Critique my tentative system/piping

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Feeling the Heat
Dec 10, 2007
After many hours of research and contemplation, this is what I have come up with for my hydronic heating system design. Would appreciate any comments before I start the install...

A few details:

Wood boiler is a Econoburn 200 sending water from a dedicated woodshed outbuilding 30' underground to two stacked, 500 gal propane tanks in the basement of the house via a variable speed pump (190F water to storage and/or to the loads, return 80F-90F water from the bottom of storage, through 2" tank headers designed to maintain stratification.)

There is a propane back-up mod/con boiler on outdoor reset feeding a 100lb repurposed propane tank used as a buffer to limit short cycling. There are large supply and return headers from this buffer which service an indirect water heater (120 gal thermostore which is also plumbed to a heat pump hot water heater for summer use), a plate heat exchanger to cycle water through a Japanese ofuro (big bath tub), radiant in floor load 1 (basement), and radiant floor load 2 (1st floor). All radiant is in concrete slabs. Pumps for the loads are constant pressure, variable speed pumps feeding manifolds with actuator zone valves. There are 8 zones (2 in the basement and 6 on the first floor). The microzones necessitate the buffer tank on the mod/con boiler. The headers are also connected to storage, which feeds the loads through an outdoor reset controlled injection pump. This common header will allow the feeding of all loads from storage if temps there are available, else from the back-up boiler when they are not. Indirect heater load and plate HX loads will be priority ordered.

The zones are controlled with wifi enabled thermostats that will allow monitoring the temps remotely. Also plan to have a wifi enabled temp logging system to remotely monitor temps at roughly 8 points in the system.

This is the simplest configuration I could come up with given our needs. I thought about a tankless coil in the storage tanks and feeding the mod/con into storage to get rid of the buffer tank and indirect water tank but the storage tanks are a fair distance from the mod/con and I like the option to use the heat pump hot water heater in the summer as it also dehumidifies. I plan on a decent sized solar electric install in a year or 2 as well, which will run the heat pump hot water heater without wood or propane.

I have completed installation and rough plumbing of all the manifolds and would appreciate comments before I start the next phase of plumbing all the pumps. Thanks!

hyeating system.jpg
Econoburn 200 is rated at 200,000BTU/hr. Underground piping is 1" pex. Pumping water at a delta T of 100F means I only need to move 4gpm at rated output. 1" pex will handle twice that. That's of course assuming my horizontal, large diameter headers, stacked tanks, low flow velocities, etc., will promote excellent stratification.
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Boiler protection is handled by pump P2 to storage. It is variable speed and controlled by a sensor on the boiler return. It pumps just enough to/from storage to keep the 80-90F return water from mixing down the 190F water in the primary loop below boiler condensing temp (140F)- it can be set to- whatever between 140F to 170F. I anticipate setting it to 150F as that will give a delta t through the boiler of 40F, which Econoburn tells me is ok. I can raise it to 160F if necessary...Tekmar 361 mixing controller.

The storage tanks have large, short headers for plumbing. The supply/return from the boiler is sent into/out of the tanks through the end of a large, short header, which continues into the tanks as a long horizontal dip tube with many small holes to lessen the mixing in the tank and preserve stratification. The supply/return to the load header comes off a tee on the tank header. Maybe my poorly drawn schematic is the issue.
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Your schematic is ok I am just trying to make sure you don't have a bottle neck with the 1"pex. There is a period during the burn when I have an output of more than 200,000 btu with my 200,00 rated boiler. It generally will last about a half an hour. You would need to maintain a 50 degree delta through the boiler with 8gpm at 200,000 which would mean a return of 140 tops. Push beyond 200k and your boiler may idle depending on what it allows for a high temperature.

I have my storage set up horizontal. It stratifies very well. I have a 1.5" supply pipe that angles slightly up and a 1.5" return in the other end which angles slightly down. I can have 80 degree water at the bottom all the way through a burn.
Even if the boiler is outputting 250,000 Btu/hr, with 90F return water, a 100F delta T will require only 5gpm. 5gpm at 90F into the primary loop would require 11 gpm of 190F water circulating in the primary loop to mix to 16 gpm at 160F. That makes for a 30F delta T through the boiler....

Good to hear you are seeing 80F temps at the bottom of the tank through the burn. That gives me some confidence that this large delta T approach, with low flow rates can work....
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Two short comments:
The math on your underground piping all assumes that you never really fully charge your storage. And I believe it also is predicated on your low temperature returns, which appears to be a safe assumption with radiant floors. I also have a system where the bottom of the tank is around 80 degrees (panel rads on ODR), and my flow rate (into storage) is about half the 4-5GPM you are calculating, which makes sense as my boiler is half the output power. But, and the big but, is what happens when the bottom of the tank reaches your anti-condensation temperature of around 140 or 150 degrees? Then the flow through P1 and P2 match, and I suspect that has to be around 10 GPM at a 40 degree rise @ rated boiler output. As the return temperature increases, so does the boiler output until the boiler controls start throttling. And throttling is not what you want.
What kinda saves you here is you hope the fire is starting to die down and the thermal output from the boiler is also decreasing. And those low return temps and hopefully a sharp thermocline. If you ever got into a situation where the entire tank was 150 degrees throughout, then you would be in quite a pickle. You likely couldn't move heat away from the boiler and into storage because of the piping/head loss limitations.
I run my system in such a way that I never go through the "second lap" of charging storage, and I use a conventional thermovar and a single boiler/storage pump. I only use about 700 gallons of my storage, even on very cold days, so the thermocline is never pushed to the bottom of the tank and the tank return is always at the return from the heating circuits. One could say the 1000 gallon tank was a waste, but that happens to be the size that tanks come in, so I have one.
Second comment - you already have an enormous buffer tank, why not use it for the mod-con? You could use just a portion of the tank, pulling the return from some mid-point, but with a condensing boiler I don't think that makes much sense. I think I see it now, you are using P8 as an injection pump, meaning the headers you show are actually ODR controlled to a lower temp than storage. I dunno, maybe I'm confused. On that topic, how is your indirect going to like being supplied with ODR temp water, especially during the shoulder seasons where the ODR target is really low.?
The system as drawn looks good, I think the next thing I'd do is write paragraph descriptions of how the system works with:
1) Wood boiler operating
2) Wood boiler and propane boiler off, and heating from storage
3) Propane boiler on
Like what is regulating the supply header to an ODR temp when the propane boiler is running - obviously it has to be on a ODR controller. So is there one ODR controller for P8 and the Mod/Con? or two controllers? Would the Mod/Con be more efficient if it were operating at a higher output temperature with the cold storage return? I dunno. And I'd think really hard about what happens when the heating load is very low in each of those situations.
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That second lap through storage can really change things up in a big hurry. And if trying to do it all in one lap, that doesn't leave much of a safety cushion between working OK, and the thermocline getting to the bottom and suddenly you have water that is too hot going back to the boiler. You would have to really make sure the fire is out before that happens, which would likely mean stopping before storage is charged and wasting a bit of storage space.
Thanks for the comments, much appreciated.

Yes, the entire system is built on the assumption of one lap through storage with excellent stratification. Indeed, if return temps from the tank climb too far above 140F, the pump will not be able to move the boiler output to storage and the boiler will go into idle if it is at or near full rated output. Clearly, there needs to be a strategy to keep this from happening....

In addition to doing everything possible to aid stratification, I plan to weigh wood when firing to make sure I am not trying to put in more btu's than there is room for. As noted, I am planning on "coasting" to the finish line, with the fire dying down and out as, theoretically, the last gallon of 90F return was is pumped to the boiler. It will probably be more difficult to figure out what is required when the loads are drawing on the end of storage. I will probably have to come up with some algorithm based on tank temps, wood weight, outdoor temps, etc. to leave as little wasted storage space as possible. I understand that practically, I may have to leave some amount of storage unheated. I am hoping it won't be too much.

It was raised that I could use storage as a buffer tank for the mod/con boiler. There are two reasons I decided not to do this: the first is simply logistical- it would be a real pain to plumb the mod/con into the storage tanks due to their location in my basement. The second reason is I don't want the return temps from the indirect DHW tank messing with my storage stratification.

The mod-con has odr and a priority function so I will use it to heat DHW. I won't be running the injection pump or load pumps when the indirect or tub circulaters are running. The injection pump has odr as well. The mod-con is for backup and dhw. The buffer tank takes care of mod-con short cycling when the loads are below the minimum mod-con output, which will happen in the shoulder seasons.
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