Need some advice (opinion?) on a ModCon installation
Last Post 06 May 2013 10:17 AM by TomWS. 77 Replies.
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TomWSUser is Offline
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10 Sep 2012 05:46 PM
Thanks Dana! No problem on the 'misstatement', I just needed to confirm so I can tweak my numbers accordingly. And, if you are in my neck of the woods, I would be most happy to buy you a coffee or your favorite beverage ;-)

We had a cool morning this morning so I tried the system with all zones open and got a 3 minute 17 second burn once it was cycling. In my earlier calculations I had come up with something in the order of 11 gallons for the total system which is why I had to re-look at the volume I had calculated for the smallest zone - 7+ gallons just for that zone didn't make any sense. I'm glad we got that resolved. Unfortunately I left my original calculations in my workshop office back home, I'll check them once I return. Sigh.

Looks like I've got a 'project' to work on in the spring!

Thanks to all!

Tom
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10 Sep 2012 06:38 PM
Welcome to hydronics 101- you now know more than the consultant who told you to run the baseboard at 180F (at least he had the primary-secondary part going in the right direction, if little else) and probably more than you ever wanted to know about heating systems. :-)

Keep the napkin-math handy- put it in a spreadsheet, even print it out, tuck it in to a dust-protected envelope on your zone-manifold board along with the boiler documentation, etc, in case you (or the next guy) decides to hack away at it in the future.

And don't forget to tweak the reset curve once you get a real heat load. You really DON'T want to run it at 150F+ all winter unless buck-a-gallon propane has made a comeback.
TomWSUser is Offline
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03 May 2013 09:15 AM
I've finally gotten around to adding the buffer tank to this system.  We've had the system shut down and winterized since Sept and were only able to return to Maine to install the Buffer Tank the last week of April.  Results are 'mixed'...

I've attached photos of the installation, but, basically, I've disconnected the boiler supply and return from the radiator loop manifolds, created a primary loop with the boiler feeding a 28 gallon hot water heater (used as a storage tank) and returning directly to the boiler.  I've installed a closely spaced tee manifold to connect the radiators as a secondary loop into the primary loop supply line feeding the bottom drain port on the tank.  The top 'Hot' port of the tank was then connected back to the boiler return, included in that line is a tee to an additional 2.1 gal expansion tank (the boiler has a 2.2Gal built-in, but calculations showed that I needed a bit more with this additional water in the system).

The secondary loop has its own circulator (Taco Bumblebee HEC-2 running in deltaT mode).  This circulator pumps into the secondary supply manifold that feeds the heating zones through zone valves.  The returns from the heating zones are gathered into a return manifold that feeds into the lower side of the hydronic separator.

The circulator deltaT sensors are placed on the shut off valve on the input side of the circulator and on the return manifold near the hydronic separator return port.

As I said, the results are mixed.  When first starting up, the boiler runs for a good long time to bring the tank up to temperature.  The time depends on outside temperature conditions due to the outdoor reset and the boiler modulation, but it's run between 16 to 25 minutes to get to a quiescent state.  Once in this state, however, the boiler still seems to burn for a very short amount of time. Again, exact time depends on outdoor conditions,  but it's almost always less than a minute. 

I measured the return from the tank into the boiler and it seems to stabilize at an almost constant temperature that is just above the boiler's low trip point.  The boiler has about 7-8 degrees C hysteresis, and seems independent of the outdoor temp (the trip points move with outdoor temp, but the difference between high and low remains the same).

The boiler, when it's burning, is definitely pumping hotter water into the supply side,  you can see it rise on the boiler's output temp gauge, I've measured it at the zone supply manifold, and the circulator adapts its speed as the temperature rises.  However, the boiler's output temperature seems to reach the high point very fast AND does not seem to be solely driven by the return temperature (which is always about 6-7 degrees C lower).

One thing I did discover, after tearing my hair out and replumbing the tank though practically every permutation (the tank has four ports which I tried to use for both primary and secondary ports), was that the boiler has an internal "automatic" bypass between the supply and return, just above the respective shutoff valves.   Thinking that too much heat was circulating in the bypass loop and that this might be adjustable, I looked for an adjustment control.  There is none.  The only thing that appears 'automatic' about the bypass is that it is ALWAYs bypassing, the only 'control' is that the diameter of the bypass tubing appears about 3/8" vs the 3/4" piping I'm using for the hydronic loops.  

Other than providing a safety in case the supply and return shut off valves are closed, does anyone know the purpose of this bypass loop?  And, is there any way I can limit it so that more of the return heat is used?

The main reason I'm getting any reasonable use from this setup is that I've modified the boiler burner 'off' time limit from 3 minutes to 6 minutes thereby limiting the number of cycles it goes through in an hour.  It sure would be nice, though, if the buffer tank actually served as a thermal mass for the boiler.  Now it's nothing more than a storage tank, proving heat to the secondary loop when the boiler is not able to burn...

Tom

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03 May 2013 01:25 PM
If Tom was to plumb the tank set up differently.
Boiler teed into the pressure relief valve. Boiler return from the lower heat element boss.
Secondary supply from the "hot" outlet. Secondary return to the "cold" inlet.
Fire the boiler using the tanks internal aqua stat. This totally isolates the boiler from the system. Just heats the buffering tank. Should allow boiler to condense at all times. Should give about 29 - 30 gallons of water for the boiler even if there is no heat demand.
Would it work? Would it work better or worse than his present setup??
TomWSUser is Offline
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03 May 2013 04:50 PM
Your suggestion has merit IF I was willing to store heat when there is no call from the zone thermostats and ignore the outside reset modulation control (since the tank temp would be controlled by the tank's mechanical thermostat). That seems counter to the supposed efficiency that this boiler should produce with existing controls.

As I said in my last post, I've tried plumbing the tank in various ways using virtually all ports except the TPRV port. You'll notice that the upper side port is at the same level as the TPRV port - in various trials I've plumbed either the supply or the return to this side port with virtually no difference in behavior.

The problem isn't with the tank or the ports used, the problem is that the boiler, for some reason, is bleeding too much of its heat output through its internal bypass tube and shutting off the burner prior to the tank temperature rising above the low set point.

If ALL of the heat output from the boiler did, in fact, circulate through the tank, the system would have the expected thermal mass and 'should' operate efficiently.

Tom
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03 May 2013 05:08 PM
I haven't read all you wrote, but you are sure that the boiler is getting enough flow to move the heat it is producing?
TomWSUser is Offline
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03 May 2013 05:25 PM
I'm not sure I understand your question. The boiler, Laars Mascot HT330, has an internal circulator pump (Grundfos UPS 15-62), which is capable of moving up to 12GPM. There is very little head in the primary loop so it should not be restricted in any way. I've also measured the temp coming from the supply side and it does increase significantly when the burner is on. Also, when the burner is off, with the primary circulator running, the tank's heated water does flow through the boiler and the boiler temperature reading matches the tank's temperature. So there is flow. It appears as if there is too much bypass flow causing too much heat to return to the boiler without circulating through the tank.

Tom
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03 May 2013 06:07 PM
A wild guess - the bypass is there to allow the output of higher temperatures. But the valve that controls it is defective.
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04 May 2013 12:28 AM
I may be missing something but why are you drawing the return to boiler off the top "hot"? Would it not make sense to draw the colder water from the bottom to be reheated?
Also it would seem that if there is a lot of bypass the must be some restriction in the 3/4 primary. The difference between 3/4 and 3/8 is pretty significant.
Also check to make sure that the tank does not have any internal baffles that might be messing with the flow. The element hole and the TPRV port will not have any.
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04 May 2013 06:53 AM
This boiler is a Baxi in disguise. It should also be properly set up with a combustion analyzer as I have see some pretty weird operating Baxis when it hasn't been set up. There is an optimum flow rate through the HX and it may be that the pump can be slowed down a bit if need be. That boiler should be able to run on minimum fire on the outdoor sensor just to maintain the temp in the tank which, given the amount of baseboard you have, is needed. Without the tank, you will have a LOT of cycles /hour. Insulate the tank really well and the primary loop piping to reduce the losses.

I think there is a theoretical point to changing the in/out on the tank top to bottom but I don't think there is any going in going from the top port to the PRV port.
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TomWSUser is Offline
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04 May 2013 08:03 AM
Mike,
thanks for the reply.  One thing I remembered last night is, when I installed the boiler, the distributor mistakenly sent me the pipe fitting set for the HT 1330, rather than the fitting set for the HT 330.  I had already plumbed the supply side when I realized the error and was able to use the valve by capping off the "cold water mains inlet". 

I wouldn't think that this would be a problem, but, at this point, I suspect anything and everything...  What do you think?

Here are the photos of the different fitting sets for reference:
HT330:  The supply valve is labeled 'K'.


HT 1330: The Supply valve is labeled 'L'.



Tom
TomWSUser is Offline
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04 May 2013 08:25 AM
Posted By FBBP on 04 May 2013 12:28 AM
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Also check to make sure that the tank does not have any internal baffles that might be messing with the flow. The element hole and the TPRV port will not have any.
The tank is a pretty inexpensive electric hot water heater.  I doubt that there would be any internal baffles.  I have plumbed and re-plumbed the supply/return on both the primary and secondary loops in multiple ways, including swapping top to bottom, without any significant difference in behavior.  The current setup is the one recommended by the consensus of the guys commenting back in Sept.

The only thing that 'helped' get some benefit from the tank at all was increasing the delay between burns. 

There is no adjustment (that I can see) for the bypass.  But, maybe, as you suggest, there is some restriction in the supply valve (all the other plumbing has been double checked or actually replaced).  See my previous post on this question.

Tom

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04 May 2013 07:59 PM
There isn't much difference between the two except for the DHW option. I guess you will need to go back and get the extras. The threads are BSP parallel thread so our fittings don't work on them except for the gas. You may get something to screw on but it is not a good fit. IIRC, the internal pump is a 3 speed and the manual MAY tell you that you can turn it down IF the gas valve has been reduced on max heating output to the approximate heat load of the house. You will have to check on this though, as i have stopped using Baxi boilers a few years ago and there may be a few changes.
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04 May 2013 09:26 PM
A pair of vise grips would reduce that 3/8 bypass flow down to about zero.
TomWSUser is Offline
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05 May 2013 08:45 AM
Posted By jonr on 04 May 2013 09:26 PM
A pair of vise grips would reduce that 3/8 bypass flow down to about zero.
Yes, you can believe that I HAVE considered exactly this 'solution'.  A small C clamp would give me 'one time' adjustability...

Seriously, since I have control over the use and service of the boiler I don't see any risk of doing that as a last resort (other than possibly splitting the tubing - that wouldn't be good - it would be difficult to replace/repair). 

I think I will remove the supply  side valve and thoroughly check the flow through the valve and port.  If I don't find an obstruction and there aren't any other recommendations, I'll probably squeeze down the bypass tube.

Tom

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05 May 2013 08:54 AM
Posted By MikeSolar on 04 May 2013 07:59 PM
There isn't much difference between the two except for the DHW option. I guess you will need to go back and get the extras. The threads are BSP parallel thread so our fittings don't work on them except for the gas. You may get something to screw on but it is not a good fit. IIRC, the internal pump is a 3 speed and the manual MAY tell you that you can turn it down IF the gas valve has been reduced on max heating output to the approximate heat load of the house. You will have to check on this though, as i have stopped using Baxi boilers a few years ago and there may be a few changes.
When I installed the boiler, the distributor did finally send the correct fitting set for the HT330 so no problem on the fittings other than I kept the 1330 Supply Valve since it was already plumbed in.  I do have the HT330 valve 'somewhere' but I think I may need to re-fit the copper to use that instead of the HT1330 valve.

Regarding changing the circulator speed, I think you may be right, I think I can change that parameter.  Are you thinking that the current speed may be too high and the bypass is bleeding because of that?  This implies that there is a pressure differential valve in the bypass loop.  Do you think there is one?  I thought it was just a passive tube.

Tom

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06 May 2013 07:05 AM
Odd that I don't recall ever having the problem of a bypass that I cannot adjust, unless there is a part missing. There is an optimal flowrate that the boiler likes with just enough turbulence without too much pump head loss. I have to find my old manuals to be sure about the values but I really think finding someone with a combustion analyser is useful as they were not designed to run without the set up.
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06 May 2013 10:17 AM
Posted By MikeSolar on 06 May 2013 07:05 AM
Odd that I don't recall ever having the problem of a bypass that I cannot adjust, unless there is a part missing. There is an optimal flowrate that the boiler likes with just enough turbulence without too much pump head loss. I have to find my old manuals to be sure about the values but I really think finding someone with a combustion analyser is useful as they were not designed to run without the set up.
Mike,
thanks for looking into this, I appreciate it.

I did have a heating contractor analyze the combustion back in Sept (he's the one who recommended removing the outdoor reset
I'll have it rechecked now that I can get the boiler to burn longer.

Tom

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