Condensing Boiler Return Water Temp Too High

ahovel

I recently had a condesning boiler installed. It replaced a 30 year old boiler. I have a 3 zone hot water baseboard system in a 1000sq ft house. There is a zone in the basement, first floor, and 2nd floor 30 miles north of Boston, MA. The system has a Taco 007 circulator pump. The boiler is a combi E110cn Rinnai. The outdoor reset is installed and does seem to be working.

The return temp is only about 10 degrees less than the supply temp. I'm wondering if there is a simple way to increase the delta T or decrease the return temp. Decrease flow rate or increase load? Other?

thanks,
Aaron

Eastman

What drives your concern regarding the temperatures?

ahovel

Good question. Forgot to explain. With the low delta T. The return water temp is quickly out of the condensing range (>130F). Would like to be in the condensing range as much as possible.

thanks,
Aaron

Gordy

This starts with a heat loss, and an existing radiation survey.

It's quite possible you may have more than enough radiation to allow lower supply temps then what you are using. Especially if envelope insulating upgrades have been done after the original system was installed.


You may need high water temps only when approaching your outdoor design temp.

With a delta of 10 you are a little strong with the circulator, or the radiation is not giving up enough btus.

ahovel

Any way for a home owner to do a heat loss calcuation? Also, any way to slow down the circulator?

Eastman

Are you using the RS100 controller?

Eastman

To answer your last question: no there isn't a simple way to throttle back a taco 007 circulator.

ahovel

Not using the RS100 controller? Should I be?

Paul48

Did the contractor go over adjusting the outdoor reset curve with you? That will take time, and tweaking. When all the zones are running with the t-stats cranked all the way up, and the house is the desired temperature, the curve is right.

Eastman

I'm reading some of Rinnai's product documentation. Their RS100 thermostat looks kinda interesting as it apparently supports indoor feedback, which can in theory optimize water supply temperatures if properly implemented. But it seems to be of somewhat limited value for multi-zone applications.

Regarding the pump: If you are willing to replace it there are pumps that have variable speed options that allow the end user more control over the flow rate. Is the boiler piped direct or do you have some kind of primary/secondary piping arrangement?

joca

does the taco2218 respond only to temp change ,and a grundfos alpha only to pressure ,or can they somehow do both?

sonofaplumber

I think the poor man's fix would be to set it up as a single zone system. It would do the trick I bet. And cost $0, take 20 minutes to switch back if you didn't like it.

njtommy

Delta T pump would be a great option to help balance the system and get your returns lower. Also once you establish your heat loss and supply water temp on a design day you could be condensing a lot more often.
Another idea you could add indirect tank or heat exchanger of some type to a holding tank and preheat your domestic hot water.

Steve Thompson (Taco)

One other idea might be to use the VR 2218 as a setpoint circ. Pump away from the tank (as always) and install the setpoint sensor on the return. Return temp goes up, pump slows down (lets more BTU's get off the train). Caution, the last zone not getting enough heat - so this solution is system dependent and might be trial and error (sorry).

We also have a Delta P circ, our VR 1816 - but you are correct, the best circ to control return water temp is temperature reactive...

SWEI

Pretty sure they both meant to type VT2218 above, BTW.

SWEI

I could actually use a VR2218. Better yet, something along the lines of a VR2240.

Gordy

I think first step is to make ODR is dialed in to the heat loss and radiation. I don't view a 10*delta as a huge handicap especially this time of year. Avg 135 water temp this time of the season

Rich_49

Beg to differ Gordy . Watch the last coffee with Caleffi . Being the anal retentive that I am I also think that moving 2 xs the water while getting no benefit from it and losing an efficiency point or 5 is not what we are or should be striving for . JMO .

Gordy

True Rich I'm not off the deep end. I suppose if the op is willing to change up circs your right. It usually starts with an initial design with the right circ to begin with. Would you have used that combi size for 1000 SF dwelling? I doubt it unless the envelope was like a submarine with screen doors.

Is the curve set correctly? Seems to be working does not mean it is set correctly. We know little except a 10 degree delta. Could mean many things. But yet we want to embrace the technology to make it do what we want instead of designing to do what we want naturally.

On the other hand if his avg water temp is 135 possibly higher now this time of year just guessing from posted info. How much will he condense seasonally if he gets down to 120 return 130 avg water temp. Just sayin it usually does nothing but get colder from here on out. Reset goes up. It's early in the season. But don't know what it's like north of Boston.


Be nice to know the particulars
3 zones for 1000 SF are pretty low load zones. Is it 1000 SF each zone, or main floor. I'll bet the 007 is screaming with only one zone calling. Why not turn it into one zone as suggested see what happens. Is it always 10 degree delta, or only when one or two zones call.

And yes a circ change of the variety talked about could solve all that.

Steve Thompson (Taco)

Oops - yep, supposed to be VT 2218, not VR 2218. Seems we all have Mondays.

I have a really stupid question. If the VT 2218 controls it's speed based on return water temp, what would control the supply water temp? The reset or boiler control?

Assuming so, if the return was set to say 135F and the building was under full load requiring say 170F to heat I can see a 35 deg delta. Assuming last zone is getting enough heat, what do you see as issues with this?

Please educate the old guy...

Eastman

@Steve Thompson (Taco)

What if the boiler control is only calling for 135 degrees or less? How would the VT 2218 respond?

Rich_49

Steve ,
If radiant floors they would suffer uneven temp across the panel . If series loop the far end units would be cold and thus the rooms . May work with cast iron rads or high temp panel rads but only if the average water temps are where they should be .

SWEI

What happens when the ODR setpoint is 95°F?

Eastman

Some background on the Rinnai E110C:

Minimum input for natural gas is 26,000 btus.

Internal pump is a UPS20-58. I can't find any information about this pump on the internet. The boiler specs indicate it comes with a single speed pump so I'm going to assume it's a single speed oem Grundfos model.

The boiler comes with a low loss header. The manual indicates the warranty is voided if this is not installed.

Page 23 of the install manual indicates we should expect a primary loop flow rate of about 6.5 gallons per minute.

That equates to an 8 degree delta-T at minimum modulation.

I suggest confirming primary loop flow rate and synchronizing the secondary flow at 6.5 gallons per minute with a flow setter or an adjustable pump. Then adjust ODR curve as required.

ahovel

Wow, what a great community! Thanks for all the response. I'll try to answer some questions that came up and then summarize.

Details... 1st and 2nd floor 500sq ft each. Each has it's own zone. Basement 400sq ft with a separate zone. Even with all three zones running, it is max 12 degrees delta T. Boiler has a low loss header, so I don't think there is a separate primary and secondary loop, but I may not understand this correctly. Contractor did not go over ODR with me, so I'm not sure if that is dialed in correctly.

Sounds like the two options are:
1. Change to one zone.
2. Change to a deltaT circulator.

Question.
1. Is there a way for me to perform the heat loss calculations?
2. Of course I'm attracted to what ever is the least expensive. So what would be entailed in changing to one zone?

thanks again,
Aaron

joca

this has been a question that's been brewing for a while i've been trying to think it thru-in a P/S system that has been designed with a certain delta T return water, it seems that the constant flow of primary loop water will be fighting the low temp return water , to try to get a low temp back to the boiler . what is happening when the designed delta T return water is injected into the high temp primary loop water. what temp is the boiler seeing.

Eastman

Hi Hatterasguy,

Regarding: Return Water Temperatures

I'm assuming the concern here again is maintaining the lowest possible return temperatures to effect the greatest possible efficiency. I think we can agree that it is the temperature of the water returning to the boiler in the primary loop that directly effects this efficiency.

The question then is how to achieve the lowest possible primary return temperature?

I argue that the lowest primary RWTs occur when the secondary flow rate is synchronized to the primary.

Eastman

@ahovel
A low loss header is one method used to create separate loops. So you do have primary and secondary loops.

Paul48

I want to discuss the RWT issue, but I don't want to hi-jack this thread.

Gordy

Hat well said.

Zman

Hatterasguy said:

Firstly, the low loss header on the boiler performs the primary/secondary function. The boiler has an onboard pump to provide the boiler with a fixed flow rate of 6.5 GPM.

Your house being the size that you stated, probably has a heatloss of not more than 30,000 BTUH on the coldest day of the year. In reality, it's probably less but let's stay with this value.

We can change the secondary pump to VT2218 or we can throttle the flow on the existing pump or we can combine all the zones together or we can do a combination of any of the three.

And, say we manage to get a ΔT of 20F. On the coldest day of the year, with all three zones calling and a loss of 30,000 BTUH, the flow rate is 3 GPM.

Now we get back to the primary loop which is flowing at 6.5 GPM, over twice the rate of the flow in the secondary loop. What do you think happens to the ΔT back to the boiler? Yep, it drops right back to under 10F.

If we run the figures in the shoulder season where the heatloss from the house is 15,000 BTUH, the situation gets even worse and the ΔT on the primary loop is now less than 5. In reality, this never happens because the boiler has a minimum firing rate of about 24,000 BTUH output and the boiler just shuts down on high limit because the load is less than 24,000.

This is the fundamental problem with most mod-cons that have fixed speed primary pumps.

Despite all your best efforts, the ΔT back to the boiler goes all to hell and the unit DOES NOT meet anywhere near the claimed efficiency because it isn't condensing most of the time. It's the "big lie" within the industry.

A manufacturer needs to step up and provide a boiler that will accept a flow rate of 1 GPM through the HX. Now you can make changes to the system side and get the 20° ΔT back to the boiler.

Finally one of them has done it:

http://www.htproducts.com/UFT-Boiler.html


In your specific case, I wouldn't spend any money on valiantly attempting to get to a ΔT of 20 because most of it is going away at the boiler anyway.

@Hatterasguy
Thank you, Thank you, Thank you.
I read this entire post and thought to myself, "doesn't anyone want to talk about the #!&^RE#^&*ing boiler circ?"
And here it is!
This one would make my top ten all time favorite posts.

SWEI

We didn't do quite that well, and the system did have about 600 gallons of water in it.

Rinnai does sell boilers with internal variable speed circs. Haven't tried one yet, but I believe the concept has merit. Time to go back to the packaged boiler paradigm? This time the circ really should be matched to the boiler.

JohnNY

I think there's too much talk about the pumps here.

I would advise the poster to consult the manual to see if the boiler allows the user to alter or otherwise limit its fan speed. If it doesn't offer that particular function, set it for "high altitude" operation and it will adjust itself.

njtommy

Well said Hat.

With these tankless units it would be great to see a lot lower flow acrossed the heat exchanger and a primary pump that can modulate down with it using a 0-10vdc signal.

Gordy

To the op there are free heat loss calculators on line through slant fin. It's padded by a lot but will see where you are at.

Rich_49

@JohnNY

If the boiler is on top of Mt Everest the pump will still flow 6.5 GPM and the problem will still exist ! I am not quite sure that advice to set for high altitude is prudent since gas and air are both gases , I do suppose one could match the 2 by performing a very good combustion analysis . We all know though that this operation is rarely performed with the exception of most of us . What if one of the hacks attempts this . Can anyone say liability ?
Al that said , the pump will still disallow proper operation . Everyone , Flow in the system loop MUST be higher than the flow in the Boiler loop to get these systems to rated efficiency , end of discussion on that , physics will allow nothing else and Mother Nature is having quite a laugh . A boiler that is flowing more GPM than the system CANNOT EVER reach it's rated efficiency . Period . Does anyone disagree ? The only way to have a shot is with a large buffer , big enough to stay stratified at all times . I like the 2 pipe buffer scenario better for this since the system water will return to boiler ahead of excess water in the buffer and boiler water will be used by the system prior to entering the tank . We need to make the buffer the in charge device and what the boiler recognizes , AWT in the buffer becomes very important in the center temp layer . Or we could recognize that there is equipment that does all this for us and has been for awhile . Now there is a second choice for those who just have to have that small wall hung boiler in the form of the HTP UFT . Lest we forget Tacos contribution to the BIG PICTURE , variable speed Delta T circs .



Thanks Dave Davis and Johnny White , your contributions are appreciated by most and we're trying to reach the rest . Maybe I only speak for myself , time will tell .

Paul48

Rich.....Can you point me at the minimum flow requirements in the UFT manual

JohnNY

Rich said:

@JohnNY

If the boiler is on top of Mt Everest the pump will still flow 6.5 GPM and the problem will still exist ! I am not quite sure that advice to set for high altitude is prudent since gas and air are both gases , I do suppose one could match the 2 by performing a very good combustion analysis . We all know though that this operation is rarely performed with the exception of most of us . What if one of the hacks attempts this . Can anyone say liability ?
.

When I asked Weil-McLain engineering why they didn't include a fan speed adjustment on the new Evergreen boiler, they told me if it becomes a concern, one could just set the boiler for High Altitude operation and that will safely and effectively limit the fan speed.

I like having the option, which is partly why I tend to default to Lochinvar for my mod-cons.

jonny88

On the contrary John I am fascinated by pump selection.Like Paul I don't want to hijack thread.In my case I have p/s.1558 on primary loop.alpha on secondary.Alpha is auto adapt speeds up and slows down as required.Would I be better with the Taco mentioned above.If I am understanding correctly delta t pump is better for controlling return temp.What scenario is the alpha used for.

Gordy

@Hatterasguy

How is the uft your magic bullet? I understand the 10/1 modulation, but what else. It still needs a minimum flow rate, and certainly is not down to 1 gpm.

Paul48 said:

Rich.....Can you point me at the minimum flow requirements in the UFT manual

Agree where is hat coming up with 1gpm min?

Paul48

If you extrapolate the chart they give for expected DT through the HX, 1 gpm puts it in the 50's. They don't show that, and you certainly can't shed that in the system. In the manual, they say you have to maintain the minimum flow requirements mentioned elsewhere in the manual, but I can't find it. Are we applying "Old School" thinking to this new technology? The boiler is attempting to apply the necessary btus/hr, based on the outdoor sensor. If the system is matched to the boiler( at whatever the boiler is being pumped at) , any change reflects the needs of the system. Exactly what it is attempting to adjust for. I think we tend to look at things as a snapshot, when we should be looking at it as a movie. The hour part of btus/hr. There is conditions in the system that will not allow for the proper operation of a mod/con, but that is the other part of the story. The square peg in a round hole problem.