WHat type of Superstor
I'm looking to close on a house with a dead oil boiler and dead electric water tank. I'm thinking that the best route is a new oil boiler and changing out the electric tank to a superstor on its own zone. Would this be a better solution then the electric tank? And it looks like there are 2 types of superstors, glass lined and stainless steel. Which superstor is better? The house is in Massachusetts. Thanks. :)
A new, high-efficiency boiler is a good match to an indirect WH. I had them install the SS Ultra (the SS version). In theory, it should last a very long time. Note that you typically can get by with a smaller tank than a typical electric because when they set the thing up, they normally make the WH a priority zone, so when needed, it gets the entire output of the boiler which is usually a lot larger than any electric tank you can reasonably buy. This means it can heat the incoming water and give more gallons in the first hour than either an typical electric or gas-fired WH.
Depending on the heat load calculation and the minimum boiler output available you may end up having to being oversized on the heating end with an oil burner no matter what, in which case you're combined-system efficiency will be much improved if you use a reverse-indirect/buffer tank (like the Everhot EA series, ErgoMax, TurboMax, etc.). By using the hot water heater as a buffer tank the partial-load efficiency performance of the heating system typically increases by double-digit percentages (and you can micro-zone the place without running into short-cycling issues.)
Originally Posted by caddyroyalty
But whatever you do, DO calculate the heat load (manual-J or other), don't guess or use X-btus/ft^2 estimates. And if you're planning to fix up the insulation or windows, use the post-fixed criteria for the calc. Most homes I see in central MA have heating systems at least 2.5-3x oversized for the heating load. Unless your oil-fired boiler can modulate to under 25kbtu/h (and it's one or two large heating zones) you'll see measurable seasonal efficiency boosts out of a buffered approach- might as well make the hot water heater the buffer if you can. The only downside to buffering with a hot water heater is that the lowest temp you can let the buffer run at is ~130F. In low-temp radiation setups (like radiant slab floors) you give up some condensing efficiency since your radiation return temps into the bottom of the tank won't drop much below120F (if you have a condensing boiler), in exchange for lower cycling losses. Buffering minimizes startup wear & tear on the boiler too, another benefit of lengthening burn cycles & eliminating short cycles.
Last, and probably least, but if it works for your configuration it's better to do it now than wish you'd done it later...
A drainwater heat-recovery heat exchanger of decent size (like PowerPipe or GFX) can effectively double your first-hour rating on any tank for showers (but not baths, since the drain & potable water flows have to be simultaneous to get the heat-exchange). This can affect boiler sizing, since peak loads on the hot water end are now cut in half- it's like having another 25-30kbtu/h of burner behind it while you're showering. It can even affect your control strategy on the indirect, since in most cases it won't need to have priority zoning (or heaing zone-suppression, with the reverse-indirect/heating-buffer approach).
For 4-showers/day use its worth ~50 gallons/year on heating oil in MA, and if it means you can drop 25K of burner size off the boiler it can result in another ~25-50 gallons lower fuel use, since a smaller boiler runs at a higher duty-cycle, running closer to it's steady-state thermal efficiency. That adds a few percent to the as-used AFUE. If done perfectly you can often exceed rated AFUE efficiencies if your radiation design-temps are reasonable (160F or under.)
It's a lot to think about, but whenever you're ripping out both the hot water & boiler, now's the time to get it right, since it'll likely be around for decades. Many/most heating systems are operating double-digit percentages lower than the AFUE rating on the boiler, often 25% lower. Getting it right means money in the bank up front (smaller boiler), and dividends in efficiency for the life of the system. Buffering the heating system, and keeping the burner as small as necessary for supporting the load has HUGE efficiency benefits in the end.
With a low-mass boiler and a buffer tank for an indirect the partial-load performance is similar to the steel-boiler w/purge control (system # 3) tested here:
Look at Table 3 carefully- note how much efficiencies suffer going from 2x oversized to 3x oversized (and how little the purge-controlled system suffers relative to the rest.) Smaller really IS better!
With a mid-mass boiler it'll be somewhat lower