Troubleshooting My Propane Fired Radiant Heating System

[Raspy]

Justy for the record, modern boilers are essentially "tankless"...they might hold a few gallons but nowhere near what the hydronic expert (hydronics institute or something like that) wants...I went through that argument with my installer and I lost...no sense keeping a large tank of water hot if the house didn't need it (even though in my shop he installed a tank system).

There is a difference between a "tankless water heater" and a high efficiency boiler, even though they both do nearly the same thing and they both have a very small amount of water in them. Water heaters are called "tankless water heaters" and boilers are called "boilers". The only tank needed is a holding tank for the domestic hot water unless someone wants to just use a tankless water heater for DWH and a boiler for the radiant. It's perfectly fine to use the boiler for both functions and heat a tank of domestic hot water with a heat exchanger.

In HillStreets case the thermostat was located in the wrong place. It wasn't a bad thermostat or relay. The computer had nothing to do with it. Sounds to me like the installer didn't understand it well enough to get it right, and then wanted nothing more to do with it when it didn't work.

In this conversation I'm trying to help HillStreet get his existing system working by identifying the problems his "plumber" built in, not redesign the whole system. BTW, plumbers are not radiant heating, or boiler contractors. But radiant heating contractors must know how to do piping. Two separate fields of expertise. The finished radiant work may look like it's just plumbing, but there's far more to laying out a proper hydronic system than simply connecting pipes. Just like any other field of expertise, it looks easy from the outside, but try to do it yourself and you're stuck.

I know of no-one that will recommend keeping a large tank hot just in case we might need heat. That theory went out the window in the sixties, or earlier. Some older cast iron boilers were wired as "hot boilers" and stayed hot all the time. That is not done now and especially not with low mass/high efficiency units. No high efficiency boiler can operate in that fashion, nor should they.

 

[HillStreet]

You are in Maine! How deep is your new snow? A traditional system would be a boiler connected to a relay (like a Honeywell) that would be started by a call from your T-stat...that relay would send signals to your boiler and your circ pump(s). If it ain't working, then I think the pros would start at the T-stat. Your relay should not close until that thing says the T-stat no longer wants heat. That would involve location (like in a area where it was getting heat apart from your system like sunshine or other heating). You are running DHW so you have mixing valves and such that I don't...my guy refused to use that and put in a free-standing LP fired water heater instead. So while I have more zones, my system is actually pretty simple.

We got about 18 inches, some drifting 24". It is nice because we are retired and I no longer have to push snow in the morning. I am reading the boiler manual now, it is making a lot of sense. I believe I will leave the outdoor temp sensor enabled for now because I want to evaluate my changes one at a time. The manual does talk about a set point for the outdoor temp sensor, so I may need to lower it to my central heat temp adjustment.

 

[HillStreet]

There is a difference between a "tankless water heater" and a high efficiency boiler, even though they both do nearly the same thing and they both have a very small amount of water in them. Water heaters are called "tankless water heaters" and boilers are called "boilers". The only tank needed is a holding tank for the domestic hot water unless someone wants to just use a tankless water heater for DWH and a boiler for the radiant. It's perfectly fine to use the boiler for both functions and heat a tank of domestic hot water with a heat exchanger.

In HillStreets case the thermostat was located in the wrong place. It wasn't a bad thermostat or relay. The computer had nothing to do with it. Sounds to me like the installer didn't understand it well enough to get it right, and then wanted nothing more to do with it when it didn't work.

In this conversation I'm trying to help HillStreet get his existing system working by identifying the problems his "plumber" built in, not redesign the whole system. BTW, plumbers are not radiant heating, or boiler contractors. But radiant heating contractors must know how to do piping. Two separate fields of expertise. The finished radiant work may look like it's just plumbing, but there's far more to laying out a proper hydronic system than simply connecting pipes. Just like any other field of expertise, it looks easy from the outside, but try to do it yourself and you're stuck.

I know of no-one that will recommend keeping a large tank hot just in case we might need heat. That theory went out the window in the sixties, or earlier. Some older cast iron boilers were wired as "hot boilers" and stayed hot all the time. That is not done now and especially not with low mass/high efficiency units. No high efficiency boiler can operate in that fashion, nor should they.

Right you are Raspy,

In my reading, the manual says I can adjust the central heat water temp to between 130 and 160 while supplying under floor radiant heat. Currently the boiler supplies water at 180, and that is stepped down by mixing valve to 110, a big difference. I do think though that I need to bump up the loop temps to 140, as recommended by dadster4. I did notice that the return water was 160 degrees or even higher, so the system does not seem to be using much energy, but the boiler is making water hot nonetheless.

The circulators were originally set at number 2 speed, but I reduced that to number 1 speed, the slowest.

Now I have a question: the manual says it is more efficient for a condensing boiler to have longer run times at a lower temp than it is to run less time with higher temps. Does that sound right, do you agree?

 

[Raspy]

HillStreet,

The outdoor sensor is a reference the boiler uses to adjust it's delivery temperature in the CH mode.

Some systems are set up to circulate constantly, or just barely add enough heat to satisfy the thermostat. In those cases, the water must be warmer when the outside air is colder. This is not the preferred way to run a house in my view. But if used with the setback thermostat, you might be able to get a small increase in efficiency at the cost of recovery. Underfloor delivery systems, as I understand yours is, are slow to respond and have little mass to even the heat out over time, but they do work.

My strategy is to have a water temperature that is adequate under all conditions. Then use the thermostat in it's setback mode to have different indoor temperatures at different times. This will give you greater comfort and a lower bill.

Each delivery system, whether it is underfloor, in-slab, baseboard, or whatever, has different characteristics and can take some fiddling to get it just right. Each person's lifestyle and heat needs are different too.

As I mentioned before, it's probably better to not use the outdoor sensor and put a thermostat program in that will give you the best comfort. In the beginning though, just put the thermostat on "HOLD" and see how it goes. Then program it later when you want cooler temps while sleeping or away.

I hope you're having fun with the boiler programming too. It's all very logical once you see it. And in your case you know the house will heat, so it's more a matter of fine tuning. If you leave the outdoor sensor installed, you can watch the boiler parameters to see it adjust the delivery temp based on outside temp. You might even be able to read the outdoor temp. Depending on how your boiler is sized, it may not be able to throttle down low enough to keep running while matching the heat load. In this case it will constantly be starting and stopping. You have already seen a lot of start ups so this may be happening. I would expect a boiler sizing of about 25 to 50 BTUs per sq ft and a load of about 10 BTUs per sq ft in a well insulated house. Do those numbers look about right to you?

 

[Raspy]

Right you are Raspy,

In my reading, the manual says I can adjust the central heat water temp to between 130 and 160 while supplying under floor radiant heat. Currently the boiler supplies water at 180, and that is stepped down by mixing valve to 110, a big difference. I do think though that I need to bump up the loop temps to 140, as recommended by dadster4. I did notice that the return water was 160 degrees or even higher, so the system does not seem to be using much energy, but the boiler is making water hot nonetheless.

The circulators were originally set at number 2 speed, but I reduced that to number 1 speed, the slowest.

Now I have a question: the manual says it is more efficient for a condensing boiler to have longer run times at a lower temp than it is to run less time with higher temps. Does that sound right, do you agree?

Yes, it is more efficient to make longer runs at a lower temp. We want the boiler to condense and the lower it's output temp the more it condenses. The lower the operating temp, the lower the exhaust temp. But you'll deliver more energy with higher temps and you'll get less cycling with higher temps if the boiler happens to be oversized. I think yours might be oversized.

Don't get carried away with your delivery temps. It's PEX and you'll get swings beyond your setpoint that may get near the upper limit of the tubing. Give the tubing some headroom if possible. The compromise is to find a practical temp that does the job, but is not so high it threatens the tube. Any properly designed system should be able to heat well within the operating temperature limits of PEX. You already know the house will heat with 110 degrees, so there is your starting point.

Turn the tempering valves up to about 140 and set the boiler to 130. Then the tempering valves are out of the loop and you still have a good boiler temp. Compare the supply and return temps to determine if the pumps are running at a good speed. In this case, probably no more drop than about 10 degrees. Your system delivers at a very slow rate, so you won't see much drop from supply to return.

Many pumps have three speeds and I don't know which model you have. And excellent choice would be a Grundfos 15-58 three speed. But a 26-99 three speed would be way too big.

 

[teejk]

There is a difference between a "tankless water heater" and a high efficiency boiler, even though they both do nearly the same thing and they both have a very small amount of water in them. Water heaters are called "tankless water heaters" and boilers are called "boilers". The only tank needed is a holding tank for the domestic hot water unless someone wants to just use a tankless water heater for DWH and a boiler for the radiant. It's perfectly fine to use the boiler for both functions and heat a tank of domestic hot water with a heat exchanger.

In HillStreets case the thermostat was located in the wrong place. It wasn't a bad thermostat or relay. The computer had nothing to do with it. Sounds to me like the installer didn't understand it well enough to get it right, and then wanted nothing more to do with it when it didn't work.

In this conversation I'm trying to help HillStreet get his existing system working by identifying the problems his "plumber" built in, not redesign the whole system. BTW, plumbers are not radiant heating, or boiler contractors. But radiant heating contractors must know how to do piping. Two separate fields of expertise. The finished radiant work may look like it's just plumbing, but there's far more to laying out a proper hydronic system than simply connecting pipes. Just like any other field of expertise, it looks easy from the outside, but try to do it yourself and you're stuck.

I know of no-one that will recommend keeping a large tank hot just in case we might need heat. That theory went out the window in the sixties, or earlier. Some older cast iron boilers were wired as "hot boilers" and stayed hot all the time. That is not done now and especially not with low mass/high efficiency units. No high efficiency boiler can operate in that fashion, nor should they.

My installer is now primarily a commercial/industrial boiler guy but in a former life required plumbing and HVAC skills. We built here in 2009 but prior to that I did some research and I stumbled across Complete Radiant Floor Heat Information Site. At the time the guy said that my Weil McLain boiler would be oversized and that storage would solve the "short-cycling". My installer called BS and did it his way...granted he was old school in certain respects (like he still uses those fused switch boxes at each pump). With that all out of the way, I'm watching...sounds like the OP is in exactly the same position I was in a few years ago. I'll yield to you.

 

[dadster4]

"Now I have a question: the manual says it is more efficient for a condensing boiler to have longer run times at a lower temp than it is to run less time with higher temps. Does that sound right, do you agree?" Yes, setting your boiler water supply temp high enough to satisfy your heating needs (130 to 140) and have the return water temp low enough (110 to 130) to provide a condensing condition in the boiler will provide for a longer running cycle and maximum efficiency.
Raspy-"Yes, it is more efficient to make longer runs at a lower temp. We want the boiler to condense and the lower it's output temp the more it condenses. The lower the operating temp, the lower the exhaust temp. But you'll deliver more energy with higher temps and you'll get less cycling with higher temps if the boiler happens to be oversized. I think yours might be oversized." You will actually get more cycling at a higher supply temp. A cooler return temp, not supply temp, will provide for a condensing condition.
The systems main problem is the supply temp is too high (180), the mixing valves are unnecessary at the manifolds, and the return water temp is too high. Once the supply temp is lowered you will know if the boiler cycles less...start there and let us know what you find. I forgot to ask if the joist spaces are insulated below the tubing, a 2" airspace is required between the subfloor and top of the insulation...it's not a real issue at the present time, but should be addressed eventually.

 

[Raspy]

"Now I have a question: the manual says it is more efficient for a condensing boiler to have longer run times at a lower temp than it is to run less time with higher temps. Does that sound right, do you agree?" Yes, setting your boiler water supply temp high enough to satisfy your heating needs (130 to 140) and have the return water temp low enough (110 to 130) to provide a condensing condition in the boiler will provide for a longer running cycle and maximum efficiency.
Raspy-"Yes, it is more efficient to make longer runs at a lower temp. We want the boiler to condense and the lower it's output temp the more it condenses. The lower the operating temp, the lower the exhaust temp. But you'll deliver more energy with higher temps and you'll get less cycling with higher temps if the boiler happens to be oversized. I think yours might be oversized." You will actually get more cycling at a higher supply temp. A cooler return temp, not supply temp, will provide for a condensing condition.
The systems main problem is the supply temp is too high (180), the mixing valves are unnecessary at the manifolds, and the return water temp is too high. Once the supply temp is lowered you will know if the boiler cycles less...start there and let us know what you find. I forgot to ask if the joist spaces are insulated below the tubing, a 2" airspace is required between the subfloor and top of the insulation...it's not a real issue at the present time, but should be addressed eventually.

dadster4,

I don't think so. The reason I say less cycling with higher temps is because you deliver more energy with a higher delivery temperature. That means the boiler is more likely to be able to modulate itself down to a point where it can match the delivery to the house with it's own BTU output. If it can match it's output to the delivery, it won't cycle. So, higher delivery temps mean less cycling. Of course, if the boiler is way oversized, it will cycle a lot no matter how it's adjusted.

Condensation happens in the heat exchanger. The cooler the average temp of the water in the HX, the more it will condense. Lower output temp means the entire HX is cool. Lower input temp doesn't necessarily mean that, because there can be a large temp rise and the outlet temp much higher, which means less condensing. It's not absolutely so, but more likely, that the output temp is more of an indicator of the amount of condensing. If the outlet is low enough to condense, for sure the whole HX is cool enough to do so. If the inlet is cool enough to condense, the outlet might not be.

I agree that the supply temp is too high at 180 degrees. That's why I wanted some clarification as to whether the same temp is doing the DHW or not and why I wanted him to reduce the setting and take the tempering valves out of the equation. This is also part of the reason to eliminate the outdoor sensor. It might cause the boiler to ramp up to 200 during cold weather. The house seems to be able to heat OK at 110 degrees CH temp. That is also a good condensing temp, but 130 is a more practical delivery temp considering the control differential and recovery time.

As far as insulation under the joist bays, or 2" below the subfloor, it would be nice to see foil backed insulation with the foil "up" if possible. It's not really required, but a very good thing to have.

I'm not a fan of "underfloor" joist bay radiant, but it's OK if it can't be done in a better way, and if there happens to be good access to the area. But even then, it's a pain to install and a lot of proper prep is required to make it as good as it can be.

There is too much we don't know about the overall design in this case to make a lot of absolute decisions.

 

[HillStreet]

Hi dadster4,

I am going to make the boiler temp changes tomorrow as our storms and cold weather passed. Then the coal stove can die down. I will also keep a log of run hours and cycles because it is something to measure. I will post my progress as time goes on.

Oh, the underside of the heat piping is insulated with batt insulation installed by the insulation contractor.

 

[dadster4]

dadster4,

I don't think so. The reason I say less cycling with higher temps is because you deliver more energy with a higher delivery temperature. That means the boiler is more likely to be able to modulate itself down to a point where it can match the delivery to the house with it's own BTU output. If it can match it's output to the delivery, it won't cycle. So, higher delivery temps mean less cycling. Of course, if the boiler is way oversized, it will cycle a lot no matter how it's adjusted.

Condensation happens in the heat exchanger. The cooler the average temp of the water in the HX, the more it will condense. Lower output temp means the entire HX is cool. Lower input temp doesn't necessarily mean that, because there can be a large temp rise and the outlet temp much higher, which means less condensing. It's not absolutely so, but more likely, that the output temp is more of an indicator of the amount of condensing. If the outlet is low enough to condense, for sure the whole HX is cool enough to do so. If the inlet is cool enough to condense, the outlet might not be.

I agree that the supply temp is too high at 180 degrees. That's why I wanted some clarification as to whether the same temp is doing the DHW or not and why I wanted him to reduce the setting and take the tempering valves out of the equation. This is also part of the reason to eliminate the outdoor sensor. It might cause the boiler to ramp up to 200 during cold weather. The house seems to be able to heat OK at 110 degrees CH temp. That is also a good condensing temp, but 130 is a more practical delivery temp considering the control differential and recovery time.

As far as insulation under the joist bays, or 2" below the subfloor, it would be nice to see foil backed insulation with the foil "up" if possible. It's not really required, but a very good thing to have.

I'm not a fan of "underfloor" joist bay radiant, but it's OK if it can't be done in a better way, and if there happens to be good access to the area. But even then, it's a pain to install and a lot of proper prep is required to make it as good as it can be.

There is too much we don't know about the overall design in this case to make a lot of absolute decisions.

Cooler return water is still the best method to provide condensing conditions in the heat exchanger (that's why it is so bad for a standard boiler to have low return temps), the supply temp will be whatever the boiler is set for.
The outdoor reset control will adjust the boiler supply temp downward from the setpoint but not increase it above the setpoint. It will not affect boiler temp below 0 degrees outdoor temp, and only modulate it down slightly with large increases in outdoor temps.
Water temp in his system was never lower than 160 degrees, so the mixing valves had no effect when set to 110 degrees. The goal is to have a supply temp just high enough to provide proper heating of the space (140ish), a return temp low enough so that the mixing valves have cooler water to mix with the supply (120ish), and a mixed supply temp at the manifold to provide a slow even heat to the loops (130ish).
I wanted Hillstreet to try these changes first to get closer to a good running condition, I still think the boiler control settings should be changed to lower the output of the burner, but making these changes will familiarize him with how to operate the controller...Let's see how it goes today!