Caleffi pump issue

[Boardroom]

Thanks Deering. I understand what you are saying and I considered putting a check valve there as shown in a page from the Caleffi Manual attached. Check out the last sentence on the left hand side of the page. I didn't do it because I didn't want to add resistance to that line as the thermosiphoning valve is my only overheat savior. Less resistance means more siphoning. Anyone know if this would actually create enough resistance to worry about?

 

[DaveBP]

It might be useful to think of the thermosiphon potential as being driven by gravity, not the buoyancy of hot water. Gravity PULLS cooler, denser water DOWN. It does not PUSH warmer, less dense water UP.

If the bottom of the storage tank is level or above the bottom of the boiler: the cooler, denser water in the tank might be able settle down into the boiler after the burn is producing water hotter than the top of the storage tank. This might be able to push water from the boiler to the top of the tank even if the top of the tank is warmer than the water coming from the boiler. Depends on the differences in the temperatures (and a busload of other variables).

You would never expect this by using the warm-water-rises perspective. So looking at your system with gravity as the driver (and gravity only pulls down) might let you analyze some puzzling behavior that would otherwise remain a mystery.

 

[Deering]

Anyone know if this would actually create enough resistance to worry about?

Your attachment didn't attach, so I can't see your exact reference. According to the class I took from John Siegenthaler, the guru of biomass hydronics, a horizontal swing check is the right solution for this application. A swing check only closes by gravity, so it takes very little pressure to lift it open. Think of an unlatched door - hardly takes any force to push it open from one direction, but just about impossible from the other.

Swing checks (for this type of application) are simple, inexpensive, and low-maintenance. Sized properly, they have low pressure drop in the system. Make sure to install them horizontally so gravity will close them.

And in a worst case scenario, you still have the boiler's pressure relief valve.

 

[Deering]

If the bottom of the storage tank is level or above the bottom of the boiler: the cooler, denser water in the tank might be able settle down into the boiler after the burn is producing water hotter than the top of the storage tank.

DaveBP has it right here. But what he's describing isn't a problem. You would want that type of thermo-siphoning since it would be extracting more energy from your boiler. It's later where the issue begins.

The boiler will cool faster than the tank because the boiler loses heat faster than the tank. Think of the boiler as being a radiator in this case (an inefficient one, but a radiator nonetheless relative to the well-insulated tank). At some point the water near the top of the boiler will be colder than the water at the equivalent height in the tank. So the boiler water will be more dense (heavier) than the tank water. It wants to sink until it reaches the same level as equivalent temp water in the tank. But the tank water has a column of warmer water above it pushing down, so it doesn't want to be pushed upward.

Once the water in the boiler cools enough, it will become dense enough that it can overcome the downward force of the tank column, and it will push into the tank, causing the warm water at the top of the tank to be forced into the boiler, where it will cool down and become denser, and the cycle continues. As DaveBP says, there are a lot of variables in play, and water is a fluid, not a solid object, so the dynamics become complex very quickly. If the boiler were higher than the tank, the problem would become a lot more acute.

Basically, it's this thermo-siphoning that you want to eliminate, allowing the boiler to become a radiator sucking heat out of your system. A check valve preventing hot water in your tank from flowing back into your boiler prevents that. I doubt that the issue would be very severe in your setup with your tanks being so much taller than your boiler. The only way to tell would be to have enough thermometers in the system to be able to monitor tank upper and lower temps along with your boiler temps. If the temperature at the top of the idle boiler consistently is close to the temp at the top of the idle tank over a long period of time, then you probably have some siphoning going on.

 

[DaveBP]

That's the primary purpose of that flapper valve in the loading units; to prevent backward flow from the top of storage toward the boiler. I would think that if one's system uses a loading unit rather than the simpler (and vastly cheaper!) thermostatic valve/bypass loop method then another check valve might be superfluous.

 

[Boardroom]

Sorry. Here is the missing attachment.
The flapper in the valve prevents flow back through the valve from the bottom of the boiler so the check valve as in the attachment would be redundant if that is the objective. But, can the flow, without the check valve in place, siphon down the bypass loop into the top of the valve and then back through to the tanks or the other way up through the boiler? The bottom of my boiler is a few inches lower than the bottom of my tanks so using BP's theory of gravity, the cool water in the bottom of my boiler would prevent this from happening?

 

[Clarkbug]

I turn my pump on when the boiler reaches 170. I should probably do a better job of heating my tanks all the way to the bottom as they are generally around 110 even when I heat the top to 175. If my boiler is reading 85 when I get up in the morning it takes me about 30 minutes to get to 170.
The end of my burn also creates the same issue. If I turn the pump off when the boiler cools, in order to avoid pushing cool water into storage, the valve again starts to thermosiphon cold water through the boiler and into storage unless I close the red valve mentioned above.

Are you burning every day? I only ask because my setup never gets that cold at the bottom unless I haven't burned for several days.

Your thermosyphon shouldnt be able to pull heat out of your boiler that quickly once you are at full fire. Have you gotten all of your storage up to full temp at this point? Or are you still warming things up?

Sorry for all the questions, but it seems like something strange is going on, not just with the loading valve. Is your bypass damper closing fully? Are you using any kindling when you build a fire?

Since thermosyphon is the only overheat protection you have, I would definitely not use that valve at all. Too much of Mr. Murphy possible.

Your initial thought about the operation of the loading valve and heating a small volume is correct, but I'm not sure if the controls on the econoburn are set to integrate with it. What is the volume of water that your boiler can hold?

 

[DaveBP]

I don't see why the hot water at the top of the tank would be driven toward the bottom of the same tank without a circulator running.
Perhaps a miniscule flow if the pipes and fitting were uninsulated and as the water in those pipes cooled to a lower temperature than all the rest of the plumbing.

In an unheated basement or similar place this might add up to something noticeable.

 

[Deering]

The flapper in the valve prevents flow back through the valve from the bottom of the boiler so the check valve as in the attachment would be redundant if that is the objective.

I'm not sure how effective that flapper valve is against backflow. It appears that its primary purpose is to shut off the unconstrained flow of cold water from the tank to the boiler during pump operation, forcing it to pass through the loading valve instead. If it's effective, then there's probably no need for another check, but at the low flow-rates that this siphoning produces, I'm not convinced that the flapper yields an adequate seal.

an the flow, without the check valve in place, siphon down the bypass loop into the top of the valve and then back through to the tanks or the other way up through the boiler?

A siphon within the boiler side of the system could be set up, flowing through the circulator and loading valve back to the bottom of the boiler. But if the pipe runs are short (as in your case) and insulated, it will be pretty insignificant. Besides, you're more worried about siphoning your tanks than the idle boiler. If the loading valve in the Caleffi is below the set temp (130 or 140), then the loading valve will be closed and there won't be a direct siphon path, except possibly through the flapper valve if it doesn't seal. Again, with your setup that doesn't seem likely.

The bottom of my boiler is a few inches lower than the bottom of my tanks so using BP's theory of gravity, the cool water in the bottom of my boiler would prevent this from happening?

Not really. It's the relative heights between the top of your boiler and the top of the tank that will be the main factors in this.

 

[Deering]

Since thermosyphon is the only overheat protection you have

I trust that there's also a pressure relief valve on the boiler. If not, then you have bigger worries than siphoning. That would be the first order of business to address immediately.

But I think your concern is to provide a path for hot water to naturally siphon to the tank if, for instance, there was a power outage during fire and the circulator between the boiler and the tanks went down. You'd still have fuel burning and producing heat. Then you'd want to provide somewhere for that heat to go w/o relying on the pressure relief valve to pop. I don't think a swing check is going to prevent that natural flow from occurring. The swing flapper will open long before you build up enough pressure to trigger your relief valve.

 

[DaveBP]

Referring back to your original post I have a few questions:

Since you say you just filled your storage tanks are you sure that some kind of crud didn't flush down into the loading unit as soon as it started circulating?
Is the water still in the bottom of your tank the original fill water? If so, what temperature is it now?
Did you try pulling the 3 thermometers out of that loading unit and holding them in a sauce pan full of boiling water to see if they all read about 212F (or 100C)?
Do you have the cast brass model? I've never been able see how those 3 thermometers in the same casting could separate the temperatures of the 2 or 3 different flows without effecting each other. To tell the truth, I've never even looked at mine. Have no idea what they show. Maybe next fire?

 

[Clarkbug]

I trust that there's also a pressure relief valve on the boiler. If not, then you have bigger worries than siphoning. That would be the first order of business to address immediately.

But I think your concern is to provide a path for hot water to naturally siphon to the tank if, for instance, there was a power outage during fire and the circulator between the boiler and the tanks went down. You'd still have fuel burning and producing heat. Then you'd want to provide somewhere for that heat to go w/o relying on the pressure relief valve to pop. I don't think a swing check is going to prevent that natural flow from occurring. The swing flapper will open long before you build up enough pressure to trigger your relief valve.

Agree, the swing check wont get in the way of the thermosyphon. I was referring to the manual ball valve that he had referenced he could close to help increase the boiler temps.

 

[Deering]

I was referring to the manual ball valve that he had referenced he could close to help increase the boiler temps.

Ah. Got it. Yeah, manually closing a valve between an active boiler and its load sounds like a bad recipe. I can think of a number of distractions, both good and bad, that would cause me to forget to open that valve... Better to control the circulator based on boiler temp. That's not too difficult to set up.

 

[Boardroom]

Good morning guys. Thanks for all the input. I didn't get a chance to reply as I was out to a hockey game last night. That's what we do up here. I will try to address the questions that have come up.
1. There doesn't seem to be any crud in the valve. I took it all apart yesterday. I refurbished a bunch of old CI rads for my install and there was lots of rust, dirt, discoloration in the system so I spent all day yesterday emptying it all and refilling it so I am off to a fresh start.
2. Yes. There is a pressure release valve on the boiler and another in the house on the electric boiler which runs on the same loop. Both are set at 30.
3. 42 gallons of water in my boiler.
4. Just so there is no confusion over how my valve works, I have attached a description. My valve is the one on the second page. The flow works the same as on page one. It is just that my protection valve and pump are all one unit. Econoburn controls are not set up to work in conjunction with it. My plan was to run it continuously while the boiler is fired. Turn it on manually before I light the fire. Turn it off manually when I am done burning. The valve takes care of all the flow so my understanding was that I did not need the boiler to control it. The valve is wired separate from the boiler. The EB's pump controls are set up for a two pump system. One pump to run the boiler loop and another to run the primary loop to storage. Boiler loop terminals are energized at boiler temps less than 150. At 150 the boiler loop shuts off and the primary pump terminals are energized. I suppose I could use the primary pump terminals to energize my pump instead of doing it manually. Would make it more convenient for sure but the only heat advantage is that I don't run the boiler loop until things are up to temp. Is that a concern given that the boiler loop is so short?
5.My tanks never really got a chance to warm on the bottom. Today I am seeing if I can heat the storage all the way through.
6.My thermosiphon concern is when the pump is off. When it has no power it is designed to allow siphon from the bottom of the tanks up through the boiler. Come to think of it, I probably cannot allow the EB controls to start the pump because as the boiler is warming, flow will be moving through it if the pump is off. I am probably better off with it running controlling the temp flow back to the boiler.
7. The check valve in the top line is probably a good idea but will need to wait until spring.
8. We have determined that the temp gauges on the valve are not all accurate. They threw me for a loop initially but since then I have mounted sensors on the pipes to and from and the valve seems to be working fine.
9. I will be a good boy and leave the manual valve open.

 

[maple1]

Since you have your Caleffi wired & switched completely separate - do you have anything at all hooked to your 'primary' & 'boiler loop' control pump contacts?

I googled up a manual for your boiler. I think I would wire the loading unit to the 'primary' control power - which according to the manual should turn the pump on at 150? Again, no EB experience here - but as far as I know every boiler should come with controls that turn a pump off & on at a given boiler temp, and that looks like it to me for the EB. With the Caleffi, you shouldn't need to use or worry about the 'near boiler' pump stuff. You could also add in a flue temp stat if you wanted to use flue temps to do it. You should also be able to utilize a flue gas stat to do end of burn fan & pump shut down.

 

[Boardroom]

Nope. Both terminals are open.
The problem I see with having no power to the pump/valve until a certain temp, say 150, is that the valve will be allowing thermosiphon from the tanks up through the boiler. Depending on bottom of tank temps it could take longer to heat the boiler this way than having the valve control the boiler loop return temps.

 

[DaveBP]

I wouldn't look too closely at what your system is doing (as long as it's safe and heated water is going to your tank) until you have that last bit of original cold water through the boiler once. When you refer to "cold" water return the difference between 10C and 35C is huge. And it will take a long time to get it up to that point. Until your return water temperature is closer to normal the loading unit won't act as you hoped.

That was my experience with my loading unit and 1000 gal. tank.

 

[ewdudley]

Nope. Both terminals are open.
The problem I see with having no power to the pump/valve until a certain temp, say 150, is that the valve will be allowing thermosiphon from the tanks up through the boiler. Depending on bottom of tank temps it could take longer to heat the boiler this way than having the valve control the boiler loop return temps.

Exactly. With a loading unit that has a backflow-preventer/fail-safe bypass valve the pump needs to run any time the boiler is 'active' regardless of 'launch temperature', up until end-of-burn shutdown. Also IMHO there should be a mechanical aquastat on the boiler supply pipe that disables the draft fan and enables the boiler circulator whenever there is a supply over-temperature condition.

 

[maple1]

Have you started a burn with the pump not running and observed your boiler top temps to see how fast they rise? I am thinking your boiler should be able to generate way more heat than thermosiphon would be able to carry away, especially since the thermosiphon won't really get going until the boiler gets a decent temp gradient going - but I could be wrong on that. My boiler, due to the natural draft, is a bit slower to get the burn up to speed than those with fans (I think), and I still am able to get up to pump start temps in 20 minutes or less starting with a room temperature boiler. I think you will see a big improvement once you get storage throughly heated. Mine gets no cooler than 100 at the bottom in heating season - usually in the 120 range. Which is only 20 less than the magic 140 figure.

 

[Boardroom]

Thanks EB - I do have an aquastat on the boiler supply line that shuts off the draft fan, I think at around 200. The pump is running continuously on a seperate circuit so no problem there.

 

[Boardroom]

What I don't have is an overheat zone on a separate circulator. There are terminals on the EB that would energize when the supply line aquastat shows overheat. I just have not used them. As long as I have capacity in storage that should be able to absorb the extra heat. What would be nice is to find a way to simply have my main pump notch up a speed when an overheat occurs.

 

[Boardroom]

I hope to get storage charged up today so I'll let you know how it goes.

 

[DaveBP]

My older forced-draft Tarm Solo has basic relay logic that turns on the circulator as soon as the system is reset to fire the next load after a normal shut-down. Basically, the circulator is always ON until after a burn when the exhaust gas goes below a temperature I've forgotten (but works to shut off the draft fan when the last coals are almost completely finished). This seems to me to be designed for use with a loading unit with that flapper valve.

With a tank still supplying well-temperature water through that thermosiphon opening it could take a long time indeed to get the boiler up to operating temperature if the circulator weren't running.

 

[Deering]

Boardroom, do you have your tanks plumbed in parallel or series? By series I mean the boiler goes into the top of one tank, and that tank's bottom connection goes to the top of the next, whose bottom goes back to the boiler. I think a series configuration might be more resistant to siphoning, but I'll have to think about that a bit more. In any event it will allow you to get more heat into the tanks when you do burn.

 

[Clarkbug]

I hope to get storage charged up today so I'll let you know how it goes.

I think this could make a big difference. Startup is always a little funky for me, so please do let us know if having "warm" storage makes things handle a little more predictably.