Variable delta-t pump as condensation protection?

gardenweasel

I have a converted gravity system that wasn't designed with any condensation protection on the CI boiler. The boiler seems to be doing OK at 25 years old (it's a 1993) but when I bought the house the chimney liner was swiss cheese and had to be replaced. I don't know how old it was at that point but I suspect the boiler was eating it with condensation, and will do the same to my new liner eventually.

I was looking at the Taco variable speed Delta-T pump as an easy retrofit that might help with this. Am I on the right track here? It seems to me like slowing down the flow through the boiler would help with flue temps - or would this be wasted money? The circ on it now is a Taco 007 that I think is also from 1993.

At this point, unless it's really cold out, my return temps don't hit 140 before the thermostat stops calling for heat and everything turns off.

hot_rod

A single pump system, from boiler to radiators, pump in set point mode?

gardenweasel

Yes, sorry - no zones, not piped p/s, just as basic as it gets right now. The pump I'm looking at has 2 sensors and you can set what you want the delta-t to be between the in/out.

Gilmorrie

What does your boiler temperature drop down to during a long heat call?

I think that Taco delta-t pump could be set up to prevent condensation in the flue, although I'm not sure that is what it was designed for. There may be simpler, less expensive alternatives to the variable speed, delta-t, pump. One possibility is to add a boiler by-pass line, with a manual control globe valve - set it and forget it. There are also aquastats that, besides controlling the burner, can be wired to inhibit your pump running if the boiler temp is below a setpoint, say 140 deg.

But, I do agree that you need to prevent the condensation in the flue liner.

gardenweasel

By the end of a long heat call the boiler temp is above 150 but it's taken an hour to get there and the thermostat has usually stopped calling. It just takes so long to heat up the amounts of water and cast iron I have.

The reason I was looking at the VS pump instead of installing a bypass is that it's so easy to do - a simple bolt-in job rather than sweating pipes (which I can do, but it takes me easily 4x longer than you pros.) Plus with my circ being 25 years old I don't feel too wasteful replacing it.

STEVEusaPA

Why not just an aquastat that holds off the circulator until you get to 140°? It will cycle frequently after a deep setback.
But you're better off with 3-way or 4-way mixing valve. And if your gas, I'd still use a stainless steel liner.

Dave H_2

The VT2218 is a bigger circ than the 007 that you already have. However than may or may not change the way the system heats up. It sounds like your radiation and boiler are closely matched and you have a large amount of water in the system.

You could use the Delta T circ here but I would use it in the Delta T mode for the results you are looking for.

You could just use one of the sensors and put it on the boiler return and program the circ for Boiler Protection mode and it would speed up and slow down to make sure that the return temp doesn't drop below 140F.
So if the temp starts to get at or near the 140 mark, it will start to speed up.

You could also use it in the delta T mode and maybe shorten the temp drop so it stays up to the higher flow rates and keeps the boiler from condensing.
These are just guesses without seeing your system, you know it better that anyone at this point

Dave H.

gardenweasel

Thanks! I'm actually looking at the variable speed version of the 007 rather than the Viridians, if that makes any difference: https://www.supplyhouse.com/Taco-007-VDTF5-007-Variable-Speed-Delta-T-Cast-Iron-Circulator-Pump-115V

hot_rod

Gilmorrie said:

What does your boiler temperature drop down to during a long heat call?

I think that Taco delta-t pump could be set up to prevent condensation in the flue, although I'm not sure that is what it was designed for. There may be simpler, less expensive alternatives to the variable speed, delta-t, pump. One possibility is to add a boiler by-pass line, with a manual control globe valve - set it and forget it. There are also aquastats that, besides controlling the burner, can be wired to inhibit your pump running if the boiler temp is below a setpoint, say 140 deg.

But, I do agree that you need to prevent the condensation in the flue liner.

What if the boiler is not hot? It starts from room temperature, as most do on a heat call. No way to mix 65F return with 65F boiler water to get to 130F return protection.

Unless the boiler is grossly oversized it will not get ahead of that load, or low return. A bypass valve, or pump, even VS will not change the thermodynamic.

You really need a thermal clutch to disconnect then load from the distribution to have absolute protection.

Snowmelt

I would put primary secondary or a bigger pump to get the hot water back to the boiler a little quicker , like a double 009

gardenweasel

hot rod_7 said:

Gilmorrie said:

You really need a thermal clutch to disconnect then load from the distribution to have absolute protection.

I assume you mean convert the piping to primary/secondary? It's definitely something I'm thinking about - I bought Dan's book on p/s and I've had Classic Hydronics for years. I was hoping for a shortcut but it's looking like there isn't really one.

hot_rod

I guess it comes down to how close to perfect or ideal do you want to get?
Many systems "get away with" improper piping and may or may not reward the owner, with expensive breakdowns and repairs in due time. 25 years is a good run, I'd suspect the boiler is oversize and doesn't condense for long periods?

Your reward was a chimney liner replacement.

But there are known ways to properly pipe a system to get all the benefits and protection required for comfort, efficiency, and long life. Some use and believe them, others not.

Plenty of installers will argue pumping away is a scam

All boilers condense on start up, determining how often and for how long they are in that mode is the key. That will have an effect on the boiler and flue life and operating conditions.

Easy to incorporate on the install, sometimes a hassle to add on.

Gordy

On a cold start boiler you are going to condense until system temps get the return water up in the 140 range. If the heat call stops, and boiler is above 140 the condensate will burn off.

If the boiler is hot start I could see some benefit.

I fail to grasp how a delta t circ will prevent this. Slowing the flow which equates to a larger delta will allow the boiler to reach higher temps faster, but also slows how fast that hotter water returns back to the boiler to get above dew point.

Higher flow, narrower delta takes longer for the boiler to get up past the dew point. Depending on how closely matched the boiler is to the load.

Seems like 6 of one 1/2 dozen of the other to me.plus a new circ that does little to fix the issue. With a cold start boiler.

EBEBRATT-Ed

I don't think a delta T pump is the answer in fact changing the flow to the radiation may alter the distribution of heat especially with oversized gravaty piping.

Installing an aquastat to hold the pump off with a low limit will not work you will just keep cycling the pump and burn a ton of fuel.

If it was me I would primary secondary with an injection loop between the primary and secondary with a 3 way valve with the sensor on the boiler return.

This would keep the boiler return at 130 or above, provide full pumping on the secondary so it wouldnt affect the heat distirbution.

Probably the most expensive fix but it will work.

But since the boiler has survived 25 years I would wait unti the boiler quits. If gas it would be a nice mod con job

rbeck

The advantage of using a variable speed pump on a boiler is the slower flow through the boiler and keeping the system flow higher. We tend to forget the water temperature is only one part of the equation. The second is water volume. If we slow the flow through the boiler we will be adding a low volume of cool water than a higher flow rate. Therefore we can return cooler water temperature and not condense in the boiler.
The interesting part of boiler bypass is this;
Let's assume we have a boiler delta-T of 20f and the return temperature is 110f. Now add a boiler bypass set to a 40f boiler delta-T with a return temp of 110f. The system temperature leaving the blend tee on the boiler bypass will be the same system temperature as what was leaving the boiler supply on a 20F boiler delta-T. This happens as we have a higher flow rate through the boiler with a 20F, slow down the boiler flow and add in a boiler bypass. We now reduce the boiler flow by 1/2 (difference of 20f and 40f delta-T). By reducing the boiler flow by 1/2 we now have a 40f instead of a 20f. We add 1/2 of the flow of 40f hotter water (110f + 40F=150F) to 1/2 the flow of 110f means the supply water temperature is the same as it was at full flow through the boiler. The difference is no condensation. To quote Gil Carlson from a white paper he once wrote, "condensation is a result of extremely cold water entering the boiler or cool water at a high flow rate". Keep the cool water at a lower flow rate to blend with the boiler higher water volume, change the ration of cold to hot, less chance of condensing.
I do like the idea of a delta-t circulator for the boiler piping if piped p/s. P/S will decouple the flow for the boiler from the system flow so there is no problem with different flows.

Gordy

You can do the same thing with a valved boiler bypass set it, and forget it. Once a house is steady state the return temps are near the same at the start of each heating cycle. Unless a deep setback regime is used.

Gordy

My old house had ceiling radiant heat from the 50's. Single circ one zone system. With a simple boiler bypass. That boiler saw return temps no higher than 90-95 degrees most times much less than that. Cold start boiler. The boiler is 25 years old no condensation damage.

Why complicate things.

hot_rod

With CI rads or radiant isn't it more likely to see a 70°F return? Add the 40F and you have 110°F return?

As a ∆T boiler pump P/S will hydraulically decouple, but I don't see a thermal component or the ability of some VS circs to slow down low enough?

Move the VS temperature sensing circ to the secondary and now you have a chance. You now have thermal decouple.

Gordy

agreed @hot rod_7 . My argument is VS pumping in a bypass set up won't work, at least not reliably . You need to use p/s.

hot_rod

Primary secondary with a buffer tank as the "closely spaced tees" still not a true thermal decoupling here.

I believe the boiler bypass or bypass pump was originally suggested by the boiler manufacturers as a prevention for cold shocking, specifically aimed at gravity conversions with high mass, high water volume. It did offer some blending of the return to the boiler, however un-predictable of a blend.

It was not intended as, nor can it provide a return protection mechanism to address extended condensation mode of operation.

A subtle difference in piping make VS pumping a reliable, predictable return protection method.

Gordy

Yes.

Now what makes VS pumping for boiler protection more favorable than a simple ESBE valve?

Let’s assume that the P/S piping is non existent on a ci boiler that needs the added protection, and there is no need for hydraulically decoupling the boiler loop from the system loop. Only a system pump is needed.

Because I’ve seen two posts now where this situation has been posted.

Why put an active soulution to a problem using electricity when a passive solution can be used, and not use electricity?

hot_rod

Exactly, what Gordy said. And now you have a single pump solution, as long as the boiler is a fire tube or cast, any low pressure drop boiler, or you size the pump accordingly.

It is the most common way they protect solid fueled boilers, usually sold as standard equipment. It is a nice, smooth way to "slip the clutch"

SuperJ

The downside to a single pump solution is that since the flow to the building is reduced to protect the boiler, you sacrifice balanced heating. It is better to protect the boiler, and have unbalanced heating, but I don't really think single pump/thermostatic bypass solutions are a good solution for new systems.
More of a Band-Aid to rescue a tormented boiler.
Those caleffi valves look sweet with all the temp gauges.

Gordy

I disagree. It’s been sucessful for years,decades.

Either way the boiler can only get so many BTus out to the system until return becomes an exceptable temp.

Gordy

Whether you slow flow at a variable rate, or bypass water at a fixed rate the outcome is the same.

SuperJ

Gordy said:

Whether you slow flow at a variable rate, or bypass water at a fixed rate the outcome is the same.

What I'm suggesting is pumping your secondary loop in such a way that flow is not compromised when you're "slipping the clutch". Even if your BTUs are limited (due to warming up your primary return) you want to evenly distribute the few BTU's that you have available, not just dump them in the closest zone that is getting all the gimped flow. This only an issue on higher mass systems. With baseboard heat, your return would come up to temp pretty quick and the bypass would allow building flow, but with a bunch of high volume, heavy cast iron rads, your return will take a long time.

Depending on the building I suppose it may not be necessary or noticeable, but some cases your farthest out zones which are often your coldest don't get any flow initially (if there are parallel flow paths to other zones).

Too be clear I'm not saying don't do a thermostatic bypass. I'm just trying to point out an sometimes unanticipated side affect. The alternative is not to remove the bypass, it's to add a more elegant pumping arrangement.

If the boiler is server a single load, I suppose who cares.

Here is an example, he's noted substantially slower, uneven, or missing heat on some of his rads after adding the valve.:
https://forum.heatinghelp.com/discussion/166259/boiler-return-temperature-too-low/p2

hot_rod

SJ I think you are missing the concept a bit. The thermostatic allows just the exact amount of bypass and boiler flow to balance the two requirements, return temperature protect and system flow. Flow is going one or both way all the time. There is no restriction to either flow path, as one port closes down the opposite opens the same amount. It is a very smooth slipping of the clutch.

It operates on the same principle of a TRV which are widely popular in Euro and other hydronic friendly markets, millions and millions of TRVs are manufactured and sold every year, KISS solutions to temperature control.

If the flow or temperature is not adequate to cover any building loads, why send it out?

A small amount of parasitic loss, so buy a high Cv valve, a 14 Cv gives you 14 gpm with a 1 psi drop, barely noticeable to the boiler pump.

You can still pipe to a buffer, low loss header, hydro sep or P/S if you need or want to blend circulators and need hydraulic separation. Or want to mix temperatures or a different operating ∆ on the distribution.

SuperJ

hot rod_7 said:

You can still pipe to a buffer, low loss header, hydro sep or P/S if you need or want to blend circulators and need hydraulic separation. Or want to mix temperatures or a different operating ∆ on the distribution.

Hydraulic separation is what I was advocating for (maybe not clearly enough).

Thanks for your explanation. I think I'm talking about a fringe problem that is basically muddying the waters.

I work commercially where maintaining balance over long distances is critical (thousands of feet of pipe installed, with many AHUs, and hundreds of zones). It's critically the AHU's with outdoor dampers get a constant supply of heat or you will start tripping freezestats or have to shut off minimum fresh air. So my perspective is a little skewed.

With cast iron boilers we always did a primary secondary loop/pumps with a temperature control 3 way valve on the secondary loop. We wouldn't start heating the secondary loop until the boiler came up to temp, but the secondary loop pumps would maintain a constant differential pressure (with VFDs) regardless of the temperature. Now that large (multiple million BTU) condensing boilers are the norm, simpler variable flow primary only systems are the norm.

With a single pump and thermostatic bypass, boiler gets constant flow, but the the bypass flow goes up at the expense of building flow. That's the the point I was trying to make, I won't say anymore since I think it is a fringe problem that only affects some high mass systems.

I love TRV's I have a bunch of them on various panel rads in my house, they work awesome.

gardenweasel

Thanks everyone for your comments - I am learning. I'll probably do a simple bypass loop since it's not a ton of work or money to accomplish, then go p/s when I need a new boiler. I've had no issue with heat getting to any part of my house so far, probably because the system is just so dead simple - all the radiators get hot at the same rate. There are no zones and the pipes are big so the water moves fast. I just want to get the flue temps up a bit because this morning I checked the cleanout plug of the chimney liner and there was water pooled in it which I can only imagine is condensation.

hot_rod

gardenweasel said:

Thanks everyone for your comments - I am learning. I'll probably do a simple bypass loop since it's not a ton of work or money to accomplish, then go p/s when I need a new boiler. I've had no issue with heat getting to any part of my house so far, probably because the system is just so dead simple - all the radiators get hot at the same rate. There are no zones and the pipes are big so the water moves fast. I just want to get the flue temps up a bit because this morning I checked the cleanout plug of the chimney liner and there was water pooled in it which I can only imagine is condensation.

You have the info and seem willing to keep an eye on how your system behaves. Make correction if or when needed. Every system develops its own habits, as do the owner/ operators.

Solid_Fuel_Man

I think we are forgetting that this was a gravity system. An ESBE valve, and one circulator. Then the supply and return can be T'd in and let the original gravity system still be gravity.

Slips the thermal clutch, and uses only one fixed speed circulator.

I'm still a fan of mod/con, then yo can leverage that cold return and not worry about a chimney liner!

ch4man

oh for gods sake, all the pumps and bypass and piping wont help at all with flue gas condensation in the chimney.

whats the flue gas temperature? im sure its too low due to the boiler being underfired.

circs and piping changes will keep the boiler chambers hot enough to prevent condensation in the boiler sections. but will do nothing to stop chimney condensation.

get someone whos trained in combustion analysis to dial in the proper input to safely raise the flues gas temperature

Gordy

Only a properly installed, and insulated liner of the right material will help that. Unless you are using a boat anchor of a boiler with high stack temps. That all depends on chimney height, and chimney temp between firings.

ch4man

underfire any CI boiler even with the best chimney installed and it will rust out the liner. you need the higher temps to create enough draft to get the flue products out of the chimney while still hot enough to prevent condensation.

now any hotter then its a waste of fuel

ch4man

the OP was about a non condensing boiler (CI) not a CATII ( or is that CATIII?)

Gordy

Who’s under firing the boiler I don’t see that anywhere?

If the liner is stainless, or aluminum how does it rust?

Insulating the liner is the better option along with proper stack temps determined by a combustion analyzer. Do that, and that’s it.

Gordy

I’m assuming by the Swiss cheese comment it was aluminum, or?

ch4man

Gordy said:

On a cold start boiler you are going to condense until system temps get the return water up in the 140 range. If the heat call stops, and boiler is above 140 the condensate will burn off.

If the boiler is hot start I could see some benefit.

I fail to grasp how a delta t circ will prevent this. Slowing the flow which equates to a larger delta will allow the boiler to reach higher temps faster, but also slows how fast that hotter water returns back to the boiler to get above dew point.

Higher flow, narrower delta takes longer for the boiler to get up past the dew point. Depending on how closely matched the boiler is to the load.

Seems like 6 of one 1/2 dozen of the other to me.plus a new circ that does little to fix the issue. With a cold start boiler.

see we agree......

my point s that all 30 some odd posts about pumps and flow will have little to do with what the thread was started about, but concentrating on maintaining flue gas temps will

Gordy

Well actually the thread is about a solution to that in which using a delta t pump may elevate the issue.

I maintain that it won’t, and is not a strap on solution with out some proper piping modifications to do so, and an extra pump.

I also maintain that there are cheaper, and passive solutions to that in which extra electricity is not needed.

However you would certainly agree which ever method used when properly executed allows the boiler to reach a higher stack temp faster past fuel dew point will most certainly benefit things farther up the chimney no?

The system is a converted gravity with high mass there for it takes some btus to get things rolling. The closer matched in size to the load the boiler is the harder it is for it to get over the dew point hump.

If it’s condensing at the boiler it’s condensing up the flue. The only saving grace is getting the boiler past the dew point as quickly as possible, and keeping there for the complete cycle.