Primary loop excess flow=low delta T

Mike T., Swampeast MO

Don't forget that the 20F delta-t target used for sizing the primary circulator is based on <I>full boiler output</I>. When the flow rate through the boiler remains fixed while the output reduces, the only possible consequence is lower delta-t. As the weather gets colder, the reset target increases and system load increases as well, you'll see delta-t across the boiler closer and closer to the design ideal. If you use too small of a primary circulator to increase the delta-t a lower boiler outputs you run the very real risk of the boiler tripping out on high safety when output increases.

Don't fret the lower delta-t across the boiler in low load conditions. As long as the average water temp is at least a few degrees below 140F or so, a rather tight delta-t actually helps keep EVERYTHING below 140F thus increasing the amount of heat recovered from the condensate.

It's the opposite situation where the effects of boiler delta-t on efficiency can get really interesting. Under high load and with an average temp requirement much above 150F or so, it's actually better to increase delta-t such that at least the return temp remains in the condensation range.

My directly connected Vitodens 6-24 (variable-speed circulator directly driving a TRVd system without LLH or primary/secondary) has an extremely wide range of delta-t across the boiler from about 2F at minimal load to about 40F when the outside temp is below design.

Before you get the idea of using a VS circulator for the primary loop, realize that the only way I can see for this to work is if the boiler and circulator actually communicate with one another. I've checked with two mod-con mfgrs now and neither can recommend the use of a VS circulator either in the primary or for a directly connected system.

Larry (from OSHA)

My Knight piping

My Knight 80 is working great, but of course I’d like to get the most out of it. I have 3 zones of baseboard with zone valves driven by a 15-58 on speed 1. I believe the system design is 20° ΔT and has been running about 10° at this time of the season.

The boiler loop is something else though. With all three zones calling, there is less than 10° rise across the HX and the inlet temp is about 10 degrees or so above the system return. With only 1 zone calling, the rise across the HX is only about 2 to 3 degrees.

The boiler pump is a 26-99 on speed 2. Currently, I need to run on speed 2 to make the flow switch which is a McDonnell Miller FS-4.

It seems to me that reducing the boiler flow would result in a wider ΔT and lower return temps at the boiler inlet. Of course, I don’t want to reduce the flow to less than what the factory wants, but with the numbers that I am getting now, I’m thinking that there might be some room to move. I have tried the boiler pump on speed 1 with the flow switch jumped out and there was only a little improvement, but not enough. I would think that a 15-58 on the boiler loop would put the boiler and system flows in better alignment, but I’d really appreciate some perspectives.

Larry

Mark Eatherton

Welcome to MY world...

The "system" 20 degree delta T is an engineers dream. Its a reference point if you will, and in my 1/3 of a century of hanging out in mechanical rooms, I have seen it ONE time, on a NEW start up with an EXTREMELY cold house.

If my memory serves me correctly, the flow switch is not required on the smaller boilers, and there are enough solid state sensors on board to know when your circulator has failed.

That said, you could replace the bigger pump with a smaller one, but I don't think you will really see that much of an improvement in overall performance. I think the majority of the fuel savings is coming form the MOD portion of the equation, as opposed to the CON part of the deal.

If you really want to experiment for yourself, jump the flow switch out, and choke one of the pumps isolation valves down until you hit the 35 degree delta T and see if it produces much more condensation. I think you may be disappointed in what you will see. But fear not, you STILL have one of THE most efficient products available on the market today. What I think you will see is that the boiler is going to see the reduced flow, and increased discharge temperatures, and will idle back the burner anyway. Remember, its attempting to achieve a given discharge temperature. Other than wasted parasitic power, I don;t think you will see much difference per se.

(my assumption is that you are using the boiler supply sensor for the reference sensor, otherwise it may not idle back)

ME

Mark_46

Hmm, same question

I read this post with great interest because I have observed the same thing on my boiler and have been wondering if it’s a problem or not.

With one space heating zone calling there is about 8-10 delta. With both space heating zones calling there’s about 13-15 delta although these values do vary at different points in the heat cycle. As you might imagine the boiler typically runs at or close to low fire rate (27,000 BTU). Good thing, right?

The Grundfos 15-58 primary circulator is set to medium speed which yields about 7.7 GPM when I plot the head feet in the primary circuit and the boiler against the Medium speed curve. Heat Transfer specifies 1 GPM per 10,000 BTU so 7.7 GPM is certainly ‘in the ball park’ for an 80k BTU capacity boiler.

BUT theoretically the primary circuit flow will never have to transfer even close to 80,000 BTUs as my entire house has only about 40,000 BTU loss or so at design temp. So I have been considering turning down to low speed. However, correct me if I’m wrong, the only gain in that case would be less electrical consumption from the pump at low speed. Is it worth it? Is there another reason to turn down or not turn down?

Larry (from OSHA)

Mark

Thanks for your thoughts. I tend to agree with you that the Mod portion is more relevant than the Con portion.

But I do have to say that I'm enjoying everything about this piece of equipment and that this is not "my father's boiler" at all.

Larry

Larry (from OSHA)

Mike

I suspect that I'll not make any changes for a while and may sit on my hands until next year, as difficult at that will be.

But, you make some very interesting points with "rather tight delta-t actually helps keep EVERYTHING below 140F thus increasing the amount of heat recovered from the condensate"

I was under the impression that the lowest return would do 2 things: make the most condensate and keep the exhaust temps as low as possible. You do remember the discussion about PVC venting a while ago, right?

Anyway, my primary motives for using a smaller boiler circ is to reduce the electrical usage (my green aspirations) and to reduce the sound level when the unit is running. A 15-58 does not really make any noise, but the 26-99 definately announces that water is moving. (It's not that loud, I'm just kind of fussy that way!)

What I have not done is accurately figure out what the flows really are and see if I'll have enough btu's moving on design day if I were to go to a smaller pump. That is the bottom line. I need about 60k into the system loop from the boiler.

At any rate, thanks for your perspectives. I appreciate it.

Larry

Mike T., Swampeast MO

Clock your meter in differing weather and keep measuring and comparing to your primary delta-t.

Give yourself at least a season of observation before you try to tweak.

hot_rod

I'm not sure why they boosted the circ size

on the small Knights. I have a dozen or more 80 and 105's running fine with the 15-58 circs they came with.

I hope the larger circs aren't to overcome another problem, like poor water quality scaleing the HXers? I hate to here fluid moving that fast through those small coil exchangers if it is not truly needed. I'd rather go the other direction, port out the HXers and reduce the needed pump power. Again a great place for a VS circ as seen on the Vitodens.

hot rod

Mark Eatherton

Inventory control...

It is easier to stock one pump that works on all boilers, hence the decision to go with the pump they are using. Scaling is not an issue to the best of m knowledge.

ME

Larry (from OSHA)

My understanding

is that there may have been some hi limit issues with certain installations (but I don't know that first hand) but I do know that some installations had nuisance flow switch lock outs. This certainly could be taken care of by using a different flow switch such as the FS-5 from McDonnell & Miller that makes with less flow.

Also, as you may suspect, systems with CI radiators and no wye strainers can lead to bits of "stuff" clinging to the HX with less than optimal results.

Larry

Larry (from OSHA)

meter clocking is \"so old school\"

That means I have to actually go outside and sit there and look at the meter.

With the luxury of computers and smart boilers, I can sit right here and watch this new fangled appliance do its thing!

Mike T., Swampeast MO

Where do I anything telling BTU input or any way to estimate BTU output? Yes, it still takes some trips to the meter--sometimes with a flashlight...

I do though agree that there is some room for improvement. 158F flue temp at only 126F supply is excessive for a well-regulated mod-con.

Mike T., Swampeast MO

YOU GOT IT HOT ROD!!!!!

How fun it is when you can tell your circulator how much energy you've added to the system.

Larry (from OSHA)

flue temp

I think 158 is a bit high too. That's why I wanted to get the inlet temp closer to the return temp from the system loop. I really don't have anything for comparative perspective here, so what do you think would be more in line for flue temps? This was combustion checked and the numbers were pretty much in the center of the ranges for O2, Co2 and CO was fairly low if I recall.

Uni R_2

flue temp

Larry, did your boiler just complete a DHW call? If so, that would make perfect sense.


I agree that you should clock it at various RPMs.


Very nice display! Is that a standard feature on the Knight?

Larry (from OSHA)

UniR

Nope, did not just come off a DHW call. Temps go quite a bit higher with supplies at 175. I had a conversation about this with the factory and they were not concerned. I would still like to see them lower.

The display is a program from Lochinvar that not only shows what's going on but also has the ability to change parameters for many things. The boiler has the output, but the program is an optional toy.