Outdoor Reset worth it?

Brad White_203

The 140F minimum, so glad you grasp that.

Here is my take on such systems:

1) We all agree that your boiler temperature ought not go below the 130 to 140F range- there is some wiggle room here, are we talking absolute minimum?, Average water temperature? Or being in the lower range for how long? Those are academic details. Point is, we want to keep the boiler as cool as possible but above the flue gas dewpoint, which varies. 135F return to the boiler is said to be safe and is often controlled to.

2) Given the above, the range of the boiler temperature is from roughly 140F to say, 180F, whatever your upper level, coldest day design happens to be. Forty degrees of range over maybe 60 to 80 degrees of outside temperature swing, more or less. Not great but this is where things must begin.

One "admits defeat" on this point when you do not have a condensing boiler. It is OK.

3) The radiation side. Ah, now we are talking.

A: Radiation output can go anywhere from the coldest day, using nearly the hottest water temperature off the boiler (if sparely sized), to lower, maybe even 140 degrees, also on the coldest day.

B: From this highest temperature, the radiation loop can modulate down to nearly room temperature. Basically the point where the room temperature, outdoor temperature and radiator temperatures nearly come together, that is the point where all heating needs cease. Thus your radiation loop can go from 180F down to maybe 80F, over the same outdoor temperature range.

If you have insulated and sealed your house and have old radiators sized for the original condition, congratulations! Your highest required temperature is much lower than the presumed boiler temperature. It may even be lower than the "minimum boiler temperature". That is also OK. This is a good thing.

Now, you have to figure out how to make this happen. Keep the boiler with it's own circulation and with reset in that more limited upper range, great. 180 to 140 F., say.

Decouple that boiler circuit from the radiation circuit,(via a mixing device, 4-way valve, heat exchanger, injection circulator, low loss header, etc.) and let that radiation enjoy the full range of reset for maximum savings and maximum comfort.

Cool.

Bob B_2

I've heard a few different sides of this...

Given my 1996 W-M oil-fired cast-iron regular old h/w boiler (not mod-con or anything fancy), will I see much savings from installing an outdoor reset control? Probably a Tekmar 260 since I have an indirect on the boiler.

I won't mention install quotes here, but if it would save me 10%, it would take anywhere from 4-6 years payback at today's oil prices. Of course when heating oil goes back up, that timeframe shrinks.

Is 10% with a non-modcon realistic?

Thanks for any opinions,

George

tim smith

I would be ?? 10%. I find reset to be a hard one to quantify

I like reset for comfort etc but the savings on an old boiler I just don't know what! I look forward to what others will say here. Tim

Paul Pollets

Savings

We've done many retrofit conversions with the Tekmar controls (over 100). In each case, savings were at least 15% and as much as 50%, depending upon heat emitters and boiler type. The return water temps must be addressed when adding the control. This means either a bypass or mixng valve, controlled by the control. Return temps into the boiler must be higher than the condensing point of either oil or gas.

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Bob B_2

Thanks - neither installer has mentioned changing any of the return piping to accommodate return water temps. Is that a red flag? I was aware of this issue, but thought that keeping the minimum boiler water temp at 140* would avoid this. Can you set up an outdoor reset so you don't have to deal with bypass/mixing valve? Does 140* minimum avoid this at all?

FWIW, I have baseboard fintube (3 zones + 1 indirect zone) on this 4 section cast iron W-M boiler.

George

Bob B_2

Thanks Brad. I have a few questions on your (most informative) response.

1. We talk about minimum 140* +/- boiler water temp, but then there is mention of return water temp. These OR controls monitor boiler supply temp, so how do we ensure 135* return temp? I imagine it is dependent on how much heat is lost as the water goes through the zone. Can one assume that there is a max of x* lost when supplying 140* water?

2. If I understand your radiation side discussion, are you talking about some sort of primary/secondary config? If so, I don't think I'm going that way right now, because from a budget standpoint, I imagine that adds $$ to this effort and I am looking for a semi-easy fix. I do understand that what you describe gives me much fuller range of reset, but I think I am admitting defeat and for now, anyway, sticking with 140-180* range. Perhaps in phase 2 (i.e. next year), I can consider that.

Given the limited reset range, is there real savings here? that is the question in my mind.

Thanks again,

George

P.S. good attic insulation (R38), very little wall insulation (slated for next year, when outside painting occurs), all new windows.

Brad White_203

Good Questions-

Yes, that "140F" does require some qualifications.

Normally, in a cast iron boiler system where this is a concern, the return water is controlled to maintain a 135F return water setpoint. (Dewpoint of flue gasses varies as stated, with CO2 content, amount of excess air and a few other variables. 135F return water is deemed "safe".)

Now, picture if you will, hot water leaving the boiler with it's own circulator pumping away.

That water temperature can be any range between 180 and 140, indexed to the outdoor temperature.

The first device that water sees is a valve, a 3-way valve. Like a traffic cop, that valve monitors the return water temperature and when it drops, approaching that 135F temperature, that valve diverts that hotter water directly to the return in order to elevate that temperature. The remainder goes off to the radiation loop/system.

Now, it may well take a lot of 140F water, maybe 75 percent or so, to raise that return water (maybe coming back at 120F) to get to 135F, but this is OK, or rather, an acceptable sacrifice.

Of course, what one assumes during this time is that the building load is lessened because the space is at or near setpoint temperature and/or the outside air temperature is milder than design. Thus less water is needed to the radiators anyway.

When the boiler return water is at or above 135F, that valve, that traffic cop valve, allows more boiler flow to the radiation loop if it is needed.

There are various ways and permutations to what I described, which is but one way. Not the only way, but you get the idea.

To your second question, yes, I am describing Primary-Secondary. I should have been more clear on that, but it is exactly what I was talking about.

Sure, it costs more but if you ever want to get there, no harm in starting with your boiler-side reset, however limited it may be.

I will offer this, if you use a 4-way mixing valve (Taco i-Valve, Tekmar, ESBE and others), you can, with one device, gain decoupling, outdoor reset and boiler protection in one device. Check out Taco for a simple device/system and see what you think.

Such valves do not address the boiler side reset (that is what the Taco 700 series controls do), but you can still get good savings and better comfort in your living space, even if your boiler is kept at a warmer constant temperature.

Do both when you can, but my point is, it may well be within reach right now or closer than you think.

The worst feeling I get, (well not the worst but I need some drama , is when I advise a client to do "X" as an energy savings measure and they call me in April saying, "I am about to implement that system change you told me about in October".... Blow the entire heating season's potential savings for what purpose I do not know. They could have paid for said change sometimes.

I think I am being clear here.

Bob B_2

Thanks again Brad.

In reading your replies, I believe that you would mandate a valve to mediate return water temp. Not to put one in is asking for trouble - hence your "red flag" title. Yes?

I have to imagine that there are many installations without these - I have two installers (one from the wall) who have made no mention of doing this. To be fair, I haven't specifically asked but I don't think they are thinking of this. And, not that I'm going this way, but if you look at something like the Beckett Heat Manager, they talk about simplicity of installation (heat sensor on supply pipe, wiring into burner, done) and make no mention of any of this.

Just when I thought I was getting close to pulling the trigger on this, it gets more complicated, or so it seems...

George

Brad White_203

I would emphasize

that the way I mentioned that I would do this (maintain boiler return temperature) above a certain level, is but ONE way to do it. Ask 20 practitioners and you will get 30 opinions. I could give you five myself.

So, absent specific questions, I would give your installers the benefit of the doubt. And just because they come from The Wall, is not a mark of infallibility. Everyone has oversights, me more than anyone I know. But the upside is, we Wallies tend to know it and want to do better. So it's all good.

Yes on the red flag, not the way it is done, but that it is done. Variable speed pumps are another, mixing valve on the return, that 4-way mixing valve with a return-side sensor, all do the same end-goal.

I am not familiar with the Beckett Heat Manager specifically, but I imagine that they to know the principles.

Each way has it's complications, each has it's simplicities.

What I would not want you to do (as I have seen too often) is outdoor reset installed on the boiler alone, an aggressive reset schedule programmed in, and a boiler with shortened life or deteriorated to an especially unsafe level.

Singh_9

Ask TOH

Did anyone see this past week episode of This old house.
The infamous Rich Trethwey (sp) installed a Tekmar ODR control on an existing
gas cast iron boiler. But did not get into control set up, or boiler protection.

He guessed at least 10%-15% in savings

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Tom S._2

I did!

I did see that, and I froze the screen so I could see what model it was (256, alas, I want/need the 260 b/c of indirect tank).

Still, I was happy to see it going in, and it helped reinforce my desire for one, but as you mentioned there was no talk of primary/secondary, return water temp, etc.

Like I said, there must be quite a few of these installed without thought to p/s etc.

George

tim smith

Regarding my post above and in light of post posts beyond it.

My comment on ? on 10% savings was in regards to just doing reset control and no mixing etc. Due to the limitations of return temps on std boilers, this is why I stated appx a 150 +or- supply temp to keep the return temp up once warmed up. Now if you do 3 or 4 way mixing you will bring your savings up but at a much greater cost and now the payback time has increased also. The cost benefit ratio to the valve and reset I sometimes question with it being an older boiler. Just my .02 worth. Maybe just the reset of boiler temp and keep within return temp limits would give you a little savings and a little more comfort until you can or want to afford a new mod/con boiler which would be a good benefit. Tim

Bob B_2

thanks to all

Thanks to all who commented (especially Brad) - I think I have the info I need now to talk intelligently with the installers. In fact I talked to one more who immediately mentioned primary/secondary, so that is nice to hear.

And of course, more comments/thoughts always welcome.

George