What is recommended return water temp

Al Roethlisberger

...my head hurts *laugh*

But bottom line, one thing that seems clear is that 140 degrees F is the minimum water temp. Correct?

Al

Al Roethlisberger

What is recommended radiator and return water temp?


One issue I've seen brought up many times is the maintaining of proper return water temp to avoid various problems.

So my question is two-fold:

1) Currently my boiler is directly connected to the system loop in the house, no pri/sec loops, mixing valves, etc. My boiler is a gas-fired ca1991 Carrier(Dunkirk) boiler of about 245K BTU. The system(old overhead gravity system ca1929) is hot water with cast iron radiators, and was converted to a sealed circulated system in 1991. I only currently have a Honeywell aquastat, no outdoor temp reset, or other complex controls.

Question: With this configuration in mind, what temps should I expect or adjust the system to reach when measured at the radiators and perhaps more importantly at the return water temp?

Given the various admonishments about condensation, etc, I wanted to find out what I should be looking for to be an optimal return water temp.


2) If/when I engineer and install a pri/sec loop setup, I assume this is hopefully(by design) less of an issue as the primary loop will keep the water circuiting the boiler closer to the optimal temperature. But then I wonder if the return temp from the secondary loop can in theory be less than in scenario 1 above, especially if being managed with a temp reset.


But for now, scenario 1 is my main concern, as I want to get the system tuned. I have the supply temp currently running at about 170-190 degrees, and will be measuring the return temps and temp at the radiators this weekend to fine tune.

BTW, I am in the center(Sanford) of NC if that helps any too with targeting temps.

Thanks
Al

Dave_4

Leave it alone.

Return water temperatures are critical to condensing boilers, but not yours. Supply water temperature is your boiler's design water temp caveat. What you have (unless you live in a 5,000+ S.F. house) appears way oversized, given the NC (temperate) address.

I would use the KISS method. Drop the aquastat to the lowest setting that:

1) limits the boiler water temperature to MINIMALLY see under 140F actual gage water temps and

2) limits the water temperature to never go above what is required to "match" the emitter output - on the coldest days of winter.

We found many aquastats set 10 to 30 degrees higher than needed to make the house 68 when at the coldest outdoor temps.

You seem aware of the "why we can't set the aquastat lower" dynamic. Condensing on this modle boiler is verboten.

Keep in mind however, ALL NON-CONDENSING BOILERS CONDENSE! The caveat is: minimize the time they take to get to non-condensing temps, hence - RA aquastats (aquastats that prevent circulation until water temps go above 140 - and the damaging effects of condensing are nil). Ideally and incredibly efficient simple design exist for what you have. That being, simple thermally actuated bypass and constant circulation.

Down and dirty design, least expensive and best option keeping what you already own; just making a minor piping addition.

Let us know what you decide.





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Unknown

Ken made some very good points, though I would be remiss if I didn't point out that a constant circ, reset loop w/TRVs would be the ultimate in comofort and efficiency and, presuming you have the ability to let flow bypass radiators (that is, that it's not direct series where all flow for the second radiator must go through the first), it would be a relatively easy and inexpensive upgrade with just some piping work in the mech room to do the primary/secondary mixing valve or injection end of things.

That makes every radiator a constant on, perfectly modulated zone. Pretty sweet.

But, not necessary right off the bat for sure, and ken's ideas are definitely good if you are happy with your current zoning.

Unknown

For nearly the same effect...

You could run a two stage thermostat, with the first stage running the pump, and the second stage running the burner.

Reset, with indoor feedback.

Noel

Al Roethlisberger

Thanks, and a couple questions


First, thanks for the thoughtful response.

To answer your question about sizing, my home was built in 1929, with little to no insulation at the time, and is 4100 sqft with 60+ windows, plus another 2200sqft in the basement that does have one radiator. I think/assume that the boiler that was installed in 1991(I wasn't the owner then) was matched to be close to the capacity of the old unit it was replacing.

We of course have improved the envelope some with insulation in the attic, but the doors/windows have "loosened up" a bit over the years(we're working on that), and all pipe insulation in the basement(asbestos) was removed a decade or two ago(which I plan to replace) so we're likely still about the same efficiency.

Of course, one day if/when this boiler has to be replaced, we'll do a proper heat loss eval to see what size new boiler we'll need.


But to your comments, and my questions:

1) You mention: "I would use the KISS method. Drop the aquastat to the lowest setting that: limits the boiler water temperature to MINIMALLY see under 140F actual gage water temps and "

Question: So, when you say "140F actual gauge water temp", measured where, coming out of the boiler or returning?


2) You then mention: "[and] limits the water temperature to never go above what is required to "match" the emitter output - on the coldest days of winter.

We found many aquastats set 10 to 30 degrees higher than needed to make the house 68 when at the coldest outdoor temps. "

Question: What do you mean by "match the emitter output", match against what? Also, how do I know/calculate what temp the radiators need to be to make the house 68 on the coldest outdoor temps? It seems to me that this could be both a function of time and temp, i.e. 150F gets to 68 in x minutes, where 140F gets there in a little longer, and 170F gets there a bit faster. How do I determine the optimal radiator heat?

Finally, can you point me a diagram or example of:

"Ideally and incredibly efficient simple design exist for what you have. That being, simple thermally actuated bypass and constant circulation."

It sounds like you are saying that I likely don't need a pri/sec loop, second pump, mixing valves, etc... and if so, I am trying to investigate all the "KISS" options, as I agree that I don't want something too complex that will provide minimal(or no) additional return and require more time/money investment and tinkering to get running correctly.

Thanks!
Al

Al Roethlisberger

Yes, my radiators allow bypass

Rob,

Indeed, my radiators are *not* in series, so I can use TRVs.

One challenge I do have is that I have some rather large piping, so TRVs are more expensive and harder to find. But it is possible. I'm looking into this down the road a bit once I get the main system safe and reasonably efficient.

I've heard "constant circulation" mentioned a few times, but I have a question. When one says "constant circ", do you mean, 24hrs a day, or just constant circulation when there is demand?

Right now my aquastat runs the circulator constantly when there is demand, and only fires the boiler when the output water temp falls below the aquastat setting(right now about 180degrees).

It does sound like you are still recommending a pri/sec loop though since you mention a mixing valve. I assume I would need a second pump then as well, which is another question, about which type of pump is appropriate to fit to the "giant pipe" solution that was an old overhead gravity system. In this case, the circulator would be feeding into a 4 inch riser.

Thanks again for all the suggestions.

I've got several people thinking about the best "KISS" solution for me, and I really do appreciate it, as I'll likely have to do this myself. And along with managing cost, I'd like to keep the maintenace/adjustment-tinkering side of the equation easy as well, while hopefully improving efficiency as much as is reasonable.

Any chance you can send or point me to a simple drawing of the setup you are suggesting.

I like your comment of "ultimate in comfort and efficiency"


Thanks!
Al

Unknown

that's not nearly the same effect at all unless you want just one zone.

TRVs means every radiator is a zone.

that's a neat idea for single zone systems though, for sure.

Al Roethlisberger

What type of thermostat, and outdoor reset question

Noel,

So, along with the varied advice I have been receiving, I've been looking at using an outdoor temp reset that can control the circulator(s), firing, temp, etc...

Right now my old Honeywell aquastat of course controls firing and circulator functions when my simple programmable thermostat demands heat.

But now you've mentioned a dual-stage thermostat that would control firing and circulator functions in stages.

Two questions:

1) Can you specify a brand/model so I can go look at the specs and learn about their function?

2) How would this dual-stage thermostat play with my aquastat and/or outdoor temp reset when/if I get one?

You mention temp reset in your post, but I wasn't sure if you meant an outdoor temp reset, ....or is there another type you were referring to?


One issue I am starting to find is that with the myriad suggestions, I am not sure which are necessarily exclusive. That is, it would also be very helpful if when recommending a solution, one could help me understand what the current recommendation might eliminate or require as opposed to other scenarios.

Thanks again for all the help. I love this place
Al

Unknown

constant circ means the system pump is on 24/7, delivering at least some heat everywhere, with the TRVs limiting output where it needs limiting. you would turn it off manually or by "warm weather shutdown" when you are done heating.

the boiler pump would not be on all the time.

You can't really do constant circ/TRV on a conventional boiler without P/S piping and a mixing device.

Pump sizing depends on peak demand flow requirements. That depends on heat load.

whether this makes sense or not primarily depends on how happy with the current zoning you are. If you're happy, then doing basic reset (if you even need more than boiler minimum temp, ever, with these radiators) is fine by itself. If you're not happy, this is the ultimate in comfort, and it has some efficiency benefit, but I wouldn't do it for payback alone unless you were switching the boiler for a mod/con where you wouldn't need mixing and the lower temps of constant circ would be of more benefit.

Unknown

More Detail...

Using what you have now for piping (assuming that it works for your building), you could use just about anybody's 24VAC, 2 stage (heating) thermostat. The first stage to come on when there is a call for heat would be wired to exactly where your current thermostat connects.

The second stage to come on a degree or so lower (room Temp) than the first stage would be wired to interupt the 24VAC signal to fire the burner. On gas burners, this wire usually is coming from the aquastat. On oil burners, this is a jumper on the oil burner primary control (most models) across the T and T terminals.

Your pump will run constantly, until the weather warms your house to ABOVE the thermostat setpoint, and the water temperature will vary up and down with the length of time that the second stage controls your burner's run time. In cold weather, the burner will run longer, thereby elevating the water temperature to the point needed. In warmer weather, the burner will stay off longer, the water temp will coast down to just what it needs to be to do the job.

No fancy piping or controls needed, provided everything works currently. It works well because of the high mass of radiator systems. TRVs can still be used, as long as there isn't one on the radiator that effects the thermostat location.

Macon makes TRVs that fit existing valve bodies. No repiping. Just swap the valve guts out.

Noel

Al Roethlisberger

24/7 circulation = high electricity bill?

Thanks again, and perhaps a dumb question(I hope you and everyone else don't mind the plethora of them): Wouldn't having the secondary loop circulator run 24/7 regardless of demand incur a noticeable inefficiency in electric cost? I have no idea what it costs to run a "typical" residential circulator constantly for 4ish months, but I would assume it is something =P

I live in central NC, where the cold really isn't that bitter, especially in the month immediately preceding hard winter. So the boiler really only fires in the evening and mornings on these borderline days. Of course once we get into Dec/Jan/Feb the cold days would likely be the norm, with very few warm days to worry about.

When you mention "warm weather shutdown", is that a function of the outdoor temp reset? If so, then I guess that answers part of my question above, which would be that the outdoor temp reset would shut off the 24/7 circulator(the whole system) at a certain temp on those days when it was just too warm to need heating at all?? I am interpreting this correctly?

I do plan to install some TRVs one day, although not everywhere in the system. So we'll end up with some "zoning".

Thanks again!
Al

Unknown

you are correct on everything, though the pump cost is roughly equivalent to a 60 watt lightbulb on most systems; hardly breaking the bank.

Al Roethlisberger

Circulator sizing


Thanks Rob,

When I asked about sizing the circulator for a secondary loop at some point, you replied: "Pump sizing depends on peak demand flow requirements. That depends on heat load."


Can you help me understand, or point me to a doc that can, how I arrive at the right sizing/type?


I don't know if my past description of the system helps, but I have a 2 story house with an old overhead gravity system. So the circulator would be pumping into a large(4 inch I believe) express riser that runs uninterrupted up into the attic, where the hot water then flows down to all the radiators, where I typically have 1-2 inch lines, mostly 1.25 and 1.5 it seems. Then that comes back to another 4 inch main return to the boiler, where the supply and return all then step back down to 1.25 inch connections.

I have typical large cast iron radiators throughout the house.

Any thoughts on circulator type/sizing?

Currently the old Carrier boiler came with a little grundfos circulator(model I don't have here at work), which I guess I'd just keep in place for the primary loop one day.

Thanks,
Al

Unknown

hi Al,

flow = Heat Load / (temperature drop x 500) for regular water systems. You would then need to calculate frictional loss at that flow rate and check the pump curve to see if it can provide X flow rate at Y frictional loss.

Unknown

for your boiler, I believe so yes.