Questions about next year's project

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You would need a flat plate heat exchanger to heat pressurized storage with an open boiler. If that is at all a strong thought (the future 250), you might be well advised to incorporate the FPHX right off in your next step, at the boiler. Then make everything else pressurized.

Not sure on your 2 horizontal tank question. Do those make up the first 500 you spoke of? I have two 330's, horizontal, stacked. Looking from the (side), flow goes from the boiler into top of top tank (top left), then out the bottom of top tank right into top of bottom tank (mid right), then out of bottom of bottom tank back to boiler (bottom left). Sort of a top down, circular flow. Stratifies very well - right now I am 3 hours into a burn & am looking at 169 mid top & 129 mid bottom. I was at 139/120 when I started. Or so. But maybe you meant a different configuration? Stacked tanks would be kind of tricky in a crawl space. If side by each, I might consider parallel piping, but think others do serial there too. I think that would be top boiler to top 1 - bottom 1 to top 2 - bottom 2 to bottom boiler. The mixing you speak of is contrary to what most look for using storage - good stratification.

I'm also reading different things about your garage. I think one spot you mention heating it from storage, then another heating it from the boiler? Could be the wine I had with supper too....

I had two, vertical, tanks in my garage. They, and the log boiler, have now moved inside the house (we built an extension, the original kitchen is now the boiler room )

Some observations from that:

"Waste Heat" from the boiler and thermal storage tanks, now inside the house, is a significant contribution. After the burn finishes (somewhat noisy and pulls cold, fresh, air from outside - don't want it pulling warm air from the house instead!) we open the boiler room door so that spare heat flows into the house. The bedroom above is always toasty warm

My thermal store tanks, in the garage, would drop several degrees a day - i.e. when no heat was drawn off, and when the dT to the ambient temperature was significant - on a cold day, for example. Personally if I was doing it again I would find a way to get the boiler and tanks inside the house envelop, or second best insulate the storage area and if possible have a means of that heat "leaking" into the house - e.g. don't insulate floor of crawl space, just the exposed outside walls and the floor.

My tanks are linked from top of primary to about 1/3rd down the secondary (NOT to the bottom) - we refer to this as the "snorkel" pipe. Return to the boiler is from the bottom (of both tanks I think ...). No idea if this is appropriate, but it was all designed by a hydraulic engineer from the company I purchases boiler and tanks from, so "hopefully" best design.

We don't, exactly, "burn to heat the thermal store". Our thermal store is not big enough to hold the whole burn, so we light the boiler around 3pm, force the central heating pump on for 6 hours (which heats the house), and at 9pm-ish, when the boiler goes out, the house is nice and warm and the tanks are at maximum temperature. This was discovered by trial and error, and replaced a simple thermostat which called for heat when the room temperature fell below threshold. What we found, with that setup, in practice was that on mild-ish days the thermal store reached max temperature before the boiler went out (because the house thermostat didn't call for much heat), the boiler fan then shut down for safety and the remaining fire wood then smouldered and was wasted, so we decided that having the house "a bit too hot" was preferable to "smouldering wood wastage". In the morning heat is used from the thermal store for the central heating; if the house is a bit warmer than normal (mild weather and/or house was overheated a degree or two the previous evening) then less heat is used the following morning. That said, our house has relatively high thermal mass, and some of the over-heating in the evening is thus stored until morning; this probably wouldn't work in a low thermal mass building.

In very mild weather we light the boiler every other day which my over-heat-house and store-max-hot-water approach allows. We find it better to have a full-load burn (on alternative days) rather than a half-load burn each day - seems to me that quite a bit of energy is taken up getting the boiler up to temperature so the mixing valve / laddomat opens to the store, and thus a bigger burn spends more of its time begin "productive" - but I fully admit that's purely a subjective observation.

Kristen said:

My tanks are linked from top of primary to about 1/3rd down the secondary

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Correction: 1/3rd down the Primary to the top of the Secondary (Pink arrow)

Found a flow diagram of where the various pipes connect to the two thermal stores, but no idea if this translates into what you are trying to do

warno said:

If I go with the flat plate heat exchanger to heat all my storage, how do I size the heat exchanger to get my storage up to usable temps. I don't know yet what the house will require but I know the garage needs at least 150 degrees for the water to air HX lower then that it doesn't work all that great.

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Here is a free HX sizer. You need to know some temperatures and flow rates to get an exact number and size.

www.flatplateselect.com

The connection to the storage tanks is important. I'd suggest a two pipe connection that allows immediate flow to the loads without first needing to have the tanks warm.

Tall skinny tanks are best for stratification, it is possible to see a 30° stratification or more. With a typical 30" diameter horizontal tank, don't expect much stratification spread.

Some tank sizing and piping ideas here.

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_17_na.pdf

warno said:

how do I size the heat exchanger to get my storage up to usable temps

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May, not be relevant, but my storage tanks are directly heated from the boiler, and pressurised. I assume that there was a benefit to doing that, rather than having a plate heat exchanger transferring heat into the tanks - perhaps, because I have a batch log boiler rather than, say, pellet it is important that the heat from the boiler can always flow somewhere and maybe a PHE would introduce some risk in that sense?

My original central heating circuit was open-vented and we did not want to change it to become pressurised (in case the old pipe joints couldn't withstand the extra pressure) so we put a PHE where the old boiler used to be (taking heat from the Store and transferring it to the Central Heating circuit). The PHE was sized to be the same as the old boiler had been

But, that said, I do have coils in the Thermal Store for the Solar Hot Water panels - but they transfer relatively little kW compared to the size of my store, and it would be a miracle if they were able to overheat the store (but I do have a heat-dump to swimming pool if that were to occur)

A consideration. While Kristen has put her storage tanks inside the home, her tanks are very well insulated from the factory. Bare tanks will work against your comfort except for the colder stretch of winter. In my case, I have 4-100 pound propane tanks (approximate 27 gals ea) laying in the crawl space. Those tanks will be like loaded freight trains. Slow to get up to temp and slow to cool when there is just a moderate demand. If there is room for insulating the tanks, go for it. My guess, even while insulated, they will be emitting some good btus. Windows open at times for temperature balancing.

BoiledOver said:

tanks are very well insulated from the factory

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Even with that they still lost several degrees, in 24 hours, on a cold day when no heat was drawn off. This was contrary to what I was led to expect, so my advice would be to assume that there will be losses, if the tank is in an outhouse. My losses are less now that the tank/boiler is within the house, because the dT is smaller than "outside", but more import, to me , is that I no longer have to worry about them because any heat-loss is now space-heating

When we had the tanks / boiler in the garage I never worried about heat loss because we had done a good job on insulation, and I just assumed that the higher-than-expected heat loss was "Sales Hype" so it was only when we moved the tanks indoors, and I then was able to compare the wood-saved, that I realised just how much it was.

We put considerable effort into insulating the tank / pipes in the garage; we did not put effort into insulating the boiler area itself (assuming that Tank and Pipes being well insulated was enough). So at the least if planning on an external installation then leave plenty in the budget for insulation, and insulate the whole area as well as both tanks and pipes - depends on what you value your time at, compared to the cost of the insulation of course. Even with plenty of wood available there is still the cut, split, stack, carry, load time

warno said:

I did notice though they only talked about vertical storage, which obviously is ideal, but like I stated above I don't have head room to do

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Seems to me, but I'm a layman, that multiple horizontal tanks, connected "top to bottom" so-to-speak, will do a good job of temperature separation (from tank to tank, rather than vertical stratification within a single tank), and that it is a single, horizontal, tank that would be the problem. The trick will be to be able to load the tanks without mixing (maybe that is not possible / necessary, if loading will be done with a lot of BTUs over a relatively short time), and more importantly to draw off the hot water without de-stratifying.

I have always planned, but never tried it, to have a valve that directed Return water to the right position in the tank so it was mixed with water of a similar temperature. When my central heating comes on there is a huge volume of cold water that is returned, from the radiators and pipes, before the return starts to be warm. That cold water could always be put into the bottom of the tank, but once the water starts to return warm I think that would be better returned a bit higher up the tank, above the cold layer. But I have no idea whether this is actually worthwhile in practice, or adding complexity, and increase numerable of things that can break / go wrong, only to save a few cents ...

I also have solar hot water panels connected to my thermal store. They always draw water from the coldest bottom of the tank, as they are most efficient heating cold water, rather than already-warm water, and they return to the top of the tank until it reaches 60C and thereafter the bottom of the tank.

warno said:

space that measures ... 32 inches deep. The tanks ... 30 inches diameter

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Presuming the tanks are not already insulated? not much space to get some decent insulation around the top / bottom ?

How hard is the digging down there?

I think in that spot, the floor space under the tanks will be quite important to get well insulated. And with the measurements your mentioning, that's not room for much. Otherwise the cold ground will be sucking up what might turn out to be a fair amount of your heat. I have a couple of patchy layers of 1-1/2" foamboard under my tanks, which are sitting on a concrete basement floor that has no insulation under it. I think when I was burning for DHW only, in the summer, that a lot of my standby heat loss was downwards, into the concrete & ground underneath.

I think with the right flows you can get decent stratification with a horizontal tank. Key being, slow load draws. I have two stacked, and regularly see 30° stratification (158/129 at this moment). And with the way the temps change quite quickly at times, it tells me there's a pretty well defined stratification layer in there. But my load flows are pretty slow. Example - one of my upstairs zones (the one our bedroom is on), has a bad end switch in its zone valve. I haven't & won't bother changing it out - when it opens, it convects naturally at slow rate (if it's the only zone open), and keeps our bedroom (first room on the zone, followed by bathroom) warm overnight just fine. Which helps the overnight stratification thing greatly. But I would think you might have challenges maintaining stratification with the draw of a WAHX - unless it was significantly oversized.

my recommendation is that you don't skimp on insulation. You want to keep every last, possible, degree inside the tanks, and to that end I would aim to use more than the recommended thickness, rather than less under any circumstances. I'd also want to keep the wind off it (the outside of the insulation). Might be that your crawl space is not open to the outside, but if it is I would put some sort of baffle between the outside and the tank - air moving over the insulated tank will steal heat faster than stationary air.

Yes that would be a hazard with digging a low spot - you would also make a sump that would collect wet. So ideally lower where the tank(s) will sit, then graded to an even lower spot that would drain, or hold a sump pump. Could make for a lot of digging & dirt slinging though.

I think I would go the parallel route also.

But at the same time, slowing down the flow is a bit more important in the heat load draw than the boiler charging push (so to speak). At least it seems so for me - stratification keep the hottest water at the top for loads to draw from. Thinking of the top ports where the boiler supply comes in, maybe something like you mentioned there but with the supply T at one end of the connecting run rather than the middle, but have a T out at the other end that loads pull from. Then the bottom connections would be the opposite way. Sort of a 'reverse return' thing. But you would want there to be a more direct path (less head) from storage to load than there is from storage to boiler, so that when loads are pulling and there is no flow from boiler, it will pull through storage rather than pull water around the cold boiler-storage loop.

(Not a pro here BTW - just someone who's cabbaged their way through something once )

Only you know what exactly you are dealing with here...but from looking at your picture it makes me think maybe the best thing would be to do this once and do it right. If you are going to have to pull down some of the wall anyways, can you get a lawn mower sized Dingo digger...
in there to dig things out for real, like maybe enough to get tanks vertical? I have seen people do this under old farm houses, leave the existing foundation alone, come in a foot or two and then dig a "basement" with a smaller footprint than existing footer, pour a concrete floor and put up block "retaining" walls all around the side at least up to the dirt level. It would be pretty easy at that point to install a sump pump crock too. Then go ahead and install a Bilco door and steps to have easy access once you are done...just throwin out ideas here...

warno said:

Would it be wise to charge both tanks in parallel from the boiler then pull from them in series for my heat load?

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I have no knowledge, other than what my installer (with the help of a hydro-designer) did for me.

My boiler has a mixing valve (back to itself) so that it gets warm water to heat at all times (for efficiency, and boiler-life)

There there is another mixing valve that draws from the middle of (both) tanks, or from the bottom (of both tanks). I presume this is so that once the boiler is running well it heats the top "half" of the tank first, until the warm water starts to get too hot, then it draws colder water in from the bottom. I expect this is so that if the burn does not fully charge the tank then the top part will be hot and stratified.

The two tanks are connected by a pipe from 1/3rd down the primary to top of secondary - so its pretty much parallel charge, but with the top of the primary being charged as a priority. We have a DHW pre-heat coil in the top of the primary, so this arrangement may have been done to prioritise that.

There is a photo, and flow diagram, here:

https://www.hearth.com/talk/threads...and-5-000l-thermal-store.151512/#post-2032757

What surprises me is that there is not the same arrangement for the return from the central heating. I think it would help if that was returned to the tank at a number of different points, up the tank height, depending on temperature, rather than "warm" return water being mixed into a point in the storage tank where the water was "cold"