Maxim 255 Temperature difference

[Cliff DeCarlo]

Hey everybody,

This was my first winter with my Maxim 255 outdoor pellet boiler (which replaced two indoor pellet stoves). It has been working great so far, except for really cold days (say below 10 degrees Fahrenheit). My first thought was that the boiler just didn't have enough heating capacity to heat the space. It is currently heating the first floor of my house through my existing furnace ducts (about 1800 square feet). I have a water-air heat exchanger installed in my plenum (rated at 120,000 BTU). The furnace sits outside pretty close to the house and there is about 15 feet of thermopex buried to get into the basement. Within in 3 feet of the where the thermopex enters the basement, I have a thermostatic valve installed with temperature gauges on both the feed and return sides of the 1" pex loop to the furnace.

I have the boiler temp set at the max (190 degrees F). What I have noticed is that the temp on the feed side of the thermostatic valve never gets above 172 deg F, and the return side is usually 130 deg F. I understand the loss of temp on the return side (the heat is being drawn out by the exchanger in the furnace...which is what you want). What I don't understand is how is it possible to lose almost 20 degrees of temp from the boiler to the thermostatic value which is less than 20 feet away (and 15 feet of that is thermopex which is supposed to lose only 1 degree for every 100 feet). When I have the propane burner on and measure the temp of the air coming out of the ducts in the house, it is about 20 degrees higher than when it is using the pellet boiler. I'm thinking that if I could get the temp of the water on the supply side to be what it is at the boiler that would make up the difference. I have replaced the temp guage at the thermostatic valve and taken the temp of the water in the boiler directly to rule out bad guage/temp sensor in the boiler. I'm using a Taco-007 circulator pump through about 120 feet of 1" pex (loop length). After the heating season is done, I'm planning on putting in a temp guage right at the supply side of the heat exchanger as well.

Anybody have any thoughts on why there is such a drastic temperature drop from the furnace to the temp on the supply side of the thermostatic valve?

I was thinking maybe air in the lines causing a low flow rate through the pex? I have not yet installed any air purge valves in the pex loop. Could there be something else going on?

Thanks,

Cliff

 

[E Yoder]

Several thoughts-
A 007 is pretty small for that loop. A higher head circulator would move more water, heat better.
Are eyou measuring temp of the actual supply water? Is the pellet boiler water jacket mixing properly with that small pump or stratifying?

An air vent doesn't work well if it's above the vent pipe on the boiler in an open system, it sucks air in. I purge with domestic water pressure. Air can hang in the top of a pipe in a high spot if flow rates are low, keeping it choked down.

 

[maple1]

If you have a high spot in your pex that air could get trapped in, there could be air in it.

Ditto on the 007 not being very big.

 

[Cliff DeCarlo]

I have flushed with residential water a couple of time with no effect. The boiler sits right outside the house and is about 5 feet higher than the lowest level for the pex loop. The highest spot of the pex loop would be the outdoor boiler itself...the thermopex goes pretty much straight down at the boiler for about 3-4 feet and curves at an angle into the basement about 4.5 feet down. The total thermopex length is 15 feet. Once it is in the basement it does have elbows that raise it up to the pex line going to the furnace (that total loop length is probably 100 feet). The highest point inside is the thermostatic valve. There are three temperature gauges in the pex loop inside the basement. One is inline on the supply side of the pex loop (right before the thermostatic valve). The second is attached to the bottom of the thermostatic valve itself and the third is inline on the return loop (right before coming back through the thermostatic valve). I know the 007 is rated for a head of up to 11 feet with max flow rate of 20 GPM. I'm wondering what pump you would recommend? Some of the taco models have higher head ratings and different flow rates. What would be more important (head or flow)? I'm thinking maybe the 0010 (11.5 ft of head and up to 30 GPM) or the 0011 (30 ft of head and up to 28 GPM). Any thoughts on either of those pumps? Anything bigger than one of those seems like overkill to me...

 

[Woodman1]

Something doesn't seem right there. Im not sure what you have for your protection valve and how you have it plumbed, but is there any chance you somehow have it plumbed wrong and are sending tempered water to the house rather than just a reduced flow of 190? Baring something horrible, there is no way you are loosing that much heat to the ground and it seems like if it is air locked the boiler would idle all the time and your house wouldn't heat well at all.

 

[Cliff DeCarlo]

Yeah, that's what I was thinking. How could I possibly be losing almost 20 degrees from the boiler to the thermostatic valve (which is less than 20 feet away through thermopex). I'm pretty sure I have it all plumbed correctly (double and triple checked the flow direction from the boiler and pump) as well as the back flow protection valve. I agree there is definitely something strange going on. The house heats pretty well as long as the temp outside is above 15 degrees F. Once it drops below that it seems to not be able to raise the temperature in the house very well. That extra 20 degrees of heat would make a big difference. Once this heating season is over, I'm planning on putting another in-line temp gauge right at the supply side of the water-air heat exchanger.

 

[Woodman1]

A picture of the valve could be helpful. Also a cheap ir gun can really help sort things out

 

[Cliff DeCarlo]

I do have an IR temp gun and the temps using that do line up pretty close to the gauges. I'm going to post some pictures of the valve/gauges when I get home tonight.

 

[maple1]

You should verify that the water leaving your boiler is actually as hot as what the boiler gauge says.

 

[Cliff DeCarlo]

I did verify the water temperature in the tank (using a thermometer on a string lowered into the tank and my IR gun) and it did match the temp on the boiler control unit. My first thought was a bad temp sensor in the boiler, but that appears to not be the case. Using the IR gun on the pump casing, the temps were closer to the 190 reading than the 170 reading. It's hard to get an exact temperature reading at the source from the boiler especially since it's all plumbed up (I suppose I could try putting an inline temp sensor right there at some point).

 

[Woodman1]

Shoot the pex line where it leaves the boiler than also right where it enters the mixing valve. It should be the same

 

[maple1]

I've never had good luck with my IR gun shooting pex. Might be my gun.

 

[maple1]

Yeah, that's what I was thinking. How could I possibly be losing almost 20 degrees from the boiler to the thermostatic valve (which is less than 20 feet away through thermopex). I'm pretty sure I have it all plumbed correctly (double and triple checked the flow direction from the boiler and pump) as well as the back flow protection valve. I agree there is definitely something strange going on. The house heats pretty well as long as the temp outside is above 15 degrees F. Once it drops below that it seems to not be able to raise the temperature in the house very well. That extra 20 degrees of heat would make a big difference. Once this heating season is over, I'm planning on putting another in-line temp gauge right at the supply side of the water-air heat exchanger.

Not sure about the need for a backflow prevention valve? Some pics of everything might help.

 

[Cliff DeCarlo]

Here are some pics of the pump and thermostatic valve.

https://photos.app.goo.gl/LgJXHdiG4PtV6HCN9 - A picture of the pump assembly (Taco 007)
https://photos.app.goo.gl/tShWyqcnr6DYtYar9 - A picture of the thermostatic valve assembly

In the valve assembly pictures, the temp sensor on the right and bottom of the assembly are on the supply side and those usually read 170-173 (occasionally when there has been no call for heat in an hour...like on a warmer day like today...I will see it up to about 178). The temp sensor on the left of the pic is on the return side of the loop and usually reads 130-140 (unless there has been no request for heating for a while, then it will creep up to near 160).

 

[maple1]

You're sure that's 1"? The check valve is stamped 3/4".

Not a good pic to tell much from but don't think I've seen a mixer like that before. What is its purpose? Normally don't see one feeding a W-A HX, and boiler protection shouldn't be needed with no storage or low temp emitters.

Thinking you have low flows going on here.

 

[Cliff DeCarlo]

The pellet furnace has a 3/4 and 1 1/4 inch supply/return outlets. It was just easier to hook everything up to the 3/4" connectors. The manual from central boiler talks about the uses for a Taco 009, 0014 and an 007. I went with the 007 because I didn't think I needed that much head pressure (since the stove is only 4 feet above the pex loop). If everybody thinks it's a flow rate issue, I guess I could try a different pump.

These are the quotes from the user manual of the stove:

"The Taco 009 is a medium flow, high head pressure pump that requires an adequate amount of pressure on the outlet side to prevent the motor from overloading."

"The Taco 014 is a high flow, high head pressure pump that requires an adequate amount of head pressure on the inlet side to prevent cavitation. Therefore, a Taco 014 may need to be mounted lower near the base of the furnace, and on the 1-1/4" bung"

"The Taco 007 is a medium to high flow, low head pressure pump. In a very low-resistance system (e.g., short length of supply and return lines, only a flat plate heat exchanger, etc.), the 007 pump may need to be mounted lower near the base of the furnace or on the 1-1/4" bung to prevent cavitation at high water temperatures."

 

[Woodman1]

Those pictures don't help me at all. I can't see how that valve is plumbed. Did you check the pex lines with the ir gun? If you put a piece of black electric tape on the pex and put the gun directly on that you should be able to get consistent readings with the circulator running. If the temperature where the water enters the valve is the same as the temperature where it leaves the boiler your pulling 170ish water.

That would be my guess since it looks like it is pulling from the bottom. I am guessing your sensor that is reading 190 is near the top of the boiler.

As far as low flow I'm running a grundfos 15-58 on low for my main system circulator with no problems at all. It is a lot longer underground run than you, and have water within a degree when it hits my hx in the house. This is coming from the top of my storage over 100' away in the boiler shed. I think a 15-58 and a 007 would be comparable

 

[Cliff DeCarlo]

From what I can tell it seems like the most commonly used pump in pellet boiler installations is the taco 009. While that pump has a high head rating the flow rate is pretty low. The flow rate on the 007 is much higher, but the head rating is much lower. I notice also that the 007 is rated at 1/25 HP and the 009 is rated for 1/8 HP. Would it be better to use the bigger HP and head rated 009 with a lower flow rate?

 

[Cliff DeCarlo]

So, the IR gun seems to be totally unreliable. My temp readings were all over the place and not consistent at all. The highest temp I got was 185 from the flange on the supply side of the pump (with the controller reading 187 at that point). The readings everywhere else ranged from 65 degrees to 140 degrees depending on where I shot the IR beam. I think the only reliable way to tell the actual temps is going to be to install a temp gauge at the boiler right at the supply and return lines there. The pump is mounted at the bottom of the boiler (and I assume it's drawing water from the bottom of the tank as well). From what I can tell from the schematics, the temp sensor in the water tank seems to be right in the middle of the tank.

 

[maple1]

You could do temps with a temp guage that has a probe - probe flush against the pipe with a layer of pipe insulation over it. I have a Maverick wireless BBQ thermometer that works good. Don't necessarily need to break into the piping, surface measuring should do.

Also not seeing how water coming out the bottom could be as hot as the top, but not sure what's going on inside the boiler.

Also still not sure on the purpose of that mixer. Seems not needed, and would be presenting head for the pump. Might be missing something though.

 

[E Yoder]

Those pictures don't help me at all. I can't see how that valve is plumbed. Did you check the pex lines with the ir gun? If you put a piece of black electric tape on the pex and put the gun directly on that you should be able to get consistent readings with the circulator running. If the temperature where the water enters the valve is the same as the temperature where it leaves the boiler your pulling 170ish water.

That would be my guess since it looks like it is pulling from the bottom. I am guessing your sensor that is reading 190 is near the top of the boiler.

As far as low flow I'm running a grundfos 15-58 on low for my main system circulator with no problems at all. It is a lot longer underground run than you, and have water within a degree when it hits my hx in the house. This is coming from the top of my storage over 100' away in the boiler shed. I think a 15-58 and a 007 would be comparable

A 15-58 on high is a higher head pump than a 007, but this loop is short? A 009 seems overkill, an 0014 definitely is. A 008 or a 3 speed 0015 would be a better medium head, medium flow pump (in Taco).
I'm really thinking there is at least some stratifying going on in the water jacket? Wonder if the mixing valve is working properly, I've heard of guys removing then when they go bad. Wouldn't be difficult to bypass it temporarily.

 

[Cliff DeCarlo]

Thanks for the feedback. I know it's totally non-scientific, but if I shut any of the valves on either the supply or return side of the loop it certainly feels and sounds like there is quite a lot of water pressure flowing through the loop (both at the pump and at the thermostatic valve). When you say "mixing valve" do you mean the one way check valve on the return side in the pump pictures? And by stratifying I assume you mean air in the lines? I've tried running the household water through the lines a few times to try and remove any air from the line (never seemed to have any effect). The idea of the variable speed 0015 certainly sounds like an interesting idea. Looking at the specs of that it has 5/10/18 feet of head and 16 GPM max compared with 10 feet of head but 23 GPM max. I was never sure if the higher head rating with lower flow rate was better than a lower head rating with higher flow rate. I'm going to guess my total head is definitely less than 10 feet (although the elbows and one way valve might be making it close to the 10 rating of the 007)? I'll order myself a decent contact thermometer to see if the temps at the boiler outlet is matching the temp reading on the boiler control unit or if they are closer to the temps I'm seeing at the thermostatic valve. Fortunately I think the really cold days are over here in the North-East!

 

[3fordasho]

Thanks for the feedback. I know it's totally non-scientific, but if I shut any of the valves on either the supply or return side of the loop it certainly feels and sounds like there is quite a lot of water pressure flowing through the loop (both at the pump and at the thermostatic valve). When you say "mixing valve" do you mean the one way check valve on the return side in the pump pictures? And by stratifying I assume you mean air in the lines? I've tried running the household water through the lines a few times to try and remove any air from the line (never seemed to have any effect). The idea of the variable speed 0015 certainly sounds like an interesting idea. Looking at the specs of that it has 5/10/18 feet of head and 16 GPM max compared with 10 feet of head but 23 GPM max. I was never sure if the higher head rating with lower flow rate was better than a lower head rating with higher flow rate. I'm going to guess my total head is definitely less than 10 feet (although the elbows and one way valve might be making it close to the 10 rating of the 007)? I'll order myself a decent contact thermometer to see if the temps at the boiler outlet is matching the temp reading on the boiler control unit or if they are closer to the temps I'm seeing at the thermostatic valve. Fortunately I think the really cold days are over here in the North-East!

Stratification means different temperature from the top of the boiler jacket to the bottom. It could be possible that if your supply water is coming off the bottom of the boiler and your temp gauge is up higher, that could explain the difference you are seeing. Hotter water will seek the high spots.

If this is occurring depends on things like boiler design and flow through the boiler (causing temperature mixing) . Most boiler designs pull the hottest water off the top and return at the bottom. Less jacket volume and good flow through jacket will stratify less, more volume and less flow will stratify more- again boiler design plays a big role here.

 

[maple1]

When you say "mixing valve" do you mean the one way check valve on the return side in the pump pictures?

I was talking about the thermostatic valve. What is its purpose in this application?

And is the 007 the only circulator in the whole system? You mentioned boiler is close to house, but you also mentioned 100' of pex.

 

[3fordasho]

You're sure that's 1"? The check valve is stamped 3/4".

Not a good pic to tell much from but don't think I've seen a mixer like that before. What is its purpose? Normally don't see one feeding a W-A HX, and boiler protection shouldn't be needed with no storage or low temp emitters.

Thinking you have low flows going on here.

Thinking you're onto something here, I'm seeing lots of 3/4" elbows, 3/4" valves and fittings, do a pump head calculation on 130ft 1" pex (3/4" ID?) 8-90els, and 10gpm and you have a pump head pushing 30ft. Well outside the 007 pump curve.

Think I'd get rid of that mixing valve too but I don't know the purpose of it.

Correction: 1" pex is .875 id. better but still could be outside a 007 pump curve. Don't know the required flow rate however, if it's 5 gpm then the 007 would be fine.