critique my heating system design

cabowen

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!

Zman

I looks like you have 2 high temp loads and 2 low temp loads which would benefit from outdoor reset. I am not sure I see the advantage to intentional stratification on the storage tanks. I think I would just heat them up anytime the outdoor boiler is running. You should be able to get more total stage out of it that way.

The attached drawing is super rough but I think it gets you there in a simpler way.


I think the ex tank would do better in the middle with all the high head circs "pumping away".

From a controls point of view, I would do mixing valves with outdoor reset. I think you could run all this with a tekmar or similar system. The controller could treat both the mod con backup and the circ coming from the storage tanks as 0-10 VDC boilers. You could make the storage circ the fixed lead and the mod con the fixed lag. The control logic would send the lowest temp water needed to the zone, maximizing efficiency.

I did not draw it but assumed the storage tanks would be tied together.

GroundUp

Well said Zman! I would also reconsider sending 90 degree water back to the Econoburn. You will destroy it quickly without a means of mixing the water temp back up to 140+ prior to re-entry to the wood boiler

cabowen

Maybe a little further explanation is in order:

Stratification is what makes this work- I can use a much smaller shuttle pump to storage and much smaller piping because I am only moving 4-5gpm from the boiler primarly loop to storage- the delta t is 100F. P2, which pumps to storage, is a variable speed pump controlled by a Tekmar 361 to a sensor on the wood boiler return- set at say 150F, the variable speed pump only moves enough water to storage to keep the mix of return water at 90F and the primary loop water at 190F from dropping below the set point of 150F. As the boiler ramps up, the P2 speeds up. As the boiler reaches the end of the burn, P2 slows down, always varying gpm to keep the temp at the boiler return at 150F. When the tanks are out of useful energy, the wood gasification boiler is fired to bring them back up to 190F, where they can supply heat down to whatever temp the outdoor reset asks for, until they can't.

I don't see any benefit in plumbing the storage tanks to the mod/con side of the buffer tank and wonder if the two pumps without hydraulic separation will interfere with each other.

The two high temp loads are priority and when they are being satisfied, the zone loads are off. They will only demand for a short time. There is so much thermal mass in the floors that they can coast for a good while.

There is outdoor reset on the back up mod/con as well as on the variable speed injection pump from storage, P8. When the tanks are fully charged, the injection pump will only be supplying a small amount of 190F water from the tanks to the supply header. As the temps in the tank drop down, it will pump more and more gpms until the tanks run out of "useful" water and are recharged.

It's all about exergy.

Zman

I see what you are doing. I am more of a generate, store, sip energy as needed kind of guy.

Those high mass slabs will eat a ton of energy when you up the temps to meet your setpoint loads. You will actually have trouble getting the water up to temp if you let them take it all. I would put in the mixers.

The way I see your system, you have 2 boilers. I would pipe them as such and take them both through the separator.

hot_rod

Here are some other thoughts. Use ECMs everywhere if you plan on PV power some day. A single boiler circ with a return protection valve would eliminate one circ and control.

Use a differential control to trigger the Econoburn, you get much better control, as shown.

Is the mod con large enough to supply the gpm you want for the tub fill? if so, maybe skip the indirect and use generously sized plate hx, an instantaneous DHW module..

The mod con could heat the very top portion of the stacked buffers, to supply DHW, and eliminate the second buffer, see drawing. You will need to maintain the tank for DHW production, 130 would be adequate with large plate HX for DHW loads..

Pull the loads with ODR to maximize the tank drawdown, these graphs show the usability better with loads pulled via ODR.

Could the two low temperature zones use one circ and ZVs, if the goal is to minimize electrical consumptio, same with high temperature loads.

cabowen

The pv system will be grid inter tied....

The tub will be filled with cold water, then the water in the tub will be circulated through a plate hx fed from the mod/con boiler on priority to heat and maintain temp. The indirect will be for normal DHW.

As I mentioned, I thought about using the top of the storage tanks as the buffer for the mod/con, with a tankless coil for DHW, but I already have a 120 gallon thermostore tank I can use for DHW, and the repurposed propane tank for the buffer tank will cost me about half of what a tankless coil will cost. Additionally, the physical layout with using the storage tanks as the buffer would necessitate longer piping runs and additional coring of interior concrete basement walls. Also, a tankless coil at the top of my storage tanks means I can only get DHW out of my storage tanks down to 130F or so, whereas I can continue to draw water out for my floor loads down to maybe 100F under some conditions, meaning my tankless coil would be useless for making DHW whenever the tanks are below 130F...

ODR on both the mod-con and injection pump are part of the plan.

It might be possible to use one variable speed constant pressure pump instead of 2 but I wanted to separate the basement from the 1st floor for both redundancy (if one pump goes out, I can still heat half the house) and because I would rather have 2 small pumps versus 1 large one as it allows me to use smaller pipes, fittings, etc.

hot_rod

I'm not sure why you are thinking tankless coil? A plate HX can be sized to provide DHW within a few degrees. So a tank at 115- 120F could supply endless DHW at shower temperature of 103- 105F. No need to keep the buffer above 120 or so for DHW production. Use either the tank capacity or the mod con if it has enough BTU to provide the DHW you want. a150 mod con should give you 3 gym or so daw.

I was just trying to simplify using so many tanks, every tank = heat loss.

cabowen

Ok, I see where you are going.

I need more than 3 gpm DHW, and the return temps would mess with my stratification.

I understand about the heat loss. The tanks will be well insulated.

Thank you for your comments.

hot_rod

I’m not sure horizontal tanks will offer that much stratification? Tall, skinny are best.

The control logic I showed will help, by having multiple sensor locations you can enhance the “stacking”

As for Dhw, you only have what you can store, with enough boiler or tank HP you can generate whatever fliw rate you size the HX to

cabowen

Sure, tall, narrow tanks would be ideal, but I don't have the headroom. Horizontal tanks will stratify just fine. I have data from several people who have done exactly as I am planning and they all have had great success with this configuration.

I have a 120 gallon indirect tank. That should be all the hot water we need, and then some. The big hot water suck will be the large soaking tub, and that is handled by a plate hx as you mention. The indirect tank allows me to heat the water with solar electricity in the warmer months with a heat pump hot water heater, which also dehumidifies the basement as a bonus. I don't have high return temps messing with my stratification as well, as I mentioned.