Couple Tepid Radiators?

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Peteman

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I’m have a couple radiators that do not get near as hot as the others (I would call their temperature “tepid”). There are 3 branches, or zones, coming off the boiler and these radiators are at the end of one of these branch lines. This branch has 5 radiators on it while the others two branches only have 3 and 2 radiators respectively. The boiler is a Vaillant GA100-12 EIVD and there is a Grundfos circulator pump that runs continuously. I have bled all the radiators several times but there has never been any air. One time, I closed circulation to the other two branches (there are valves on the cooler water returns for each branch) and I could get the temp of those problem boilers up to a normal level. Any ideas? Pump undersized? Sediment? This is driving me crazy as one of the problem radiators is in the kitchen so morning breakfasts are cold while the rest of the house is toasty!
 

NY_Rob

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Is this a new behavior or has it always been like this?

Have you made any recent changes to the system?

Do you have three zones on one circulator, does each zone have a zone valve?
 

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If you have ball valves on the other branches, rather than closing them completely just throttle them back so that the branch with the tepid rads gets more flow. You may be able to find a reasonable balance where all branches & all radiators are comfortable. Most systems are over-pumped, but in your case one branch is (apparently) under-pumped, but it's only a matter of how much, and whether the excess flow on the other branches can be put to better use.

Given that Grundfos makes only about 1000 different pumps of different capacities & characteristics, what model number is yours?

It may be counter intuitive, but lowering the temperature of boiler can also help. With hotter water more of the heat is emitted in the part of the branch taking the supply from the boiler, leaving less for the radiators down stream. When the thermostat is being satisfied quickly due to the heat emitted from the rads at the beginning of the branch rads on the other end never come up to temp. As you drop the water temp, the longer it takes for the thermostat to be satisfied, and less heat is emitted early in the loop, making the radiator temperatures more even. At some low enough temperature there isn't enough total heat being emitted to keep the house warm, and with a cast-iron gas-fired boiler going too low can result in corrosive condensation inside the boiler.

From this document and your "-12 "model number, I'm inferring that this is a cast iron boiler that takes 120,000 BTU/hr in, and puts ~100,000 BTU/hr out?

Run a fuel used based load calculation, and measure up the total equivalent direct radiation square footage of all the rads (keep it room by room, branch by branch for later use, but for now just the total EDR is fine.) With the ratio of load per square foot EDR you can determine roughly the water temperature needed to deliver the heat, using the nomograph on page 2 of the radiator document. Not all rooms/radiators are going to have exactly the same load/EDR ratio, so running it on the whole-house numbers will probably be a lower bound. But estimating that number is a useful data point for analysis.
 

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Thanks for the replies. RobNY - we just bought the house in November and there doesn't appear to be any modifications to the system. The pump works all three branches/zones. Below is a pic of the valves I was closing to get all the flow to go one branch (used channel locks to turn). Dana, I assume by reducing the temp you mean lowering the setting on the temp limit switch? Couldn't find a model number on pump but have
included a pic.
IMG_7383.JPG IMG_7385.JPG

I have heard about "flushing" the system may remove some scale blockage but I'm not sure how one would do that. I'm wondering if that might cause further problems anyway.
 

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If you have no air in the system, you may just need to balance the zones- that is make sure one or two zones aren't getting all the flow and the third zone is hardly getting any. The zones with the lowest head (resistance to flow) will get the most flow, the zone with the highest head will have the least flow.
 

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Those bronze Crane valves with the square heads are almost certainly ball valves(?), that turn completely off if turned 90 degrees(?). Ball valves can be set to partially open in order to adjust the flow rates, and balance the flows between the branches. I sincerely doubt you have a blockage issue- it's 95% chance that it's primarily a flow balance issue, which can be tweaked into balance at those valves by opening up the loop with the tepid radiators to full flow, and throttling the others back a bit.

The pump is a UP 15-42F, a fairly common circulation pump suitable for moderate flow low-head applications. The pump curve for it can be found on page 20. It's common to find much bigger (and completely oversized) pumps on high volume radiator systems with multiple branches plumbed in parallel, but that's not to say this pump can't work.

Cast iron boilers typically have both a high and low limit temperature aquastats, sometimes integrated into a single control, sometimes not. I didn't quickly find any online documentation for that series with details about the controls. But yes, reducing the high limit will reduce the average temperature of the water going out to the radiators, which reduces the rate that heat that can be radiated into the rooms, but also reduces the temperature difference between the first and last radiator on a loop. Try rebalancing the flows first, but report back what the temperature settings on the boiler are. The aquastats' part numbers might be useful too.

If you bought the place in November it means you already have a month's worth of fuel use, at least one meter-reading & billing cycle, which is good enough to ball-park the load using fuel-consumption against heating degree-day data. If you can share the exact meter reading dates, the fuel used between those dates, and a ZIP code I can run that arithmetic for you.
 

Peteman

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Those bronze Crane valves with the square heads are almost certainly ball valves(?), that turn completely off if turned 90 degrees(?). Ball valves can be set to partially open in order to adjust the flow rates, and balance the flows between the branches. I sincerely doubt you have a blockage issue- it's 95% chance that it's primarily a flow balance issue, which can be tweaked into balance at those valves by opening up the loop with the tepid radiators to full flow, and throttling the others back a bit.

The pump is a UP 15-42F, a fairly common circulation pump suitable for moderate flow low-head applications. The pump curve for it can be found on page 20. It's common to find much bigger (and completely oversized) pumps on high volume radiator systems with multiple branches plumbed in parallel, but that's not to say this pump can't work.

Cast iron boilers typically have both a high and low limit temperature aquastats, sometimes integrated into a single control, sometimes not. I didn't quickly find any online documentation for that series with details about the controls. But yes, reducing the high limit will reduce the average temperature of the water going out to the radiators, which reduces the rate that heat that can be radiated into the rooms, but also reduces the temperature difference between the first and last radiator on a loop. Try rebalancing the flows first, but report back what the temperature settings on the boiler are. The aquastats' part numbers might be useful too.

If you bought the place in November it means you already have a month's worth of fuel use, at least one meter-reading & billing cycle, which is good enough to ball-park the load using fuel-consumption against heating degree-day data. If you can share the exact meter reading dates, the fuel used between those dates, and a ZIP code I can run that arithmetic for you.

Sure thing - between 10/25/17 and 1/28/18 my 2 bill say a grand total of 387 therms (very cold from end of Dec until a couple weeks ago). Monthly breakdown is 162 therms from 10/25 to 11/26 and 225 therms from 11/27 to 1/28. Square footage of heated space is around 1800 sq ft. Zip is 55105.
 

Peteman

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If you have no air in the system, you may just need to balance the zones- that is make sure one or two zones aren't getting all the flow and the third zone is hardly getting any. The zones with the lowest head (resistance to flow) will get the most flow, the zone with the highest head will have the least flow.

Thanks Rob. I'm going to try and tinker with the valves based on number of radiator fins.
 

NY_Rob

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Keep in mind that all boilers have a minimum flow requirement through the heat exchanger, if you close the valves down too much you can fry the heat exchanger. When working with/adjusting flow through your loops... keep a keen ear on the boiler- if at any time you hear sizzling, percolating, boiling... immediately open the valves again to prevent damage.

If adjusting/balancing flow remedies the situation, you would probably be better off installing "globe valves" for permanent use. Ball valves are generally used for on/off not flow control because their shape can cause turbulence/noise and actual metal erosion of the valve itself over time.
 

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Since there were no days that didn't have a heat load between 10/25/17 and 1/28/18 let's use the 387 therm total and the daily HDD65 from station KMNSAINT20 in St. Paul (which is within your ZIP code.) To deal with the time of day the meter was read, sum the HDD from 10/25 through 1/27, and sum the HDD from 10/26 through 1/28, then average them.

10/25-1/27 HDD: 3772

10/26- 1/28 HDD: 3803

Average: 3788 HDD

Assuming the boiler's nameplate efficiency (DOE output divided by input) is 80%, the heat that went into the house was 0.8 x 387= 310 therms or 31 million BTU (MMBT).

So the BTU per HDD is (31,000,000/3788= ) 8184 BTU/day, or (/24=) 341 BTU per degree hour.

The 99% outside design temperature in Minneapolis is -11F, which is 76F colder than the presumptive 65F base, so you have an implied design heat load of:

76F x 341 BTU /F-hr= 25,916 BTU/hr

If using base 60F (appropriate if it's 2x6 /R19 with l0w-E windows) it would come in a bit more than that, but the load at -11F is still going to be under 30,000 BTU/hr (which makes the boiler ridiculously oversized for the heat load, but until and unless you're planning on replacing it we won't go there.)

As a sanity check, 26,000 BTU/hr for 1800' of fully above grade conditioned space is 14 BTU/hr per square foot, which would be possible for a VERY tight 2x4 framed 1800' house with decent low-E windows, or reasonably tight 2x6 /R19 house. If some amount of that 1800' is below grade the description of the insulation and air tightness could be quite a bit looser. (I have about 2400' fully above grade 2x4 framed house, 1600' of mostly below-grade insulated- conditioned but unfinished basement that would come in at 11 BTU/ft-hr @ -10F if I include the basement in the square footage numbers, 19 BTU/ft-hr for the whole-house load if I only include the above grade square footage.)

If you normally keep it 60F or colder while your away, and only 65F when your there pr something, tell me what your temperature setback schedule is and we'll pick a different temperature base. Using 65F as a base temp is appropriate if it's normally kept 68-72F when you're there, with maybe a setback to 65F at night or when you're at works/school.

So, we'll see how that works out in BTU per square foot EDR and figure out a reasonable starting point on operating temperatures.

Most cast iron boilers don't really have a minimum flow rate the way low-mass modulating boilers do. Many of them have a minimum temperature that's maintained by firing up even when there is zero flow and no load. As long as the in-to-out temperature difference (delta-T) isn't chronically more than 50F most will do fine, and if your flow is that low that it has delta-T for long periods of time (which can happen with under-fired boilers with high-thermal mass radiation) there is usually a bypass branch in the near-boiler plumbing to mix some of the boiler's output directly into the return water stream to keep the delta-T bounded.

With iron plumbing and an infra-red thermometer you can get quick temperature measurements accurate enough for this analysis. (A $30-$50 pistol-grip type IR thermometer from a box store is fine.) Copper, bronze or galvanized piping would give you a falsely low reading due to the low emissivity of the metal finish, but a daub of paint on the point where you're measuring would fix that. But black iron or rusty iron is perfect on it's own.

At the beginning of a call for heat the radiator water is tepid, and delta-T will be at it's highest, and is likely to be over 50F difference (90F back, 160F out or something). At the end of a call for heat it should be much lower. Measuring the individual delta-Ts on the radiator loops will tell you something too. Ideally with 150F+ entering water temp at the manifold you'd see a 20-25F delta-T on the return loop before it enters the manifold for most of the call for heat. If it's much more than 30F delta on all individual zones there may be a case for going with a bigger pump.

What I suspect is happening is that on the less-radiated loops you're getting a 10F delta, and on the one with the tepid rad it's more than a 50F delta. If you dial back the flows on the low-delta zones to bring them up to the 20-25F range, the high-delta zone should exhibit a much smaller delta than when you started.
 

Peteman

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Since there were no days that didn't have a heat load between 10/25/17 and 1/28/18 let's use the 387 therm total and the daily HDD65 from station KMNSAINT20 in St. Paul (which is within your ZIP code.) To deal with the time of day the meter was read, sum the HDD from 10/25 through 1/27, and sum the HDD from 10/26 through 1/28, then average them.

10/25-1/27 HDD: 3772

10/26- 1/28 HDD: 3803

Average: 3788 HDD

Assuming the boiler's nameplate efficiency (DOE output divided by input) is 80%, the heat that went into the house was 0.8 x 387= 310 therms or 31 million BTU (MMBT).

So the BTU per HDD is (31,000,000/3788= ) 8184 BTU/day, or (/24=) 341 BTU per degree hour.

The 99% outside design temperature in Minneapolis is -11F, which is 76F colder than the presumptive 65F base, so you have an implied design heat load of:

76F x 341 BTU /F-hr= 25,916 BTU/hr

If using base 60F (appropriate if it's 2x6 /R19 with l0w-E windows) it would come in a bit more than that, but the load at -11F is still going to be under 30,000 BTU/hr (which makes the boiler ridiculously oversized for the heat load, but until and unless you're planning on replacing it we won't go there.)

As a sanity check, 26,000 BTU/hr for 1800' of fully above grade conditioned space is 14 BTU/hr per square foot, which would be possible for a VERY tight 2x4 framed 1800' house with decent low-E windows, or reasonably tight 2x6 /R19 house. If some amount of that 1800' is below grade the description of the insulation and air tightness could be quite a bit looser. (I have about 2400' fully above grade 2x4 framed house, 1600' of mostly below-grade insulated- conditioned but unfinished basement that would come in at 11 BTU/ft-hr @ -10F if I include the basement in the square footage numbers, 19 BTU/ft-hr for the whole-house load if I only include the above grade square footage.)

If you normally keep it 60F or colder while your away, and only 65F when your there pr something, tell me what your temperature setback schedule is and we'll pick a different temperature base. Using 65F as a base temp is appropriate if it's normally kept 68-72F when you're there, with maybe a setback to 65F at night or when you're at works/school.

So, we'll see how that works out in BTU per square foot EDR and figure out a reasonable starting point on operating temperatures.

Most cast iron boilers don't really have a minimum flow rate the way low-mass modulating boilers do. Many of them have a minimum temperature that's maintained by firing up even when there is zero flow and no load. As long as the in-to-out temperature difference (delta-T) isn't chronically more than 50F most will do fine, and if your flow is that low that it has delta-T for long periods of time (which can happen with under-fired boilers with high-thermal mass radiation) there is usually a bypass branch in the near-boiler plumbing to mix some of the boiler's output directly into the return water stream to keep the delta-T bounded.

With iron plumbing and an infra-red thermometer you can get quick temperature measurements accurate enough for this analysis. (A $30-$50 pistol-grip type IR thermometer from a box store is fine.) Copper, bronze or galvanized piping would give you a falsely low reading due to the low emissivity of the metal finish, but a daub of paint on the point where you're measuring would fix that. But black iron or rusty iron is perfect on it's own.

At the beginning of a call for heat the radiator water is tepid, and delta-T will be at it's highest, and is likely to be over 50F difference (90F back, 160F out or something). At the end of a call for heat it should be much lower. Measuring the individual delta-Ts on the radiator loops will tell you something too. Ideally with 150F+ entering water temp at the manifold you'd see a 20-25F delta-T on the return loop before it enters the manifold for most of the call for heat. If it's much more than 30F delta on all individual zones there may be a case for going with a bigger pump.

What I suspect is happening is that on the less-radiated loops you're getting a 10F delta, and on the one with the tepid rad it's more than a 50F delta. If you dial back the flows on the low-delta zones to bring them up to the 20-25F range, the high-delta zone should exhibit a much smaller delta than when you started.

Dana - thanks so much for the reply. Sorry for the delay but got off track onto some other things. A few comments:

- I looked back on the bills and I forgot a month (duh!). Total therms from 11/27 thru 1/29 is 557 therms
- We set the thermostat to 69 during the day (someone is here all day) and set back to 62 at night
- Yes there is a valved bypass line from supply to return just outside the boiler (open)
- I played around with those branch valves and the only way to get any significant flow to the tepid rads is to completely shut the other two

One of the main things I wonder about are the separate zone valves. The system is set up with two zones/two thermostats - one for the basement (newer baseboards) and the other for the main house (big old radiators). When I moved in, we didn't care about heating the basement so I disconnected that thermostat and closed the isolation valves completely closing off that part of the system. Below is a pic of the two zone valves. My concern is, with these zone valves, when the main thermostat shuts the heat down, it also shuts that main valve (and basement zone when it was working). In effect, this doesn't allow the always-on pump to keep circulating the heated water throughout the main house radiators between heat cycles. It just keeps pumping the water out the boiler through the bypass and back to the boiler. Seems very ineffecient. The valve has a lever to hook it open but every time the system goes through an on/off cycle it resets that valve to obey the open/close signals. I hate to remove the valve - I assume someday someone will want to heat the basement again.
 

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Dana

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Most zone controllers also control the pump, and turn it off when there is no call for heat from any zone. This sounds like it was a bit of a hack

Throttling back the bypass branch will improve the flow to the zone radiation too, but keep an eye on the entering water temperatures (EWTs) at the boiler at the beginning of a call for heat. Chronic operation with EWT< 130F could potentially damage a cast iron boiler, as could delta-Ts >50F from out to in. A the beginning of a cold start after a long overnight setback it would be normal to have some period of time before the EWT was >130F, and the delta-T dropped to <50F, but if that's the condition during most of the call for heat you'll have to open up the bypass branch valve a bit more.

So, you actually used 387 + 557= 944 therms between 10/26- 1/28? That would make the implied heat load 944/387 x 25,916 BTU/hr (the previously caluculated load)= 63,216 BTU/hr, quite a bit higher.

For an 1800' house that's a ratio of 35 BTU/hr per square foot, which is on the very high side, which means there may be some low hanging fruit to pluck on the building efficiency front, probably some serious air leakage, and no foundation insulation(?). Even if you're not actively heating it and it drops to 50F on cold days, any above-grade exposure on the foundation loses more than 50 BTU/hr per square foot when it's -11F outside. The band joist and foundation sill air leakage usually adds up to more than all windows & doors combined. Tightened up, and with foundation insulation I'd expect most 2x4 houses to come in under 25 BTU/hr per square foot @ -11F outside, 70F inside.
 

Peteman

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Most zone controllers also control the pump, and turn it off when there is no call for heat from any zone. This sounds like it was a bit of a hack

Throttling back the bypass branch will improve the flow to the zone radiation too, but keep an eye on the entering water temperatures (EWTs) at the boiler at the beginning of a call for heat. Chronic operation with EWT< 130F could potentially damage a cast iron boiler, as could delta-Ts >50F from out to in. A the beginning of a cold start after a long overnight setback it would be normal to have some period of time before the EWT was >130F, and the delta-T dropped to <50F, but if that's the condition during most of the call for heat you'll have to open up the bypass branch valve a bit more.

So, you actually used 387 + 557= 944 therms between 10/26- 1/28? That would make the implied heat load 944/387 x 25,916 BTU/hr (the previously caluculated load)= 63,216 BTU/hr, quite a bit higher.

For an 1800' house that's a ratio of 35 BTU/hr per square foot, which is on the very high side, which means there may be some low hanging fruit to pluck on the building efficiency front, probably some serious air leakage, and no foundation insulation(?). Even if you're not actively heating it and it drops to 50F on cold days, any above-grade exposure on the foundation loses more than 50 BTU/hr per square foot when it's -11F outside. The band joist and foundation sill air leakage usually adds up to more than all windows & doors combined. Tightened up, and with foundation insulation I'd expect most 2x4 houses to come in under 25 BTU/hr per square foot @ -11F outside, 70F inside.

Hi Dana - here's an update ...

I ended up calling a heating contractor I have worked with in the past and he spent some time going over the system. Here's a summary:

- He rewired the pump so that it doesn't pump continuously now. He didn't see any practical reason to have it continuously running so now it kicks on only when there's a call for heat
- He lowered the high temp limit to 180 down from 200. He thought perhaps it was set up for 200 when the basement zone (copper baseboards) was being used
- As for the tepid radiators, he thought it mostly had to do with # of rads and size, length of piping in that branch. He re-did what I had done previously and throttled back the valves on the other 2 branches. He also thought, if I really wanted to get full even heat, I'd have to install a small supplementary pump in that third branch line. He really didn't push the idea, just a suggestion.

As for the gas usage, it's actually total 557 therm for that period. I was checking my last bill and the utility compares gas usage with similar sized houses in the neighborhood for that period and I was within the top 20% usage efficiency group (they rated it "great") so that makes me feel somewhat better. I think a lot of that has to do with the fact that the previous home owner installed new windows and had the walls blown with insulation.

Pete
 

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The retrofit insulation definitely helps, but it's also worth insulating the foundation walls, especially when you have an oversized boiler and all that uninsulated 180F plumbing running around the basement. If you have 2' of above grade exposure on that R1-ish foundation the heat lost from the basement is going to be well into double-digits as a percentage of your heating bill, could even be as much as a quarter of it or so. Putting a code min R15 continuous insulation (or the thermal equivalent thereof) on the interior of the foundation walls would cut that portion of heat loss by more than 90%, even while raising the temperature of the basement to something similar to the fully conditioned space above.

Seriously, a 63,000 BTU/hr load at -11F is high for a tight 2x4 framed house with updated windows. It should be ~50K or less with an insulated foundation. Also what is the BTU in & BTU out numbers on the boiler nameplate?

MN code on foundation insulation strays pretty widely from the IRC specifications in very particular and sometime odd ways, but for retrofit situations most inspectors are flexible. The rationale for the fairly complicated MN amendments to the code on foundation insulation is unclear. Before the most recent versions the letter of the code would have made foundations poured in insulated concrete forms technically a code violation. But the code has been evolving. I believe code min for foundation in MN is now R15 continuous insulation, but previously (and maybe still) R10 of that has to be on the exterior, and the interior R can't exceed the exterior R, etc. Before insulating the foundation draw up the plan and get pre-approval from the inspectors. If they'll allow continuous R15+ foam insulation on the interior of the foundation, or R7.5 continuous foam trapped to the foundation with an R13 studwall, either of those would be good options. (Reclaimed 3"-4" roofing foam is often cheaper than the foam + fiber-insulated studwall solution.) There are lots of details to get right to avoid turning it into a mold farm, with multiple threads on this site covering it, but if you are considering it I can walk you through most of those. It looks like St. Paul's local building code would allow continuous R15 on the interior as a retrofit. (See p.2 of the document.) It appears even R15 fiberglass would be allowed, with a 4 mil interior side vapor barrier, but that's a riskier proposition than rigid foam from a mold & moisture point of view.

Independently of whether or when you insulate the foundation, insulating all 120F+ heating system plumbing with at LEAST 1" fiberglass pipe insulation (the overpriced half inch stuff from box stores need not apply) would also be a good idea, and may even help with the tepid radiator symptom. Current IRC code-minimum would be R3 (3/4" fiberglass), but for high-temp systems like yours 1.5-2" (R6-R8 ish) would still be financially rational. The good stuff can be bought online, if local supply houses don't want to deal with small-scale purchases.
 

Peteman

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The retrofit insulation definitely helps, but it's also worth insulating the foundation walls, especially when you have an oversized boiler and all that uninsulated 180F plumbing running around the basement. If you have 2' of above grade exposure on that R1-ish foundation the heat lost from the basement is going to be well into double-digits as a percentage of your heating bill, could even be as much as a quarter of it or so. Putting a code min R15 continuous insulation (or the thermal equivalent thereof) on the interior of the foundation walls would cut that portion of heat loss by more than 90%, even while raising the temperature of the basement to something similar to the fully conditioned space above.

Seriously, a 63,000 BTU/hr load at -11F is high for a tight 2x4 framed house with updated windows. It should be ~50K or less with an insulated foundation. Also what is the BTU in & BTU out numbers on the boiler nameplate?

MN code on foundation insulation strays pretty widely from the IRC specifications in very particular and sometime odd ways, but for retrofit situations most inspectors are flexible. The rationale for the fairly complicated MN amendments to the code on foundation insulation is unclear. Before the most recent versions the letter of the code would have made foundations poured in insulated concrete forms technically a code violation. But the code has been evolving. I believe code min for foundation in MN is now R15 continuous insulation, but previously (and maybe still) R10 of that has to be on the exterior, and the interior R can't exceed the exterior R, etc. Before insulating the foundation draw up the plan and get pre-approval from the inspectors. If they'll allow continuous R15+ foam insulation on the interior of the foundation, or R7.5 continuous foam trapped to the foundation with an R13 studwall, either of those would be good options. (Reclaimed 3"-4" roofing foam is often cheaper than the foam + fiber-insulated studwall solution.) There are lots of details to get right to avoid turning it into a mold farm, with multiple threads on this site covering it, but if you are considering it I can walk you through most of those. It looks like St. Paul's local building code would allow continuous R15 on the interior as a retrofit. (See p.2 of the document.) It appears even R15 fiberglass would be allowed, with a 4 mil interior side vapor barrier, but that's a riskier proposition than rigid foam from a mold & moisture point of view.

Independently of whether or when you insulate the foundation, insulating all 120F+ heating system plumbing with at LEAST 1" fiberglass pipe insulation (the overpriced half inch stuff from box stores need not apply) would also be a good idea, and may even help with the tepid radiator symptom. Current IRC code-minimum would be R3 (3/4" fiberglass), but for high-temp systems like yours 1.5-2" (R6-R8 ish) would still be financially rational. The good stuff can be bought online, if local supply houses don't want to deal with small-scale purchases.

Thanks for the reply Dana - insulating those basement walls is on the radar. You can tell the heat loss by simply looking at the snow melt on the outside around the perimeter. I've read some about mold issues ... getting basement insulation right in old homes seems tricky but doable if done carefully.

To correct a previous post, the total usage from Nov to end of Jan is actually 577 therms (not 577 + 387 = 944 as you posted just 577 total). Going through your original calc procedure gives me a 38,621 BTU/hr number. These seems more reasonable. The boiler label plate says:

Input Cap = 120,000
Heating Cap = 100,000
Net Rating = 87,000

Hope this makes more sense. Also, it seems as though the adjustment of the branch valves has had some impact. Not totally even between all the branches but about half way better if that makes sense. Not sure I want to go down the money road of putting in a supplemental pump for the "weak" branch.
 

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OK, if it's 577 therms total, the implied load is the previously estimated 25,916 BTU/hr x 577/387= 38,640 BTU/hr , and a ratio of 21.5 BTU/hr per square foot of fully conditioned space, which would be more within expectations.

But that doesn't mean you can't knock it back another 5-10K with foundation insulation.

With a DOE output of 100K and a load of 39K it's an oversize factor of about 2.6x, which is about twice the ASHRAE recommendation. A boiler with about 50K of output would be more appropriate for the actual load, which would then have twice the duty cycle, and substantially less standby loss (since it's not standing-by when actively burning.) The IBR Net water rating isn't relevant here, since even though the basement isn't insulated or actively heated, the boiler isn't isolated from the fully condition space with an insulated wall or ceiling.

If you're keeping the oversized beastie boiler for more than a couple of years it's worth adding heat purging boiler control such as an Intellicon HW+ (DIY-able for those with some electrical skills), which will reduce the amount of standby & distribution loss to the uninsulated basement, which lowers the actual load a bit since the basement won't be heated as much by the standby & distribution losses. It won't be as big an effect as insulating the foundation, but at an oversize factor of 2.6x load it'll still likely hit somewhere between 5-10%, sometimes more in fuel savings.

For more on the efficiency effects of oversizing, see Table 3 in this document. At 2x oversizing you'll typically have an efficiency hit of 6-8% below it's steady state rating (in your case, 83% steady-state), but at 3x oversizing it'll be 10% or more. You're at 2.6x now, with an as-used AFUE something like 8% lower than it's steady state numbers, and would be more than 3x oversized if you insulated the basement. A heat purge control will reduce the effects of oversizing by quite a bit, which is what's going on with boiler system #3 in that table. A retrofit heat purge control isn't quite as effective as one specifically designed with algorithms tuned for a particular boiler, but it's still a big step in that direction.
 

Peteman

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A quick update on the cool radiators issue ... as previously posted, we had someone come out about a month ago to look things over. At the end of his visit, he mentioned that the company guru (owner) would be in touch to see if there's anything else that could be done. The owner came out last week and readjusted the branch valves a little bit more and closed the bypass valve between the boiler hot out and cool return. Problem solved. He did suggest, if I wanted, I could install a new, higher capacity 3 phase pump (he indicated that I could order and install myself without any problem) to help. I asked him about keeping the bypass valve closed and condensation issues but he seemed not be overly concerned about that and it wouldn't be a problem. Are these bypass valve absolutely necessary to have open? I have a friend who just had his boiler serviced and the technician closed his also.
 

Dana

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Measure the entering water temperature of the return line at the boiler. If it's chronically under 130F, it's potentially a problem. If it's chronically under 120F it's definitely a problem for a cast iron boiler. At low entering water temperatures there will be acidic gas condensation on the heat exchanger plates, eroding the turbulence inducing features (which lowers efficiency) and ultimately reducing the operating lifespan of the boiler.
 

Peteman

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Measure the entering water temperature of the return line at the boiler. If it's chronically under 130F, it's potentially a problem. If it's chronically under 120F it's definitely a problem for a cast iron boiler. At low entering water temperatures there will be acidic gas condensation on the heat exchanger plates, eroding the turbulence inducing features (which lowers efficiency) and ultimately reducing the operating lifespan of the boiler.
Thanks Dana ... I'll check that. I think my IR temp sensor can read from the black pipes.

I assume that on system start ups, after the boiler has been off quite a while (like during nighttime set backs or spring time when PM outside temps get warm), that return water temps will be below 120 until the whole system has cycled through all the radiators - not long enough to corrode heat exchanger?

Pete
 

Dana

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If it's a few minutes it's not a problem if it's many tens of minutes it's a problem.

For high thermal mass systems it's sometimes necessary to install a thermostatic mixing valve on the bypass branch to establish a minimum entering water temperature (EWT) on a cast iron boiler. An EWT of 130F is usually safe for a gas fired cast iron boiler's heat exchanger plates, but sometimes result in excessive flue condensation in masonry chimneys if the flue is oversized, or located on an exterior wall (where the masonry is cold) rather than inside of conditioned space. Setting the EWT to 140F is usually enough to prevent the flue condensation too (with exceptions to prove the rule.)
 
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