Retrofit Conversion Help - Replacing Outdoor wood furnace with Propane Boiler

MisterEd

I am converting my radiant floor heating system outdoor wood furnace to a propane boiler arrangement.

1. current furnace is a Central Boiler CL17 with 170 gallon tank; btu output
unknown
2. House has 3 Taco -007 zone pumps; workshop uses 1 Taco -009.
3. i use 7-8 cord of wood a winter; dependent on how much i use the workshop (700 sq feet traditional frame)
4. home is full round logs - 1500 sq foot with 23 foot vaulted ceiling and large windows.
5. 180° dgf supply water; 16° delta T between supply and return
6. location: north central Arkansas Ozark Mtns.
7. system does not provide domestic hot water needs

Question 1: is a 50 btus per sq foot rule of thumb, or 35, more realistic for my location?
Question 2: does transitioning to a propane boiler allow me to use a smaller storage tank than the one on the wood furnace?
Question 3: i am thinking of a condensing unit boiler with something having a 10:1 turn down ratio. Is this reasonable for this application?

The Central Boiler unit is 21 years old and a workhorse but I am getting too old to comfortably bring the wood in off the property and cut, split, and stack it. Just had double knee replacement surgery. Lol

thanks for any input fellas,

Mister Ed

Jamie Hall

The best first thing to do is to make as close an estimate of the heat loss of the house -- Manual J -- as you can. https://www.slantfin.com/slantfin-heat-loss-calculator/ has an excellent calculator for the purpose, and can be used with your construction. Then size the boiler to that.

I'd say off hand that both of your "rules of thumb" are high, perhaps ridiculously high.

I am very concerned, however, about the temperature you are running your radiant floor at. Does it have to run at that water temperature? Most radiant floor installations are designed to run at 120 or less, but with the water circulating all the time. Can you provide more detail on that?

If you need to run at 180, a condensing boiler will save you exactly nothing.

GroundUp

The CL17 output is approximately 75,000 BTU sustained or 150,000 surge. Only burning 7-8 cord through it tells me your heat load is far less than you think it is, those smoke dragons will burn 2-3 cord of hardwood a year just to keep themselves hot with no load at all. Flow rate can be roughly calculated using you length and diameter of the underground lines to and from the CL17, which will give us the actual BTUs used if the 16* delta is accurate. As Jamie stated, some supply temps would be great information to have in order to select a boiler. A heat loss calc would be best, but 35 BTU/sq ft is probably too high already

MisterEd

Mr. Hall,

"I am very concerned, however, about the temperature you are running your radiant floor at. Does it have to run at that water temperature?"

I realize concrete floor applications use the lower temps but in my case I am dealing with roughcut 2 x 10s on 24" centers with a 3/4" t&g pine flooring on top. There are two evenly spaced tubing runs between the joists with the tubing affixed to the underside of the flooring. This was per the spec laid out by Central Boiler.

"Most radiant floor installations are designed to run at 120 or less, but with the water circulating all the time. Can you provide more detail on that?"

My zones pumps work off a common supply and return manifold in the basement and the only time water is moving is when a stat calls for heating. I have four individual loops for my 900 sq foot Great Room zone, using 1 Taco -007 circ pump. My other areas are much smaller with just 2 loops on my master bedroom and 1 loop on the bathroom/hallway area, each using a -007.

"If you need to run at 180, a condensing boiler will save you exactly nothing."

Can you expound on this? Are you saying a non-condensing boiler would be a better fit for my application?

thanks,

Ed

MisterEd

Groundup,

"The CL17 output is approximately 75,000 BTU sustained or 150,000 surge."

Not sure what you mean. Are you saying with a newly loaded firebox burning at peak there is a 150K output capacity?

"Only burning 7-8 cord through it tells me your heat load is far less than you think it is, those smoke dragons will burn 2-3 cord of hardwood a year just to keep themselves hot with no load at all."

Determining my particular heat load may be problematic as neither my log home or workshop are entirely traditional structures. I have 5 large sliding glass door size windows in my great room and a 23 foot vaulted ceiling. My workshop only has clear plexiglass in the gable ends to allow maximum natural lighting. This was a big tradeoff but copious natural lighting for my woodworking requirements was needed.

In the 21 years operating the Central Boiler furnace I have never needed much wood to maintain setpoint if there was no demand/calls for heating. I realize that is anecdotal but that is my experience.

"Flow rate can be roughly calculated using you length and diameter of the underground lines to and from the CL17, which will give us the actual BTUs used if the 16* delta is accurate."

The underground supply and return lines feeding the house basement headers are 1" diameter and approx. 60 feet in length each. The lines to my workshop are also 1" and about the same length, just no manifolds involved.

The differential temp is fairly accurate if we can reliably reference the Taylor themometers at the manifolds.

"As Jamie stated, some supply temps would be great information to have in order to select a boiler. A heat loss calc would be best, but 35 BTU/sq ft is probably too high already."

Typically with a well stocked firebox the temp at the basement supply manifold runs at 180° and the return leaving the other manifold is 164°.

One aspect with my situation is that if I am not working in my woodshop and cut the stat back to 48 or 50 degrees that lessens the demand considerably. This aspect makes me interested in the gas boilers that have the 5 to 1 and 10 to 1 turndown ratios that allows reduced gas usage and less possibility of excessive cycling.

thanks for your input,

Ed

JUGHNE

Have you considered a boiler at each location?

There must be some heat loss in the UG piping which would be eliminated.
Plus it is 21 years old and has run at fairly high temp.

Condensing boilers will not achieve the touted efficiency unless the return water is 120 or lower. The cooler water causes the exhaust gasses to condense and draw that extra 10% out of the process.

What type of tubing is in your house that handles the 180 degrees?

How do you heat your domestic water?

Jamie Hall

The Slant/Fin calculator has enough "wiggle room" in the inputs to account for your building structure. I highly recommend playing with it.

Radiant floors -- yours included -- work best when there is continuous circulation of relatively low temperature water. If nothing else, this minimizes the temperature change stresses on the floor. In fact, the ideal -- which you will be able to do with a gas boiler, but couldn't with the forest eater, is to modulate the circulating water temperature so that it is always going and so that the floor is just warm enough -- but no more -- to hold the space temperature where you want it. This technique is referred to usually as "outdoor reset" and depends on the boiler water temperature -- or in some applications a mixing valve -- being controlled by the outside air temperature.

If the boiler itself can modulate to maintain the target temperature, rather than cycling the flame on and off, so much the better.

Do you have any record of how long your present setup actually does circulate water, and then how long it goes turned off before it starts again? And do you have any way to measure what the floor temperature actually is and how it varies with when the circulation is happening and is not?

As @JUGHNE said, to gain the benefit of a condensing boiler the return temperature must be below 140 -- and the farther below the better. It is quite possible that with continuous circulation that may be achievable.

MisterEd

JUGHNE,

"Have you considered a boiler at each location?"

Not practical for my situation.

"There must be some heat loss in the UG piping which would be eliminated.
Plus it is 21 years old and has run at fairly high temp."

The insulation jacket is fairly robust and a section I did check a few years back appeared to be still completely intact.

"Condensing boilers will not achieve the touted efficiency unless the return water is 120 or lower. The cooler water causes the exhaust gasses to condense and draw that extra 10% out of the process."

I had not considered this aspect with the higher temps. I know the boilers I have looked at online show a 180° setpoint capability and they never mention efficiency loses from running higher temps. This makes me think I may not want to invest in a condensing unit if efficiency savings will not be realized.

"What type of tubing is in your house that handles the 180 degrees?"

1/2" orange PEX.

"How do you heat your domestic water?"

Traditional electric wtr heater.

thanks,

Mister Ed

MisterEd

Mr. Hall,

"The Slant/Fin calculator has enough "wiggle room" in the inputs to account for your building structure."

Will definitely use that calculator.

"... is to modulate the circulating water temperature so that it is always going and so that the floor is just warm enough -- but no more -- to hold the space temperature where you want it. This technique is referred to usually as "outdoor reset" and depends on the boiler water temperature -- or in some applications a mixing valve -- being controlled by the outside air temperature.

If the boiler itself can modulate to maintain the target temperature, rather than cycling the flame on and off, so much the better."

Not sure how I would use a continuous flow system with the independent zones having separate stats and demand loads. I am sure it might be feasible but I want to avoid any modifications inside the home or shop.

The 'outdoor reset' feature definitely appeals to me though. During milder weather if I could have the boiler take the supply temp down 30 or 40 degrees that would be great.

"Do you have any record of how long your present setup actually does circulate water, and then how long it goes turned off before it starts again?"

No record. For the Great Room the circ pump will run an hour or two in cool weather and when it gets into the 30s/40s it runs nonstop pretty much. There is a lag effect with the thermal mass of the flooring initially getting heated up and then it radiates for a time even with no circulation.

"And do you have any way to measure what the floor temperature actually is and how it varies with when the circulation is happening and is not?"

The floor temp runs around mid-eighties, according to an infrared temp probe, after the lag period is over. It takes several hours for the latent heat in the floor to dissipate with no water circulation.

Jamie Hall

Oh good. Forward progress here. Your last paragraph there tells you that the floor doesn't need to be in the mid-eighties all the time -- that you can and do let it drop for several hours. This indicates that you can achieve the same heating overall with a lower floor temperature, perhaps as much as 5 to 10 degrees lower -- provided it is continuously maintained at that temperature. Which, following along, means that the water temperature circulating can be much lower and maintain the space temp. Quite possibly low enough to make a condensing boiler feasible.

Independent zones can be managed using mixing valves to provide different water supply temperatures.

However... as you may have already discovered, radiant floor systems really aren't happy with trying to follow varying temperature setpoints in the space. They do have a lag built in -- they can't help it -- so they may take hours to days to react to changing a space temperature thermostat -- so cranking the thermostat up or down for a few hours just isn't helpful at all.

Now -- there are ways to cope with that. Mostly by using the radiant floor to maintain a base temperature which is comfortable, but not warm, and using other radiation -- panel radiators running off the same boiler, but on a different circuit, for instance -- to warm the space up further when you want it.

GroundUp

MisterEd said:

Groundup,

"The CL17 output is approximately 75,000 BTU sustained or 150,000 surge."

Not sure what you mean. Are you saying with a newly loaded firebox burning at peak there is a 150K output capacity?

- I mean it's capable of producing roughly 75k continuously, but will produce 150k for the first hour due to the stored volume of water. A gas boiler will have no water storage, so the actual load needs to be figured out.



"Flow rate can be roughly calculated using you length and diameter of the underground lines to and from the CL17, which will give us the actual BTUs used if the 16* delta is accurate."

The underground supply and return lines feeding the house basement headers are 1" diameter and approx. 60 feet in length each. The lines to my workshop are also 1" and about the same length, just no manifolds involved.

-1" pex in a 120ft loop with an 007 will move roughly 5 GPM and with the 009 roughly 7 GPM. Is this 16* delta with all zones calling simultaneously? That equates to approximately 40,000 BTU in the house.


The differential temp is fairly accurate if we can reliably reference the Taylor themometers at the manifolds.


One aspect with my situation is that if I am not working in my woodshop and cut the stat back to 48 or 50 degrees that lessens the demand considerably. This aspect makes me interested in the gas boilers that have the 5 to 1 and 10 to 1 turndown ratios that allows reduced gas usage and less possibility of excessive cycling.

- With 180 degree SWT like you're using now, a condensing boiler is a losing venture from day 1 IMO. The turndown ratio is neat in theory, but not really for high supply water temps like your system requires. Yes it would likely cycle less, but gas usage would be pretty similar in the long run with a whole lot more maintenance involved. A cast iron traditional boiler with outdoor reset would make you a much happier person down the road, in my opinion.

Ed

MisterEd

Mr. Hall,

"... Which, following along, means that the water temperature circulating can be much lower and maintain the space temp. Quite possibly low enough to make a condensing boiler feasible."

I have a heating system that has worked very well for over two decades and hopefully you can understand my great reluctance to go about changing what is a very straightforward simple system to one involving additional zone pump controls, mixing valves, and auxillary heaters.

My goal is to simply replace my wood furnace with a gas boiler and leave the rest alone if at all possible. This leaves me with two issues needing to be addressed:

1. The correct size boiler and its specific features like outdoor reset. Is a non-condensing boiler considerably less efficient? Are there other drawbacks I am unaware of about non-condensing units? Keeping the "turn down" capability in mind is it prudent to oversize the boiler capacity by 10-15% just as a margin of safety? I did the Slantfin calculations off 32° outside air temp; it occassionally gets into the teens but maybe for only a few days.

2. Do I need to keep a separate water storage tank that is equivalent to the 170 gallons I currently have or would something much smaller work just as well in conjunction with a gas-fired boiler?

thanks,

Ed

Jamie Hall

I can understand your desire to keep what you have and know well. So long as you are aware of the potential drawbacks in comparison with more modern systems -- and, I might add, some of the shortcomings of more modern systems.

That being the case, I would suggest that since you have a system which works to your satisfaction, but are only wanting to move from working with wood so much -- which, as an older person, I also quite well understand -- probably the best move will be to a really good cast iron boiler, non-condensing, not modulating and keep the basic system intact. There will be lower efficiency -- condensing gains 5 to 10 %, but that's a maximum. On the other hand, a high quality cast iron boiler will last longer and take a good deal less maintenance.

The boiler still should be sized for the heat loss of the building, however. While you shouldn't oversize, undersizing can be a problem. In your situation I would suggest rerunning the heat loss for the temperature which you get from time to time for a day or so. Otherwise, the boiler simply won't be able to heat the building. On the other hand, perhaps on the very cold days you could forego heating the shop for a few days. Work it out both ways.

I'm not sure that implementing outdoor reset will be particularly beneficial. However, you might consider controlling the circulation with a floor temperature sensor rather than a space thermostat, and running shorter, but more frequent, cycles to keep the floor temperature more constant.

MisterEd

Mr. Hall,

" ... probably the best move will be to a really good cast iron boiler, non-condensing, not modulating and keep the basic system intact. There will be lower efficiency -- condensing gains 5 to 10 %, but that's a maximum. On the other hand, a high quality cast iron boiler will last longer and take a good deal less maintenance."

Do you have any recommendations as to specific manufacturers to look at who offer this type of cast iron boiler? And are there any specific features to pay attention to? My only experience with boilers is as an industrial maintenance electrician and it was a large steam unit.

"On the other hand, perhaps on the very cold days you could forego heating the shop for a few days. Work it out both ways."

I do have a backup in the form of a massive fireplace. Once I get it going it has a huge amount of stone to radiate heat and the entire structure is inside the home except for the portion above the roofline.

Do have any input to offer on the sizing of my hot water storage tank?

thanks,

Ed

GroundUp

You need zero water storage with an LP boiler. The only reason the CL has any is to make up for its lack of performance during full-out burns