FRUSTRATED WITH SUPPLY TEMPS

[91LMS]

i dont have a gasser or the most popular choice in boiler by any means but am looking for some direction with my frustrations. i have a 1000 gallons of storage, boiler charges storage, loads pull from storage. my circ is hook on return side after danfoss valve to limit return temp to 140 deg.. after 2 weeks of the boiler running i am getting frustrated with my storage temps. i seem to have a hard time to push it to 150 deg. boiler output only 145-160 and staying on the lower side for the most part. i figured with the danfoss installed i would be able to push higher outlet temps with return water being hotter. 3 spd pump on low and even tried throttling with ball valve. it helps but not seeing anywhere the results i am hoping for.

any thoughts, hopefully the pics will give a bit of insight on my plumbing

 

[Bob Rohr]

i dont have a gasser or the most popular choice in boiler by any means but am looking for some direction with my frustrations. i have a 1000 gallons of storage, boiler charges storage, loads pull from storage. my circ is hook on return side after danfoss valve to limit return temp to 140 deg.. after 2 weeks of the boiler running i am getting frustrated with my storage temps. i seem to have a hard time to push it to 150 deg. boiler output only 145-160 and staying on the lower side for the most part. i figured with the danfoss installed i would be able to push higher outlet temps with return water being hotter. 3 spd pump on low and even tried throttling with ball valve. it helps but not seeing anywhere the results i am hoping for.

any thoughts, hopefully the pics will give a bit of insight on my plumbing

Sounds like you just don't have enough "horsepower' to get over the hump? Any idea on the rated output of the boiler and the actual heating load your home presents to the boiler?

How well insulated is the tank, any energy escaping to the outside from the tank location?

 

[henfruit]

You want to close the bypass valve half way. If you shut off the house load and just charge the tank what happens?

 

[Chris Hoskin]

what are you seeing for stack temperature? If you are seeing stack temps below about 300 you are not getting a good burn (most likely culprit is wet wood); If you stack temps are in the 400 to 600 degree range then the heating load may be too big for the boiler. Install a stack thermometer if you don't have one and shut off all heating zones and just heat the tank to see what happens. Keep us posted.

 

[maple1]

How far apart are your boiler & storage? Have you done any head calcs between them?

If you've got a 15-58 pump, it will only pump about 2 gpm at 10' of head on low speed.

That would mean that with your loads NOT drawing from storage, and with that much head, it would take 500 minutes before your boiler would see ANY rise in return temps from storage. But it would actually be more than that because some of the pump flow is not going to storage but rather around the bypass loop, until you can get your bottom of storage and return from it up above the 140 point.

You're kind of in a catch-22 with the boiler you have, if it's anything like my old one - heat transfer efficiency is so poor that you need slow flows to get a reasonable 20° delta-T. I can't find a cutaway of yours, but the lit does say no tubes - which they might consider to be good but I would consider to be bad. I would think that if you could get your storage up with a long period of warm weather with nothing going to loads, it should be able to maintain something a bit better. But you're up against it with the boiler you have, I think - assuming it's tuned in as best it can be with things like chimney draft

 

[91LMS]

i am seeing stack temps between 400-550 with the draft adjusted the way i have it. no tubes, just one big water jacket and a baffle that runs across at the top of the chamber. worked great with low mass/no storage but wanted longer use of the wood boiler and not having to supplement with oil.

storage is right next to the boiler, only 4 feet vertical over to tank, down into tank then 8 feet up on return and drops 8 feet to boiler return.

i can throttle return right before it enters boiler, before supply enters storage or where hot leg supplies to danfoss. will it matter where?

storage has 6" min of fiberglass and trying to stuff more where i can(under studding for enclosure where insulation is held in between studs). still working on tightening that up more but seems to have no bearing on tank temps so far.

95k btu cap but rated at 104k

 

[maple1]

I'd try throttling the hot leg to danfoss (that's the bypass line - right?), quite a bit. If you haven't already. Definitely not the other two.

 

[91LMS]

what are you seeing for stack temperature? If you are seeing stack temps below about 300 you are not getting a good burn (most likely culprit is wet wood); If you stack temps are in the 400 to 600 degree range then the heating load may be too big for the boiler. Install a stack thermometer if you don't have one and shut off all heating zones and just heat the tank to see what happens. Keep us posted.

of course i can make better headway with the tank this way. in the warm weather lately i can get storage to creap upwards of 130 bottom but then i am in the night hours of no fire. wood isnt the best but have loaded with really dry hot fuel and get the same scenario.

 

[91LMS]

but if i throttle that back and yes that is the bypass leg, wont it shut the t-stat in the danfoss as it has to mix hot with the cold return to open that valve. or is there enough temp there to keep the element open and the volume doesnt matter?

 

[Fred61]

You only need enough flow through the bypass to allow the expansion bulb on the automotive thermostat to bathe in 140 degree minimum water. I would at least start with it open only a little more than 1/4 open. I don't know the size the pipe or valve but with the ball valve only open 1/4 the way you will still get a lot of flow.

Or try this:
Hey gang! stop me if you think I'm offering bad advice here. If you have good reliable gauges, fire up the boiler and close the bypass valve completely and watch the boiler temperature. DO NOT LEAVE THE AREA When boiler reaches 175 or circulator launch, crack the bypass valve and you will be able to tell by the gauges and perhaps the sound when the zone starts flowing. That may give you an indication of where the bypass valve should be set. I've done it.

 

[91LMS]

my gauges seem to be spot on (within 5 deg of each other) in doing this though am i defeating the danfoss and allowing too cold of water to return to the boiler? i checked last night and return was over 140 and pulling 120 from bottom of storage. i must be useing a fair ammount of supply water to mix? i would think if i was just looping and supplying 140 to the boiler return i could push the supply temp higher.

 

[Fred61]

No you're not defeating the danfoss because it will not open 'til 140degree water flowing through the bypass is sensed by the expansion bulb on the thermostat It will stay closed until you send a slug of water through the bypass by opening the ball valve. I only have a 3/4 inch bypass and throttling valve and my sweet spot is a little over 1/4 open so with the size of your bypass the correct opening could be even less. If your return temperature is 120 the thermostat will close and be sensing the loop water again and will continue to modulate 'til the return water reaches close to 140 and then will act more like a mixing valve.

 

[91LMS]

with the bypass leg restricted, i am wondering if the danfoss will allow the cold to flow and allow colder than 140 to return to boiler. right now i am mixing all the time, havent seen storage 140 but once in the 3 weeks i have been running.

 

[Fred61]

No 140 degree water must flow through the bypass in order to open the thermostat. That's why the bypass is there. It's there to send a sample to the bulb on the thermostat in order to make it open. You might be just spinning your wheels by doing this but it will take one variable out of the picture. I f your air supply damper is staying open and your boiler temp doesn't reach your upper limit, Bob Rohr may have stated the correct diagnosis above.

 

[ewdudley]

You only need enough flow through the bypass to allow the expansion bulb on the automotive thermostat to bathe in 140 degree minimum water. I would at least start with it open only a little more than 1/4 open. I don't know the size the pipe or valve but with the ball valve only open 1/4 the way you will still get a lot of flow.

This doesn't seem to add up to me. I think the bypass leg needs to supply as much hot water as it takes to bring the average temperature up to 140 degF.

If pump on low speed is flowing 8 gpm, boiler supply is 150 degF, and tank return is 120 degF, then in order for boiler return to be 140 degF there needs to be 5 1/3 gpm recirculating through the bypass leg and 2 2/3 gpm coming from storage. Boiler output in this example is 40000 btu per hour.

If bypass valve is closed down substantially then the end result will be that the gpm through the pump will be throttled and boiler supply temperature will increase, but boiler btu per hour would stay about the same. For instance if the pump flow in the above example is changed to 4 gpm, the boiler supply goes to 160 degF, boiler return stays at 140 degF, bypass flow goes to 2 gpm, flow from storage goes to 2 gpm, and boiler output stays at 40000 btu per hour.

Here's a program for calculating boiler btu per hour given pump gpm, storage return temperature, boiler return temperature, and boiler supply temperature. The calculations could be set up in a spreadsheet if Perl is unavailable.

use strict;

my $TBoilerSup;
my $TBoilerRet;
my $TStorageRet;
my $BTU;
my $GPMPump;
my $TBoilerDelta;
my $TStorageDelta;
my $TMixDelta;
my $GPMStorage;
my $GPMRecirc;

$GPMPump = 4;
$TStorageRet = 120;
$TBoilerRet = 140;
$TBoilerSup = 160;

$TBoilerDelta = $TBoilerSup - $TBoilerRet;
$TStorageDelta = $TBoilerSup - $TStorageRet;
$TMixDelta = $TBoilerRet - $TStorageRet;

sub calc_btu {
  $GPMRecirc = ($GPMPump * $TMixDelta) / $TStorageDelta;
  $GPMStorage = $GPMPump - $GPMRecirc;
  $BTU = $GPMStorage * $TStorageDelta * 500;
}

sub pr {
  printf("%9.2f GPMPump\n", $GPMPump);
  printf("%9.2f TStorageRet\n", $TStorageRet);
  printf("%9.2f TBoilerRet\n", $TBoilerRet);
  printf("%9.2f TBoilerSup\n", $TBoilerSup);
  printf("%9.2f TBoilerDelta\n", $TBoilerDelta);
  printf("%9.2f GPMRecirc\n", $GPMRecirc);
  printf("%9.2f GPMStorage\n", $GPMStorage);
  printf("%9.2f BTU\n", $BTU);
  printf("\n");
}

printf("Calc BTU:\n");
calc_btu();
pr();

 

[maple1]

This is why I could never grasp the concept of throttling bypass flow to a danfoss-never did make sense to me in how it could increase flow to storage if the danfoss was doing what I thought it should be doing.

 

[Fred61]

Modulating thermostat should take care of that by allowing more or less return water from the return. I will agree that there needs to be enough flow through the bypass to keep the pump from cavitating when the thermostat closed and blocks return water.

 

[Fred61]

This is why I could never grasp the concept of throttling bypass flow to a danfoss-never did make sense to me in how it could increase flow to storage if the danfoss was doing what I thought it should be doing.

Because water will take the route of least resistance and you don't want it going in circles.

 

[maple1]

Because water will take the route of least resistance and you don't want it going in circles.

Yes, but if the danfoss is designed to mix hot and cold supplies to an output of 140, it would seem to me that throttling one of those would lead to the danfoss throttling the other to maintain its 140 output, so the end result of throttling the bypass would just be increasing pump head and reducing overall flow. That's what I could never get my head around. Still can't I guess.

 

[91LMS]

interesting reading the input on all of this. so after the weekend of being able to have exteneded burn times, this is what i got........

bypass leg throttled down and open probably 1/8th-1/4 my boiler output increased to a steady 150 deg with the same wood, damper and draft settings... stack temps of 500 deg average. after about 10 hours of burn, occasional loads calling (25-30 deg. days) i was able to get storage 140 bottom and 145-150 top. this seems to be my shortest charge time and best charge temps so far. however i am still puzzled why i cant push supply temps over 150 when my return temp from bottom of storage 140. i can get a delta of 30 deg with cooler temps. i was hoping to see the same change when the bottom of my tank came up.

forgot to mention that my ir gun show storage return water accurate with my gauge, bypass leg accurate to boiler supply and return water to the boiler maintaining between 130-140 deg. it does seem like closing the ball valve on the bypass leg is helping.

on another note, thanks for the help thus far. without all the help i would have never gotten this far. i have made changes and found issues but for the most part this has been a success so far.

 

[Fred61]

Looks like you've got all there is to get with this exercise. It's now time to look at your wood as Chris Hoskin suggested or you have limited horsepower as Bob Rohr stated.

 

[maple1]

How are you measuring your stack temps, or what with? Sorry if you posted already & I missed it.

I guess where I was going is that if you're measuring with a magnetic guage, I think those are pretty high temps and a lot of your heat is going up your flue. You might be able to improve that some with a flue damper - but a lot of it is likely due to poor heat transfer efficiency of your boiler.

 

[91LMS]

i agree with poor efficiency of the boiler. that is measured with a magnetic gauge and usually close the damper about 25 percent with a full load to give me 350-400 stack temps. i have tried to open it more but lose burn time in trade for a bit hotter supply temp. with it being conventional though i think i should be 350+ to avoid creosote issues?

 

[maple1]

I ran a boiler that sounds a lot like yours for a long time, trying to get the most heat I could out of it.

What I ended up doing was damping it down as much as I could with a key damper in the pipe, more or less leaving it open just enough that smoke wouldn't come out my draft door - and cleaning my chimney as often as that dictated. Very poor way to run things, but if instead I opened the damper enough to keep a good hot burning fire going in the box that would prevent or reduce creosote buildup, most of my heat would go up the flue. Along with making more creosote, it also builds more coals in the firebox - so along with cleaning my chimney more often (I got by with 2 mid-winter cleanings, along with one at beginning & end of heating season), I also had to shovel out coals to get more wood in on occasion.

Catch-22's all around with a unit like that.

 

[Chris Hoskin]

I can only speak to how the Termovar mixing valve operates, but I assume the Danfoss is similar: In order to assure a minimum return temperature of 140F, port one of the valve/element needs to see at least 160F and does not begin to open until then. Port one is the inlet from the bypass leg, port two is the return from the zones/thermal storage and port three is return to the boiler circ/boiler return port. This is why we do not turn on the boiler circulator until the boiler has reached 165F. The balancing valve is to introduce some flow resistance to the bypass leg so that the path of least resistance is out to the zones, not through the Termovar mixing valve. If there is not enough resistance in the bypass leg you will see high boiler temperatures and not enough heat out to the zones. The other thing that happens is that, normally, you will eventually see return temperature from your zones/thermal storage ABOVE 140F, so no mixing is needed. However, port one never really closes all the way so the balancing valve encourages flow to the zones instead of through the valve in this circumstance too. One of the advantages of the Loading Unit is that port one will positively close so no balancing valve is necessary. As I say, I'm not certain that the Danfoss works this same way, but assume that it does. Hope this helps.