I'm a DIYer looking to install a condensing boiler. I have an engineering background, and I've concluded that there are few things I'd enjoy more, and getting the average contractor to get it right is hard. One wanted to run an indirect tank off of the hot water connection of a combi boiler, and another didn't understand that the system need a buffer tank.
Based on a quote from one contractor I've become very familiar with the Laars Mascot FT, and plan to use a Laars MFTHW-199 with a Techtanium TT-79. (I've read the Laars install manual several times, and have gotten a bunch of other information from their fabulous customer support, in part to convince a contractor that they were proposing the wrong things, so it's the easiest brand for me to use at this point.) This is massively oversized for my heating needs, but I'll soon have 5 adults and two children taking showers, and it's only modestly oversized for HW demands. I'm not a strong believer in buying the smallest boiler possible, though I realize there are many advocates here. I can't buy one small enough to avoid a buffer tank, and the extra cost is minimal. (My smallest loop--which runs all the time--emits 7000 BTUs. It will emit even less if I modulate the temperature down.) Only the 199 has a 10:1 turndown, so my minimum firing rate will actually go up if I stick with Laars and choose a smaller boiler.
1) The Lochinvar Knight WHB boilers seem like roughly the same thing but for twice the price. I don't want any of the extra features except for the programmable ramp-up. (As an answer to question 3 below.) Is there some important difference in quality and reliability, or is it mostly different features that explains the price difference?
2) Is the HTP UFT basically identical, except for the lack of a pump and 10x turndown for almost all of their models?
3) My plan is to have a buffer tank attached as a secnodary loop via a circulator. This will allow me to switch it on only when the loads are less than the minimum firing rate for the boiler.
It seems like every buffer configuration that I've seen--including mine--will cause the boiler to operate at max power. In the classic case, where it acts as a hyraulic separator between the primary and secondary loops, you have a bunch of cold water just sitting in the primary loop. The primary loop will pump it out at high speed into the boiler, and the boiler will work as hard as possible to bring it up to its target temperature. Once the boiler shuts down, the buffer tank it does it's work, letting it's heat out into the small load and preventing short cycling. This is bad in two ways: a) if the cold water could be parceled out just quickly enough to get the boiler to fire at it's minimum rate, you could get away with a tank about half the size and b) the boiler will fire at its max rate, lowering its efficiency. I was thinking about using a Taco 007-VDTF5 to make this happen. (Unlike other delta T pumps that I'm aware of, it can operate in an inverse mode, where the pumping rate increases as delta T goes down.) It could switch the pump on as required to maintain a delta T of 5 degrees across the boiler. The Laars pumps at a fixed speed, with a delta T of 20-25 degrees at max power. (I know this because of their excellent customer support.) This doesn't quite get the job done, as this will target about 20% power, not the 10% minimum firing rate.
Has anyone ever solved this problem?
I'll may punt entirely, and just use an HTP SSU-20B as a hydraulic separator like normal people. I think I'd have to switch away from the Laars, which has a built-in primary circulator, so that the buffer tank won't be used while the HW loop is operating.