may need to learn to sweat pipe but I am mechanically challenged. This causes me to lean on the good graces of a fellow Hearth reader. If you go back a month or so you will find a thread along the lines of "replacing Calculators with Zone Valves" (yes spell correct put "calculators" in there and I can't figure out how to edit it. I think if I went the Zone Valve route I would pull the Circs and have 5 Alpha's for sale at a great price. I am hoping that Grundfos gives me a call, I would like to get there thoughts on the ideal setup. For know increasing pump speed on the zones has increased demand enough that unwanted flow is prevented (keep in mind that the way I run I shoot for 3 of my zones to call for 24x7. The other 2 have hydronic blower heaters so I don't like those running non-stop (noise is annoying) so I set those at higher flow rates so they kick on and off).

 

[velvetfoot]

Yeah, I'm only running 2 zones. Generically, is this a result of running a 2-pipe system? Mine is 2-pipe, I forgot yours.

 

[JohnDolz]

Yeah, I'm only running 2 zones. Generically, is this a result of running a 2-pipe system? Mine is 2-pipe, I forgot yours.

I have a 4-pipe in my tanks, if I had to redo I would make it a 2-pipe. Assuming you are asking if the flow in the unwanted zones is a result of 2-pipe I would say no. I am no engineer but I would simply say it is a question of resistance. Once I installed Alpha's I could start seeing some #'s (understand they are not accurate but assume they should read consistently across aplha's or close enough) and I could see that my main circulator was pushing more than my zones were moving. It made me wonder where the extra water was going, turned out to be going through the bypass and into my return flow. Since I heat mostly from storage I figured I wanted to stop design temp water from going into the return without going through my zones. My hypothesis was that my Alpha, set to auto, would drop down the volume out due to the new pressure created by the closed bypass. Turned out not to be the case, output remained the same and water found a new place to go.

 

[maple1]

Did you try changing from auto, to speed 1? (Same as your zone circs). Thinking it should match gpms. Maybe?

 

[velvetfoot]

It's additive, though.

 

[maple1]

Ya, not sure how those things would act, with their changing flows & pressure drops happening. They might do some bouncing around off each other. Sounds like a round of zone valves would help - I could use a couple more Alphas if you do that.

 

[JohnDolz]

Ya, not sure how those things would act, with their changing flows & pressure drops happening. They might do some bouncing around off each other. Sounds like a round of zone valves would help - I could use a couple more Alphas if you do that.

Yes, if I drop the speed to 1 it solves the problem (because it decreases supply) but then when all of my zones kick it (i.e. Recovery from Setback) I am way under-powered. Yes it sounds like a round of zone valves would be in order (not sure my installer's wife agrees, nor mine) but if I had any commonsense I would just leave them on speed 2 or walk downstairs, put the alphas on speed 1, push the little lever on the bypass zone valve to open and live with the minute loss of BTU's that I was losing when I started this latest round of thinking.

 

[TCaldwell]

your installer likes coffee and pizza, i wonder f there is a way for the zone circs to talk to the supply circ, being that its all alphas there must be a common voltage, current or rpm signal. A question for grundfos.

 

[JohnDolz]

your installer likes coffee and pizza, i wonder f there is a way for the zone circs to talk to the supply circ, being that its all alphas there must be a common voltage, current or rpm signal. A question for grundfos.

Thanks for the thought, I will add that to my list for Grundfos. It is my installer wife that I worry about, it is the happy wife, happy life theory.

 

[TCaldwell]

i broke her in properly spending countless hrs in the boiler barn, to the point of her joking if there was another woman out there. She's not fazed that you to have become obsessed. Although I keep the honeydoo list current

 

[JohnDolz]

i broke her in properly spending countless hrs in the boiler barn, to the point of her joking if there was another woman out there. She's not fazed that you to have become obsessed. Although I keep the honeydoo list current

Just don't let her know that I am encouraging you to tilt at windmills.........

 

[maple1]

What about the zone circs on speed 1 and the supply circ on speed 2?

They're supposed to ramp up & down to meet demand - so was thinking the supply would do that, as more zones opened. But then that would change the pressure diff on the zone circs - there would maybe be some yo-yoing going on there to some extent? Interesting...