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We prefer more flow to the system. If the boiler, or the pump serving same, is grossly over-sized, the net results are hard to correct.
Too low of a flow would do several things: not heat the room unless the inlet temp was higher, or produce less even heating. There are a lot of tradeoffs between, efficiency, comfort, and a long-lasting, trouble-free install. It's hard to overcome a too big boiler's part in this balancing act.
Jim DeBruycker
Important note - I'm not a pro
Retired Defense Industry Engineer
Thanks to this forum (mainly Dana's advice), the boiler is not over sized. Here's a link to the sizing thread: http://www.terrylove.com/forums/show...load-calc-help
Everyone locally recommended 120 or 150k boilers, I went with 85k-despite dire predictions of freezing this winter which has not turned out to be the case. The boiler often often cruises at 20% (min fire) on milder days in the 30's when only one zone is open. Even with 4 out of 5 zones open (the most I've seen at once) it runs at 70-80%. So I'm glad I didn't listen to the local installers.
The pump serving the boiler came packaged with it, so I would hope it is correctly sized.
Last edited by Handymaner; 03-17-2013 at 04:56 PM.
Well, I've found a way to measure delta t that's accurate enough. I have 2 of those digital thermometers that have a lead to place out through the window (for outside temp). By putting the lead under the pipe insulation in the area I want to measure, it's consistent enough. It's not a true representation of the actual temp inside the pipe, but for for delta t should work.
I have proved my theory that I'm getting significant recirculation in the primary loop. I've got one sensor on the return line just prior to the closely spaced tees, and one just a few inches up the return side of the primary loop. With just one zone open, I get 7-9 degrees difference (hotter in the primary loop than the return water). The difference decreases as more zones open, with 3 open there is 3-4 degrees of difference. I'm assuming with more than one loop open there is less head pressure and therefor more flow in the radiation loops thus recirculating less of the already heated water back to the boiler.
I think this will significantly effect my efficiency. At times it keeps me completely out of the condensing range, and it always reduces efficiency. I'm not sure the best way to fix it. The boiler (Lochinvar Knight) has a readout of primary pump percentage. Last I looked at it, it was at 35%. Does anyone know if it actually controls the speed of the primary loop pump? It's the pump supplied with the boiler, Grundfos UPS 12-28 with 3 selectable speeds (a slip of paper in the box with the pump said to use speed 2 for my boiler size, but I've got it on 1). I tried Dana's suggestion of throttling the ball valve in the primary loop and I could increase the delta t but the inlet water temp did not decrease, the output temp of the boiler increased-which does not meet my goal of increasing efficiency.
I think the ultimate would be a var speed pump for the radiation loops (they are all on one pump) with a simple controller that measures the temp of the return water and the output temp of the boiler, and adjusts the pump speed to maximize the difference between the two. No matter how many (or how few) zones were open the flow would be adjusted to optimum, finding the best combination of lowest return water temp and least recirculation in the primary loop. But I am not familiar with the var speed pumps, what controls them, cost, etc.
Any ideas? Dana, is this similar to the fix you hinted at a few posts back?
By the way, I finally had the city inspector come out for my inspection (I did pull a permit). He was pleasantly surprised, said that hardly any of the homeowner installs pass on the first try! So I was really happy about that. He only had a couple small suggestions for me, easily taken care of. And as to the original subject of this thread, cycles-I'm VERY happy with that. My average burn time since first firing up the boiler (including the short shoulder season cycles) is at 43 minutes!
First, the Knight condensing boiler is too "big" for the load. It is, like so many, bumping off the bottom.
You may slow down the system pump, but if the boiler pump runs faster your problem may not go away.
The Knight is a fire-tube condensing boiler and as such, will suffer a 40°F delta T across the heat exchanger (Many water-tube and aluminum condensing boiler will also). The higher the better, from an efficiency standpoint. If the boiler flashes-to-steam, a.k.a kettles, you can turn the pump up. Fin-tube baseboard is not the perfect emitter for any hydronic system but it has a predictable heat curve and may be properly sized and balanced to any boiler...as long as you have enough to heat the space at design temperatures. We prefer European style panel radiators.
For the next guy:
How to fix this problem on the Knight or any other condensing boiler?...ACCA Manual 'J' heat load analysis and review of the hydronic system as a whole, to include radiation and zone loads, by an experienced hydronic designer. (No, I don't care how long your installer has been throwing in boilers.) Getting a bigger boiler is not the answer, nor is getting a bigger pump. Getting a bigger brain is the thing.
Reading, understanding and following the installation manual is the thing.
Primary/secondary pumping was designed to keep boilers hot! It is not required for most Mod/Con installations (even though Buderus, WeilMclain and others still insist on it). It is, by definition a waste of energy. We have installed many Lochinvar Knights here in Minneapolis and repaired/re-piped many more, with perfect results. We do not use primary/secondary piping for any boiler unless it can't be avoided (My own driveway snow/ice melting system is driven by an condensing boiler and a single pump). We always consider the highest load zones and the radiation therein when designing hydronic control systems. We always pump into low-mass boilers, this includes all copper and electric boilers.
Or you can, I guess, get a boiler that is too big for the space/load and install a buffer tank to lengthen the burn cycle.
Proper ACCA Manual 'J' heat load $500.00
Buffer tank with labor design help $1500.00.
New, properly sized boiler installed. $8800.00.
"Getting a bigger brain is the thing."
I gawdda git me wunna doze sumday, eh? :-)
But seriously, there seems to be a dearth of boiler installers with a grasp of the fundamentals. How tough is it to pick up the book, really? It's not rocket science, but it takes more than napkin-math to dial in the last 5%, even if the napkin model can easily find the (all too common) gross errors.
Do you honestly think it's oversized? On what do you base that decision? With 43 minute avg burns and only "bumping off the bottom" with mild temps and a single zone open that seems pretty good to me. Am I to understand that a boiler properly sized for -6 deg design temp would never run at min fire on a mild day with just one zone open? That seems to be a tall order. I don't believe the next size down would cut it on design day, let alone colder. I'm just ecstatic I didn't (thanks to this forum) accept the multiple recommendations I got from local installers for 110 or 150K boilers, and went with the 85K instead.Originally Posted by BadgerBoilerMN
I've come to the conclusion that I need to speed up the system pump, not slow it. That's what I was hoping to get insight on, before I proceed with changing out the system pump. My goal now is to optimize the system I have and get the return temps down as low as possible. Perhaps I can find out from Lochinvar if I can use a smaller pump on the primary loop to help my re-circulation problems. That's the only way I imagine a smaller boiler would help my situation, if it came with a smaller primary pump.
I also would prefer different emitters, I hope some day for warm board or similar. Thanks again for all insights!
You don't need Warmboard for a retrofit like yours. I would look for a good deal on European style wall-hung panel radiator properly sized to low temperature applications and controlled for optimum comfort and efficiency.
PS Yes, the pump that came with the boiler is over-sized, and no, the boiler does not control the pump "speed"...yet.
Glad you and Dana got it all worked out.
Last edited by BadgerBoilerMN; 04-03-2013 at 01:06 PM.
Hey Thanks Badger. I'll look into a smaller pump and check out those panel radiators. If I could come up with some of the old cast iron base boards I would use them, but I've never seen any up here. Shipping too high for that kind of weight probably.
You can buy panel radiators online if you know exactly what your going for. PexSupply and others even have free shipping on them to US addresses. I'd assume they would honor an AK order, but the shipping might be slow.
They're a fraction of the dry-weight of cast iron, but the can be even higher in thermal-mass. Steel & iron have about 1/9 the specific heat of water, so every 100lbs of iron is the same thermal mass as about 11lbs of water, or about 1-1/3 gallons- it doesn't take a lot of water volume in a panel radiator to have the same thermal mass of 20' of cast iron baseboard.
Do these panel radiators work with low temps suitable for solar? I have some solar ambitions.
They have a predicable output down to very low temps, but of course the lower the temp, the lower the output. Active solar thermal isn't exactly a great match at your latitude, since the solar input is pretty pathetic at about the same time that your loads are peaking, and in the snowier parts of AK keeping the panels clear of snow is even an issue.
From a net-benefit point of view photovoltaic panels have now hit a price point that PV + air-source heat-pump technology is more financially viable than solar thermal, and there are ductless mini-splits with fully specified output at outdoor temps as low as -25C/-13F, that run at a coefficient of performance of about 1.8 at that temp. (That's 1.8x more heat per $ than electric resistance heating.)
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