firetube modcons piped full flow with zoning

Eastman

So firetube modcons have been on the market for awhile now. At least a few manufacturer's provide full flow (no primary/secondary) reference installation diagrams in their install manuals for systems that are zoned with valves or pumps. Are there people out there installing firebubes in such a manner? How are they holding up to zoned variable flow applications without hydraulic separation?

hot_rod

I think the manufacturers are clear on the minimum flow required on those direct piped applications. I think Lochinvar has charts showing the "available" pump when you have a model with their onboard circulator. I'm not sure the risk is worth it?

I wonder that it will take some run time maybe a few years before problems related to low flow in the FT heat exchangers start to show up, just like the water tube designs.

Water quality, hard water scaling will certainly be part of the HX life, or death cycle as the tube scales the metals will be under more thermal stress. Stainless steel HXers are pretty thin gauge, doesn't take much thermal stress to destroy one.

Eastman

Hi hot rod.
Yes, the I+O manuals have clear minimum and maximum flow rates. I am not concerned so much about staying within those requirements. What is not clear to me is: what effect does variable flow within those specified parameters cause? It seems like burner modulation control would significantly lag the flow rate reduction caused by a zone shutting down.

Regarding water quality: do you think water quality is even more important at minimum flow rates?

hot_rod

One of the keys to copper tube boiler life expectancy was flow velocity. Keeping velocity high certainly helps keep the surfaces from getting thick scale build up. I would apply the same to modern mod con designs.

Back in the day the boiler tech support answer to a failed copper tube boiler was "it's not our fault, and you need a bigger pump" That applied to DHW boilers and heating copper tube.

I'd worry most about high operating temperatures combined with low flow, low velocity. Low flowoften equates to less or lower turbulence, which is the key to good heat exchange in a pipe, boiler, heat emitter, etc.

Minerals come out of solution as temperatures increase, looking at the dynamics at play. That high flame temperature against a thin gauge, with an insulative mineral build up = $$ disaster in the making. See attached. Probably not a warrantied failure.

I see Lochinvar offering ∆T circulators, on some models, I'm not sure what the logic is behind that as a boiler circulator? Seems modulating the burner and pump together would be a better approach? If, as you mentioned the modulation of the two got along together, no lags.

flat_twin

That cut away firetube pic is U G L Y ! Wonder how that system's water tested? I have a fire tube hx piped direct but on a single zone system. WM says no direct piping with multi zones controlled by circulators. A bypass pressure regulator is required for a direct system with zone valves.

Eastman

@hot rod_7 I think those loch pumps are actually being controlled by the boiler directly in harmony with the rest of the control system.

@flat_twin I wonder if the bypass would still be needed if a delta P pump was used instead.

flat_twin

"I wonder if the bypass would still be needed if a delta P pump was used instead."

While that might work, the word "MUST" in the I/O manual probably means your voiding the warranty by doing so.

hot_rod

flat_twin said:

That cut away firetube pic is U G L Y ! Wonder how that system's water tested? I have a fire tube hx piped direct but on a single zone system. WM says no direct piping with multi zones controlled by circulators. A bypass pressure regulator is required for a direct system with zone valves.

That one wasn't all that scaled up, but enough layer to cause the metal and welds to fail.

This one had more build up, public water, no make up water connected, as I recall. So the minerals can be put into the system with the original fill water.

hot_rod

Eastman said:

@hot rod_7 I think those loch pumps are actually being controlled by the boiler directly in harmony with the rest of the control system.

That would make more sense that the boiler keeps the ∆ within acceptable range by varying the pump speed, not locked in to one ∆?

@flat_twin I wonder if the bypass would still be needed if a delta P pump was used instead.

Eastman

That last one makes me think watertube all the way.

flat_twin

To me it reinforces the need for good fill water kept within spec.

Eastman

Yeah, but I was hoping firetubes would require less maintenance. Seems like this style might be more prone to scale, as the velocity would typically be less.

Gordy

I think piping direct in low head systems is a great advantage. Especially old gravity systems.

Loch gives minimum flow rates at low fire down to 1gpm, and high fire. Others are a bit more cryptic claiming minimum of 1 gpm.

1gpm going through an 80k boiler 10:1 tdr is 8 k heating 1 gallon of water every minute. I don’t see this as crazy low flow. Especially if the hxers water content is above that.

I also see that minimum flow rate as an advantage in zoning with small loads. Especially radiant. So long as the modulation, and flow rate play well together down in the weeds.


Water quality has always been the monkey on the back of HX wes, and systems in general.

Gordy

Seems to me the stick in the mud is coupling the flow, and modulation communication break down.

Eastman

@Gordy
I feel like the super low flows are too risky. Can a Loch down modulate quickly enough to handle zones closing? It seems like full fire must be assumed for flow rates. (Or whatever max fire is capped to, at least.)

I like how U.S. boiler's Sage zone controller let's you set max modulation rates on a per zone basis. It seems like that would give the boiler instant direction as to what to do.

Gordy

Hmmm it would be nice to know how the low flow was laboratory tested. I suppose risk has been factored in for market, so long as the I/O manual has been followed no? If not, and risky why bother boasting the ability? Risk is a matter of willingness. Seems others are willing also.

I do like the control of directing fire rate per zone requirement, but that still leaves a wide range depending on the load for that zone. 50 or -10 OAT is a wide range for directing fire rate.

Gordy

What I’m saying is let the manufacturer live up to their products advertised abilities. If the installer plays it safe. As in above the exceptable parameters out lined in the manual then how will field testing ever produce any exceptable data, to either improve the product, or bring in the parameters to exceptable of same product.

Eastman

It still uses ODR for the target temperature, but the max modulation cap is dependent on the sum of individual calling zone firing rate limits.

Henry

We have had a number of firetube modcon failures near the top. It seems the expansion of the tubes create the failure. I am talking of commercial boilers. We have not had any failures on HTP EFT or UFT boilers. Depending on the application, we now still prefer water tube modcon as we size the piping not to connection size but to flow for a 25DT with the proper pump.

Gordy

True. I keep going back to mass, and boiler water content. What’s a gas cook top burner 8k? Throw a pot of water on with the volume content of the selected firetube boiler of choice at room temp.

Now try to heat it at 8k minimum fire for an 80k at 10:1 tdr with removing, and replenishing 1 gallon of water every minute. The replacement water is of increasing temperature as you go.

I know the HX wall is thinner the transfer is a bit more intense, but I just don’t think it’s that terrible on the HX as is the water quality..

Gordy

Observe water content. Where I watch is the content is the same for two sizes, and then steps up to next level same two sizes, and so on. Maybe safer with smaller size yet same content as the next biggest size up.

Eastman

@Henry

Henry said:

...we now still prefer water tube modcon as we size the piping not to connection size but to flow for a 25DT with the proper pump.

For only commercial your preference is watertube? Or residential, too?

Henry

I prefer HTP EFT for residential. It has the Polish heat exchanger. The UFT has

Eastman

What kind of delta T do you size for residential? Still 25?

GroundUp

I ran into this yesterday. Customer supplied Laars Mascot FT smashed into a closet with no room for P/S piping. 8 loop radiant floor in a single zone, should be ~5 GPM at all times. Same Kiturami exchanger as the UFT which allows direct piping, very similar elsewhere, I don't see why piping direct would hurt the boiler with a flow rate like that

Eastman

@GroudUP
Single zone systems are not a concern as long as the flow is within spec.

I'm more concerned about possible modulation control issues during flow rate changes as zone valves close.

Gordy

^to me that falls into the design side.

Know the possible equipment selection, and it’s capability. Design zoning around that. 8k is as low as you can go presently with the 10:1 tdr in the smallest boiler. 8k will surely require the minimum 1 gpm. So a single zone should not be less than that. Or there is the buffer tank as a work around.

Eastman

@Gordy
Hypothetically let's say one has 3 zones that run at 5 gpm each. So every combination of flow is: 5, 10, and 15gpm. Let's assume these are all within spec for the boiler. Let's also assume that each individual zone's current btu demand exceeds boiler minimum modulation. What happens when zones shut down and flow drops from 15 to 5gpm? Demand is still above minimum modulation rates, but will modulation actually be able to ramp down quickly enough? Or will the boiler shutdown on high side differential? How should I design accordingly to prevent inadvertent short cycling due to variable water flow?

Solid_Fuel_Man

I've installed a few FT boilers. Just put in my first NTI trinity 110. Same AIC brand (Poland) hx as the Weil-mclain's I've been using. The W-M shows the air purge and ex tank on the supply and pumping away as conventional. The NTI shows ex tank on return and pumping away and into the return. This is the same hx mind you. Also WM states different allowable fill water and required the use of Sentenal X100, I've been dosing the X100 based on system volume, as the kit that comes with the boiler only treats 25 gallons.

Its interesting how two different manufacturers using the same outsourced hardware (including the burner in both boilers) allow and suggest different piping etc. Risk management?

NY_Rob

Eastman said:

@Gordy
Hypothetically let's say one has 3 zones that run at 5 gpm each. So every combination of flow is: 5, 10, and 15gpm. Let's assume these are all within spec for the boiler. Let's also assume that each individual zone's current btu demand exceeds boiler minimum modulation. What happens when zones shut down and flow drops from 15 to 5gpm? Demand is still above minimum modulation rates, but will modulation actually be able to ramp down quickly enough? Or will the boiler shutdown on high side differential? How should I design accordingly to prevent inadvertent short cycling due to variable water flow?

The Mod-Con has SWT and RWT temp sensors, since zones don't generally slam shut with high DT's due to the fact the the DT narrows more and more when the call for heat is almost satisfied- the boiler will be tracking the steadily decreasing delta between SWT and RWT. If it was on anything but the lowest fire rate it will already be modulating down to prevent setpoint overshoot. If the control program is written properly, when one of the zones closes it will sense quickly rising SWT and rapidly modulate down and even allow modest overshoot for a few min to prevent unnecessary shutdown/restart cycles.

HTP had a bug on one revision of the UFT-80W control boards that would activate Boost even it it was disabled in the installer menu.
They fixed that bug in the next revision, but didn't address the continuing issue where the reaction time was too slow when a high mass zone (like cast iron baseboard) would close leaving low mas fin-tube zones unable to shed the excess BTU's quickly enough causing high limit bounce and shutdown only to start a few min later because the fin-tube zones were still calling for heat.
The current revision control board finally addressed that issue and reacts much more quickly to narrowing DT's and sudden rises in SWT like when a zone closes on a high fire rate setting. It also now allows a few deg of overshoot for a few min to prevent excess cycling vs. shutting down on any overshoot at all in earlier revisions. As a matter of fact, the new board performs so well a former member here is successfully using a DT ECM CH pump zoned/piped direct on the UFT-80W with excellent results.

NY_Rob

There are tools in the mod-con installer menu to help prevent over fire/under flow situations that may damage the HX.

If you know your heatloss on DD is 35K BTU's for example, limit your fire rate to 50% so you max out at 40K BTU's on an 80K boiler when spaceheating. Enable ramp delay yo you slowly build up to full (or limited as mentioned above) output over time vs. throwing everything your boiler has at the HX as soon as it starts up.

Keep a close eye on your CH pump (especially if it's a Alpha with a GPM display) and ear on your boiler and work through all combinations of zones under high fire (and this is where limiting fire rate really helps) and listen for crackling/percolating, etc.. from the HX and increase pump flow accordingly or de-rate the max fire rate for spaceheating a bit more.

Modern automobiles have had knock sensors since moving from the carb to fuel injection back in the 80's. If the sensor hears knock/ping/detonation it retards the spark advance a bit to prevent engine damage. Boiler manufacturers could literally do the same thing if they wanted and place a sensor/mic on the HX and lower the combustion any time it hears percolation or anything else that indicated too little water flow for high fire rates.

Eastman

@NY_Rob

Yes, I am aware of ramp delay, fire rate limiting, etc. Those are all great tools, but typically one can not specify fire rate limiting on a per zone basis. As you mentioned, there undoubtedly will be situations where the boiler is firing 100%, and then flow will drop by possibly a third, or two thirds, after one or two zones shutdown. That's really the scenario I'm curious about. Can controls handle that transition appropriately without shutting off and then turning back on the burner unnecessarily? Some equipment can? Some can't?

Gordy

I think you are over thinking this, or at least questioning the design parameters of the controls which do vary amongst manufacturers some what. @NY_Rob explained it well.

Most good installers are worried about cycling. You are just worried about smoking the HX because of loose modulation controI.

I think most manufacturers that tote low flow rates (1 gpm) at minimum fire would have more concern, and more to lose since so long as the proper installation, and control is adhered to money is out of their pocket.


I think it’s interesting what @Solid_Fuel_Man has observed form two different boiler manufacturers sharing like hxers. Maybe the people to talk to are the HX manufacturers directly. However all they would possibly share would not be control related which falls on the boiler manufacturer.

Eastman

@Gordy
I've seen one forum discussion where the boiler was piped direct with zones, and the control failed to manage modulation correctly. I think this was the same issue that Rob was referencing. I don't know of any examples where someone posted: "Hey, I did that and everything just worked like we all thought it should."

I'm not concerned about super low flow rates or micro zoning. I'm concerned about systems that are zoned appropriately, boiler sized appropriately, all possible combinations of flow meet max fire minimums, etc. Reference diagrams for such installs are included in the manuals, but they seem to always illustrate the use of a DPB valve. Why is that? Is there a secret reason I couldn't opt for a deltaP pump and ditch the DPB valve?

Gordy

I/O illustrations are some times cut, and paste from previous years since most piping principles are same. Saves time, and money. I see no reason a DP circulator wouldn’t accomplish same task. That is their goal. DBP is old school, but they still make them.

Rob is directing to problems with the software which as he noted is updated as problems seem to arise. That’s good HTP support in recognizing, and fixing the issue. Maybe frustrating, but it got resolved. Haven’t heard of other brands with said issue here at least.

With the price point of that UFT some bugs can happen, and dealt with when support is on point.

I think some issues can arise when a DT circ is strapped in without understanding the total package. Anytime sensors for temperature are involved especially different brands, how sensors are in the system (strapped, or well) , and different control boards from boiler to circulator directing them things don’t play together as well.

It is actually said a DT circ can perform same function as DP in some situations. I’m on the fence. Sensor Reaction time from pressure is going to trump temperature most times.

NY_Rob

Eastman said:

I've seen one forum discussion where the boiler was piped direct with zones, and the control failed to manage modulation correctly. I think this was the same issue that Rob was referencing.

If you're referring to the discussion by the former member who was using a Delta Temp pump on the UFT-80W... back then, with the early versions of the control board it was problematic as the pump and boiler were basically fighting for control and it became a tug-of-war between them. With the latest incarnation of the UFT-80W control board he recently told me the two work really well together now.

If you're worried about overfiring the HX, de-rate your max fire rate to just slightly above your DD heatloss, balance your zones best you can and observe your pump and boiler while running to find a DP setting that works for all open/closed zone combinations.

Surprisingly, HTP recommends a minimum 1GPM flow through the UFT-80's HX even at 80K BTU output! As a non-heating professional that even scares me, so I run my boiler at 50% output ensuring I never get more than 40K BTU from the HX no matter if one or all three zones are open, regardless of the DT or OAT. In my mind there's a huge difference between running 1GPM through a HX firing at 40K BTU's vs. 80K BTU's.

Gordy

^Yes, and the person spear heading that discussion had very much influence with HTP on its control board issues.

Edit: this is also why installer feed back to manufacturers is crucial to product improvement.

Most don’t create designs that take products to the edge of their performance band intentionally. It’s mostly by accident especially when they can’t even follow pretty pictorial instructions. Squeezing the most performance out of its design is just good design, and getting the most money’s worth out of a product.

Eastman

So you have an HTP directly piped in a zoned application?

NY_Rob

Eastman said:

So you have an HTP directly piped in a zoned application?

Me?
Yes... I'm going into my 3rd winter with it now.

Three zones running on an Alpha in DP mode, speed 1 setting. 30 Gal SS Ultra on a 15-58 pump.

1,700 sq/ft 1960's construction home, DD temp 15F. Two fintube zones one CI BB zone.

It's been wonderful so far!

Eastman

Have you observed it ramp down after a zone valve closes?

NY_Rob

^ certainly, and with the newer control board I have it ramps down quicker to prevent setpoint overshoot. And, if it does overshoot the boiler is tolerant of it and just runs on low fire for a few min till it settles back down to setpoint again vs. just shutting down the fire like the old control board did only to restart it min later.
I'm pretty happy with the performance of the new control board.

That being said, HTP still needs to work on the clumsy transition from spaceheating to DHW production back to spaceheating.... but that's another story.