New mod-con Indirect DHW heat transfer problem

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James Robins

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Decided to post my problem here as I have benefitted greatly from Dana D's clear advice on various mod-con challenges posted, not to mention how others have chimed in, as well.

In late March 2021 I installed a Bosch Greenstar freestanding 151 with Laars 50 gal. double wall indirect (rebadged Bradford White - 43 gal actual capacity) heating a Victorian style 2,500 sq.ft - cast iron radiators. Turn down on the Bosch is 34kBTU/hr minimum, and eventually I learned of the poor thermal transfer on the DW tank - 23.5kBTU/hr. max at very high temps. Single wall is not allowed in my jurisdiction.

Many installer return visits involved the following limited remedies: 1) check valve on radiator send (solved the radiator runaway heat problem in shoulder heat season), 2) check valve on boiler return from DHW tank - did nothing for high temp - 142F at faucet - and slightly helped correct quick loss of hot water with moderate use; 3) move sensor from normal mid-tank to above tank with wrap - very little improvement; 4) add mix valve - turn up indirect heat to 142F, then 147F - with mix down to 125F at faucet. This is good in terms of finally getting adequate hot water storage and proper temp at faucet. HOWEVER, the DHW process still took at least 1:3o hours for daily recovery with excessive DHW short cycling to bring the reported temp up to satisfy the boiler. For the first hour, the DHW temp actually drops several degrees before slowly recovering. Inspector correctly predicted that in full heating season, too much heat will be lost due to the long draw, and full solution will need to be found.

The system is set up as a single zone and heating parameters of heat and DHW are not separated. It became apparent (at least to me) if there is no hot water demand going on, the sensor does not really know that the tank has fully heated... so the wasteful lengthy process of overheating is taking place. The hot water has two branches... and a total of about 70 feet of mostly 3/4" pipe in the basement alone. My proposed solution is the Taco SPE-1 set to pulse (5 min. on, 10 min. off) with the 006e3 variable circulator set on the low end. PEX return to the bottom of the tank. PEX at basement level only - evenly divided between the two branches so no balancing valve needed. So "storing" some heat in the pipes, flow giving the sensor an accurate temp reading, and bringing back warm water to the bottom of the tank might mitigate the recovery problem. I get the benefit of quicker hot water at the tap too. The installer plans to get more expert advice, but with current cold spell he's dealing only with emergency situations.

In the meantime, I decided to drizzle drip the laundry faucet on the 2nd floor so that overnight I don't get 5F extra temp drop (which was showing up on 15F nights)... just because the tank recharge was cycling so long. It worked. Even last night, at -12F (design temp) it all worked out fine. So, for several days now, I haven't seen the long tank recharge problem. I was fairly confident that this make-shift solution would work because I had been able to turn on the basement hot faucet and consistently halt the lengthy short cycling-recharge problem several times in a row before the truly cold weather arrived.

I'm not fond of the solution I've proposed (circulator pulse water draw endlessly) because such a complicated solution shouldn't be necessary for what seems like a fairly simple problem. Of course, dripping hot water out the laundry faucet isn't a practical long-term solution either.

Any expert advice will be greatly appreciated!

Bosch-Laars XT206562.jpg
 

wwhitney

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Uninformed comments:

Whether the indirect loop is circulating and the mod-con is heating it, that is controlled by a temperature sensor for the DHW tank, which is located outside of the tank (that grey wire coming off the lefthand top connection to the tank)? Is there no way to put the sensor inside the tank?

Cheers, Wayne
 

James Robins

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Thank you, Wayne. Correct - the white wire is going to the sensor on the hot supply above the tank. Installer indicated that the sensor cannot go inside the tank - I had the exact same question as he was putting it in. Obviously, this is the source of the problem - and a Bosch tank would be the normal install not needing any workaround, but Bosch doesn't offer a single wall. Not legal here (as our local water services department apparently knows more than the folks who write the national code). Even the inspector said the policy is wrong, but what would he know with 20+ years on the job.

The sensor does seem to do its job from the perspective of satisfying the boiler - but only when some water is flowing... and I have found that it takes very little flow (light drizzle) to set things right. In any event, the sensor had been located down in the aquastat area originally where you see the cover held on by three screws (originally only one, but it got opened and closed so much that the single screw stripped, so I added two more).
 

wwhitney

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OK, I know very little about these products (but am interested in learning), so I looked up "Laars double wall indirect" and I got to this brochure for them: https://www.laars.com/document-library/file/11027 which says "Immersed Adjustable Honeywell Aquastat—All Laars-Stor indirect tanks are supplied with a fast acting immersion aquastat for precise and automatic temperature control."

So are you saying that the factory immersed aquastat down at that access panel wasn't providing responsive temperature info? Was it possibly defective? Or are you saying that for some reason yours didn't come with an immersed aquastat?

Cheers, Wayne

P.S. I do wonder a bit--the brochure shows the indirect coil at the bottom half of the tank, with the aquastat there, so perhaps originally the problem was that the water in the top half of the tank wasn't getting heated enough before the aquastat was satisfied? Moving to the upper external sensor avoided that problem at the tradeoff of not getting an accurate reading unless you have flow. Seems like with a lower indirect coil you'd want an upper, immersed aquastat. Although that's just from first principles, I don't have any experience with indirects.
 

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Wayne, you've raised good questions. The installer had the sensor in the normal place down there at first - and I'm pretty sure that the sensor is the one that was provided by Bosch. I'm guessing that Bosch does not want you installing the non-OEM tank's sensor due to whatever proprietary differences that might exist.

How that sensor gets placed in a Bosch tank (not immersed) is not a question I recall asking at the time (very early on in the process). What was determined - as the basis for moving the sensor - is that the normal location was thought to have too cool of a temp reading that far down near the boiler return. Your questions make me wonder if the original sensor location wasn't more appropriate - allowing the sensor to report actual tank temperature changes that include R/S fluctuations, as opposed to just heat coming out of the tank.

The installer has many years of experience, and at the time these initial changes were made, I was pretty sure that he had a deep understanding of the situation. He indicates that he puts in similar systems often, including some double walls each year due to the code requirement. He's a boiler-only guy who has very high consumer ratings. He is easy to work with, but I'm running out of fingers and toes counting the number of return visits.
 

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I'm not sure if this is the correct installation manual residential_indirect_powerstor2_series_dw_double_wall_dw_2_iomanual_45085.pdf but this tank has an internal aquastat if it is the one you have (it looks like it in the manual). So, instead of using a probe from the boiler, it is designed to control a circulator or zone control relay directly which would turn the boiler on when the indirect needed heating. You will NOT get good control over the tank's temperature by sensing it on the outlet pipe, especially if there is a heat trap in there. FWIW, where I live, code requires a tempering valve on the outlet of the tank itself to prevent overly hot water getting anywhere.

My indirect has an internal well for the boiler's temperature sensor, so there's no electronics or electrical equipment in it at all...it's all done by the boiler.

To get timely domestic hot water, most of the time the indirect is setup as a priority zone which prevents the space heating from operating until the tank reaches the set temperature. You can still do this with one circulator, but you'd need two zone valves...one for the dwelling, and the second one for the indirect.

It's also not uncommon to run the boiler at a lower temperature when doing space heating versus heating the indirect. Depends on what controls are there for your boiler. Most have the ability to deal with a priority zone.
 
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James Robins

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Thanks, Jim. Yes, these units are same - and the most recent Laars manual (4/21) has the exact same specs and identical instructions.

You raised the same questions I had at the time of installation. I still don't understand the reason for not having two zones, but was told that the Bosch boiler would not function properly with more than a single zone with the non-OEM tank. Even that single zone relay (SR501) you see on the side (near the tank circulator) is not operational. The Bosch is controlling the DHW function directly. What has happened, at times, is the Bosch goes into Eco mode on its own. That apparently allows the switch to Heat mode if tank charging takes too long. Once the Eco light goes on, it won't go off unless I switch it to off.

The mixing valve is located directly above the sensor - that tan knob about a foot above the tank.
 

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I have a Greenstar wall mount unit, but the tank I have does not use an aquastat.

I guess I wonder if you could remove the aquastat and its probe, and insert the Bosh probe into that same well. Lots of indirect units would have been directly compatible with the boiler. On mine, I use an external Taco zone controller for a water/air heat exchanger zone, and another zone for radiant floor heating. The boiler controls the indirect by itself as a priority zone. If you do move the probe, it sometimes works better if you use some heat transfer paste so you get faster response on the probe in the well.

If you link the exact boiler you have and its installation manual, I'll look at it. Sensing at the outlet of the indirect is unlikely to ever work properly. If you piled a lot of insulation around it, it might work better, but still not great.
 

John Gayewski

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Why don't you want faster hot water? It seems it would solve your problem and make your life more comfortable. If toy don't want a pump you could do it by gravity, but the inspector won't like it.

I also think this problem could be fixed in a better way. There's something off here. Your boiler should be able to handle more than one zone. Any modern boiler does.
 

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Jim, I think you have a good idea regarding the aquastat - and I didn't realize it was removeable. The installer had the sensor originally just snuggled in next to the aquastat mechanism surrounded by fiberglass. As for the concerns about how exactly the set up would work without multi-zoning, it was one of the first questions I raised. The installer had experts in (the wholesale folks) who were correct in saying that the mixing valve would help (it did, but not with the very slow recovery); he talked with the Bosch senior techs and the inspector also looked at the set up - and no one else raised the multi-zone issue.

When the sensor was moved up above we filled the surrounding pipe area with dope (transfer paste). So, that would make sense putting dope in below, as well.

Here is the manual for the Greenstar FS KBR-3A (among other models):
https://www.bosch-thermotechnology.us/us/media/country_pool/documents/downloads-for-bosch-products/gas-fired-condensing-boilers-manuals/6720810590_greenstarfs_installation_instructions_en_02.2021.pdf

Here are the various possible application options which I studied after the install... And, yes, I did mention to the installer way back when that none of these designs fits with how he put it together... and his response was that you have to go with single zone design if the tank is not an actual Bosch... He's done it this way with the Bosch boiler numerous times, but hasn't run into problems until this time, apparently.
https://www.bosch-thermotechnology....sch_greenstar_applications_manual_07.2020.pdf

I'm sure he would push back hard at the suggestion of going to multi-zone after so many alterations - and knowing that the system seems to be working fine as long as some flow in DHW takes place. My inclination now would be to suggest moving the sensor back to where it was - but get it deeper into the well, as Jim has suggested. That would be quick and simple, and might solve the problem. (Finally!)
 

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Why don't you want faster hot water? It seems it would solve your problem and make your life more comfortable. If toy don't want a pump you could do it by gravity, but the inspector won't like it.

Believe me my wife wants faster water. Anything less than hot in an instant is not good enough (her thoughts). Just a matter of wanting to keep things simple with fewer moving parts, less electricity, fewer piping connections, and avoiding the added cost. As it would involve asking for faster hot water and the added return tubing - I'd expect to have to pay for it. At least the inspector (who listened to my pulse circulator idea) said he would hold the application open so I wouldn't have to pay for the added permit fee. Essentially, he said the job isn't done. And he turned out to be correct about that.
 
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Jadnashua

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You've essentially got two zones already as there are two circulators...one on the indirect, and another for the rest of the heating. The boiler knows how to control each of those circulators, and you have the option to set the DHW up as a priority zone if you wish. The boiler has more than enough heat output to maximize the recovery rate of the indirect and minimize how long it would hold off space heating.

On mine, the Taco zone controller makes each of my space heating zones look like a single zone. Zone controllers are available for pretty much any number of zones and will only 'look' like one zone to the boiler. Wiring it up is pretty simple. The end switch ends up going into the boiler as the single thermostat input, while you can have as many thermostats on individual zones as you wish. Because the total flow can vary considerably, it can take some bypass to keep the head within specs for the circulator pump. It's simpler if it's an all or nothing situation. If your boiler's output is larger, you may not want the indirect as a priority zone to keep the load up on the boiler, but they system allows separate operating temperatures depending on whether it's trying to reheat the DHW (generally max) versus the space heating, which may not need max and is more efficient if it can be lower.

If you can relocate the boiler's DHW sensor into the well now used by the aquastat, it should work much better.

I am on my second hot water recirculation system. The first one lasted 20-years and it was an all-in-one box. They used a proprietary check valve and at the age, I didn't feel like trying to retrofit a new, different one, as I couldn't get a direct replacement. I figured 20-years was enough. I ended up with this Laing unit as a replacement. Laing LHB08100092 AutoCirc Recirculation Pump with Timer - Power Water Pumps - Amazon.com the price has gone up a lot since I bought it, but this does not require any dedicated return line. I've found that flushing the toilet in the bathroom where it is located pretty much flushes out the warm water in the cold line it uses as the return. Being the furthest from the DHW tank, things closer also get hot quicker, but if you have many branches, you may need more than one, and then, a pump at the DHW and cross-over valves or dedicated return lines would work better with lower costs. This pump is quite small, and only draws about 9W. The length and diameter of your pipes would change the timing, but when the timer on mine first turns on in the morning, it might run 2-3 minutes. Then, during the time it's been on, the pump wil only run about 45-seconds maybe 4-5 x per hour. If your pipes aren't insulated, it would run more often and you'd be losing more energy. In the heating season, it's not too bad, as that heat is going into the building, but means more cooling costs when the a/c is on, so insulating to slow the heat transfer is better.
 

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Other than the insulation I added in the basement, there's no pipe insulation on the two DHW branches in wall or crawl space. I'd seen the cheap cold-water return devices - and I'm not real comfortable with the idea of running hot back through the cold side. The Laing looks more substantial. I'd possibly go for it, but we're just two of us - I'm semi-retired - and neither of us have regular schedules. There's no consistent rhyme or reason to when we are drawing hot water, but we don't draw much overall. We are cooled by window ACs - have upgraded a couple of the high-use old ones last year - and that is saving considerable energy costs.

We probably could have done fine with the 131 kBTU, but decided to go with the 151 because the minimum input is exactly the same (so the larger unit essentially has a larger modulating turn-down ratio). So, yes, we have more-than-enough to balance between the heat and DHW. The problem is the sensor isn't sending the temp change info the boiler needs to trigger off recovery.

Jim, your analysis is very helpful, and you're right that the boiler is essentially functioning between the two zones... but the installer expresses it as a single zone (in his thought process). You may not be "a pro" but I'm going to guess that you would out-troubleshoot the vast majority of those who make a living in the business.

For now, moving the sensor to the "normal" position in the well - sans aquastat and with the heat paste - should be my next move. Hot drizzle at the furthest faucet has kept the system working just fine for several days now with most overnights below zero. On the coldest day the boiler ran nearly continuously - but kept up - and that's the coldest day we've had in a couple years. The ODR and design curve appear to be working nearly perfectly.
 

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All WH have an aquastat at the bottom of the tank. AN electric one generally has a second one near the top since they control two heating elements. Hot air/water rises, so if you're not sensing the temperature at the bottom, a good portion of the tank will not get to the set temperature as the cold will pool at the bottom. In your situation, the sensor is above the tank, so it's relying on convection of the water in the tank to get up there. Most tanks out of the factory come with a heat trap, preventing that convective flow out of the tank, so having it up there would be very unreliable.

A dual-walled tank would mean the first location was insulated quite a bit from the actual water temperature. The existing aquastat's sensor is in a well, surrounded by the tank's DHW, so it can actually measure it accurately.

Most of the system hot water recirculation systems shut off prior to getting the water fully up to the supply temperature unless you're in a commercial building where it could be running full hot constantly. Those that use a pump at the tank tend to have cross-over devices at the end of each branch, and those close at 105F. That means, you'll have warm water there, and hot not far away. The Laing unit shuts off at 96F, and they have one model that has an adjustable aquastat in it rather than the fixed one. This means again, that there's warm water available nearly immediately, but it also means it's not pushing very much warm/hot water into the cold-water return. This means that there are fewer standby losses. The higher the temperature in the pipes, the more radiant heat is lost when it's just sitting there, but it also means it takes longer to flush it out and warm up the pipe if it's not insulated. So, in the process, rather than circulating full hot water through the pipes constantly that could be done, these systems purge the cold out of the line to the point that it's warm, keeping the cold line cooler. The Laing unit turns the pump on/off, and only runs a few minutes an hour if the pipes are insulated in my place. Other systems run the pump constantly but stop the flow with a thermostatically controlled valve. Those have a bit more standby losses because the pump runs more. There are ways to just turn the pump on when you want it...an occupancy sensor could be used to trigger the pump on, or other manual methods. The Laing unit has a multi-window timer you can set, or just let it run, and it will turn on/off as needed. Depending on how the hot water line is run, in my place, everything branches off that one line, and the vanity at the end is where I have the unit. Everything closer to the tank has hot faster, depending on how long their branch off of the trunk is. If I (rarely) get up before the timer has turned recirculation on, it takes nearly two minutes to get full hot in the shower. That's about 5gallons of water wasted, and 5 gallons less hot available versus when both the tank is hot and the lines to the fixtures are as well.

The price has jumped a bunch in the last year on the unit I have, but there are others out there at various price points that likely work as well. Many of them put the pump at the tank as there's generally power more readily available there, but it was easy for me to put a receptacle in the vanity, and around the tank was already kind of crowded. But then, you need a sensing, cross-over device, ideally, close to the point of use rather than back at the return side of the tank. Any system will require a check valve, so you don't try to pull from both the hot and return side of the loop. On an engineered system, that tends to be built-in, and on my first unit, that's what failed after 20-years, which is why I replaced it. One friend has a Chilipepper (sp?) unit, but it has a huge pump in comparison. That wastes a lot of energy, and exceeds the Copper Institute's recommended hot water velocity, which can literally erode the pipes over time. IMHO, smaller is better unless you insist on a demand control where you have to remember to turn it on, and then wait for the lines to be purged of cold.
 

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All WH have an aquastat at the bottom of the tank. AN electric one generally has a second one near the top since they control two heating elements. Hot air/water rises, so if you're not sensing the temperature at the bottom, a good portion of the tank will not get to the set temperature as the cold will pool at the bottom. In your situation, the sensor is above the tank, so it's relying on convection of the water in the tank to get up there. Most tanks out of the factory come with a heat trap, preventing that convective flow out of the tank, so having it up there would be very unreliable.

A dual-walled tank would mean the first location was insulated quite a bit from the actual water temperature. The existing aquastat's sensor is in a well, surrounded by the tank's DHW, so it can actually measure it accurately.

Most of the system hot water recirculation systems shut off prior to getting the water fully up to the supply temperature unless you're in a commercial building where it could be running full hot constantly. Those that use a pump at the tank tend to have cross-over devices at the end of each branch, and those close at 105F. That means, you'll have warm water there, and hot not far away. The Laing unit shuts off at 96F, and they have one model that has an adjustable aquastat in it rather than the fixed one. This means again, that there's warm water available nearly immediately, but it also means it's not pushing very much warm/hot water into the cold-water return. This means that there are fewer standby losses. The higher the temperature in the pipes, the more radiant heat is lost when it's just sitting there, but it also means it takes longer to flush it out and warm up the pipe if it's not insulated. So, in the process, rather than circulating full hot water through the pipes constantly that could be done, these systems purge the cold out of the line to the point that it's warm, keeping the cold line cooler. The Laing unit turns the pump on/off, and only runs a few minutes an hour if the pipes are insulated in my place. Other systems run the pump constantly but stop the flow with a thermostatically controlled valve. Those have a bit more standby losses because the pump runs more. There are ways to just turn the pump on when you want it...an occupancy sensor could be used to trigger the pump on, or other manual methods. The Laing unit has a multi-window timer you can set, or just let it run, and it will turn on/off as needed. Depending on how the hot water line is run, in my place, everything branches off that one line, and the vanity at the end is where I have the unit. Everything closer to the tank has hot faster, depending on how long their branch off of the trunk is. If I (rarely) get up before the timer has turned recirculation on, it takes nearly two minutes to get full hot in the shower. That's about 5gallons of water wasted, and 5 gallons less hot available versus when both the tank is hot and the lines to the fixtures are as well.

The price has jumped a bunch in the last year on the unit I have, but there are others out there at various price points that likely work as well. Many of them put the pump at the tank as there's generally power more readily available there, but it was easy for me to put a receptacle in the vanity, and around the tank was already kind of crowded. But then, you need a sensing, cross-over device, ideally, close to the point of use rather than back at the return side of the tank. Any system will require a check valve, so you don't try to pull from both the hot and return side of the loop. On an engineered system, that tends to be built-in, and on my first unit, that's what failed after 20-years, which is why I replaced it. One friend has a Chilipepper (sp?) unit, but it has a huge pump in comparison. That wastes a lot of energy, and exceeds the Copper Institute's recommended hot water velocity, which can literally erode the pipes over time. IMHO, smaller is better unless you insist on a demand control where you have to remember to turn it on, and then wait for the lines to be purged of cold.
Being of the mind that lukewarm water in piping creates slime that is full of bugs I generally aim to keep full hot water in the system piping. Slower is definitely better, but constant and slow is better yet. If not constant a timer is better than pulse or hot button.

Recirculating water is a luxury. Stand by losses on fully insulated pipe aren't something to write home about, but pneumonia is. Especially if you are an older person. In the winter there really isn't a such a thing as stand by losses.
 

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The way I look at my situation is that while the water isn't full hot along the entire hot water distribution, the water has already been 'cooked' in the tank, and with every hot water use at a fixture, I'm flushing the line with that full hot water. I've got over 20-years here, and when any work has been done to the lines, the hot ones have been clean and nobody has gotten sick. Maybe we've been lucky. There's more crud in the cold supply lines.

Flushing my toilet seems to flush all of the warm water out of the cold that's used as the return, so warm water hasn't been pumped very far along the cold supply line. In a configuration where each line wasn't used at least daily, I could see there may be an issue, but likely rare.

Maybe I've been lucky...but I'm not changing things!
 

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The way I look at my situation is that while the water isn't full hot along the entire hot water distribution, the water has already been 'cooked' in the tank, and with every hot water use at a fixture, I'm flushing the line with that full hot water. I've got over 20-years here, and when any work has been done to the lines, the hot ones have been clean and nobody has gotten sick. Maybe we've been lucky. There's more crud in the cold supply lines.

Flushing my toilet seems to flush all of the warm water out of the cold that's used as the return, so warm water hasn't been pumped very far along the cold supply line. In a configuration where each line wasn't used at least daily, I could see there may be an issue, but likely rare.

Maybe I've been lucky...but I'm not changing things!


No change suggested.

That is odd. Here the cold lines are generally fine it's the hot that has crud. Although I've cut into some city mains that were slimy.
 

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I'm more concerned about the cold side contamination given our relatively low use overall. Toothbrushing with any lukewarm water would really give me the creeps. On the hot side, the mixing valve allows us to bring the tank temp up, and we are seeing a bit more than 160F after showers (Bosch FW200 on the boiler provides a lot of useful temp info in real time).

Yes, Jim, the double wall tank was a big part of the impetus to shift the sensor from below to the area above the tank. I'm pretty sure that no effort was made to get that sensor deep into the well before making that move. No doubt, the double wall design requires positioning as deeply into the well as possible to get the proper readings. There is virtually no possibility that the problem is on the boiler side because the DHW burn does indeed cut off once the temp reaches a few degrees short of the preferred setting @ 147F. Once the sensor is functioning correctly, I'll likely raise the target setting a little higher - depending on how quickly the tank recovers. As you have implied, we won't know that until things are set right because the tank probably has never been fully heated. One of the reasons for considering the Taco SPE-1 variable circulator with smart plug was that the return water comes in at the bottom of the tank (drain plug) - so you are assured of getting better circulation and more even water mix.

Thanks guys for helping me get this right. I'll report back when the fix is completed. While I could work on this myself, of course, I really think that the installer has to do it just in case something goes wrong - and a warranty issue could be challenged. Given the high season, it might take some time as I'm not having problems needing immediate attention. We do have a forecast of a -16F overnight a few days off - so that will tell me how the design temp (plus a bit more) works out.
 

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FWIW, as I said, flushing the toilet for me purges the cold lines so that the normal cold water is there. NOw keep in mind, it may not be warmer than what it could be in some houses in the summer, depending on how and where the lines are run. IOW, it's not a big deal. Washing your hands can usually purge the line, too. But, if you have a dedicated return line, most all of those will still work, you just hook it up slightly differently. On some, you'd require a different model. If you had one with an adjustable aquastat, you could have it shut off at the low end. It would take a little longer to get full hot, but you'd put less warm water into the cold line. That Laing unit I have is available with an adjustable aqusatat, versus the fixed temperature one I have.
 

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One of the benefits of an indirect over a stand alone tank is that when it needs to be reheated, it gets the full output of the boiler which is almost always larger than the burner or heating elements in a gas or electric tank. This assumes that you've selected to have the boiler treat the tank as a priority zone. For a typical shower, it's often really hard to run out of hot water with one, but maybe if you had a big soaker tub. I can shower with mine for 1/2-hour and never run out, or fill my 6' long soaker tub without issue. Now, mine is slightly larger than yours, it's a 60g, but I could probably have gotten by with a smaller one.

I had an issue with the boiler at the beginning of a weekend, and by taking short showers, I was able to take a hot shower for three days before the part I ordered came in to fix it. The insulation and volume kept things warm. I probably could have gone another day, but the shower would have been warm instead of hot.

I'd just pull the aquastat out, and use the same sensor well to insert the boiler's temperature probe there. Mine came with a spring clip to hold the sensor against the side of the slightly larger diameter well. You want it in contact versus just floating in there plus I used some heat transfer paste on it as well. You could probably find something to wedge it in there so it makes good contact.
 
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