Calling all heating gurus - Considering System Change

[RoryMcQ]

Despite the appearance of being a first time poster, I've been here for years, the forum didn't want to recognize my old name and password. At any rate, this is my ninth winter with a Tarm Solo 30 using unpressurized storage for a heat and DHW solution, with my old oil furnace on standby with a Johnson setpoint controller in my homemade 4x8 storage tank. The tank has a primary heat exchanger for the heating system and another 180' coil for DHW, with a tempering valve as it exits the tank. It works quite well, but I'm considering a couple of changes.
The first would be to add a heat pump /hot water heater in line after the tank, assuming it would dehumidify my cellar and supply my DHW for those few weeks a year we get around here that we're not heating the house.
Having done that, it seems to me I should be able to change my oil furnace from essentially heating the tank when required to sending its output directly to my baseboard system.
In other words, I would still need my setpoint probe in the tank to let the Tarm heat the house when desired, but if the tank water isn't hot enough for my baseboards, I'd like my furnace to kick in and send its output directly to the baseboard system.

Does this sound reasonably doable?

 

[RoryMcQ]

I forgot to mention, I first will install a heat pump/DHW unit in the HW line after the tank. With my DHW taken care of, I would like to be able to heat the house without relying on the tank.

 

[__dan]

Not sure exactly what you asked or how much change you want to make, but ...

I set up a primary secondary piping system. The distribution system is secondary and each boiler, wood and oil, are independent primary pumped loops and tap into the secondary loop with separate sets of closely spaced tees.

Rightly or wrongly, the boiler primary loops are completely independent with no overlap. I was concerned about the miniscule heat loss and took steps to arrange no flow or ghost flow over the other primary. It works but I have not burned a drop of oil since putting the wood online,

If your load distribution is set up as a secondary loop, essentially you would have two primary loops, the storage tank or the oil boiler, and may possibly separate them by tapping each independently into the secondary loop with closely spaced tees.

 

[Bob Rohr]

there are a number of ways to pipe that.

Pipe the boilers in parallel, primary secondary as Dan mentioned, or a hydro separator.

Want you don't want to do is have the oil boiler warm the storage tank.

Some ideas here. Also the "Biomass" sticky above.

http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_10.pdf

 

[RoryMcQ]

I guess my biggest question is whether my oil boiler control system can be set up so that IF the house is calling for heat AND the tank is below the setpoint (say 150F), it will come on and supply hot water for the distribution system. Currently, I think my setpoint controller is essentially serving as the thermostat for the oil boiler, which runs until it warms up the tank enough.
Thanks for the link, I'll check it out.

 

[Bob Rohr]

I guess my biggest question is whether my oil boiler control system can be set up so that IF the house is calling for heat AND the tank is below the setpoint (say 150F), it will come on and supply hot water for the distribution system. Currently, I think my setpoint controller is essentially serving as the thermostat for the oil boiler, which runs until it warms up the tank enough.
Thanks for the link, I'll check it out.

I use a differential controller for that , a solar control. Page 38 & 39 of that Idronics 10 shows how that would work, along with a wiring schematic.

 

[RoryMcQ]

The tank cooled off below the setpoint this morning while the house was calling for heat, so I went down and felt the pipes to see where the heat was going (very briefly). The line headed to my baseboards was definitely hotter than the line going to the tank, but the tank was definitely receiving heat as well. I think this elaborate plumbing scheme helps to direct the water to the best place. All of which has nothing to do with my original query, but I thought it was of interest.

 

[peakbagger]

My setup has some of the attributes you want but not all. I have ancient Burnham wood boiler with 550 gallons of storage with a single coil. Hot water is from a "hot water maker" external tank with coil fed from zone off the heating system. The system is set up so that its an oil fired system if the storage tank is below 140 degrees. There is also a relay on the oil boiler burner that locks it out of the wood boiler jacket water is above 140 degrees. This temperature probe on the jacket also serves double duty to turn off wood boiler circulator pump if the jacket temp is less than 140 degrees. I have three zones, DHW on priority, first floor zone and second floor zone with individual zone pumps on the return side of the loops.

With the tank above 140 degrees, If a thermostat calls for heat, the zone pump turns on and a bypass valve close on the common return header of the zone pumps this bypasses water around the oil boiler. The water is diverted to the storage tank coil where it picks up heat from the tank and then is routed to the common supply header of the oil boiler upstream of where the three zones are tapped off. This is how the house is heated until the storage tank temp drops below 140. At this point the bypass valve around the oil boiler opens. If the wood boilers jacket water is below 140 degrees a the oil burner lockout relay is closed so the oil burner runs and the zones operate as normal.

In order to understand the operation of the system when the wood boiler is over 140 degrees you need to understand how I charge the storage. The wood boilers hot water outlet goes to a 3 way motorized valve. If I want to charge the storage, the hot water is diverted to the storage coil and then routed back to a Tee, one leg of the T goes back to a common return header that goes between the oil boiler inlet and wood boiler inlet and the other side of the tee goes to a Tee located below the safety relief valve on the oil boiler. The wood boilers circulator pump is on the inlet of the wood boiler. If there is a call from a zone, the zone pump pulls water through the zone and off the common supply header of the oil boiler, the return line from the storage coil is hooked to the T on the safety relief valve so I am heating with somewhat lower temperature water from the return coil. In this mode the oil boiler is not totally bypassed from the flow. The reasoning on this is that the pipe friction in the return line to the boiler through the bypass valve and the friction through the heat exchanger of the boiler is much higher then the friction in the line that goes directly to the wood boiler circulator pump so there is minimal flow through the oil boiler.

The last mode of operation is when the storage is at high temp. In this case the motorized three way T shifts the flow to the common return line where it cycles back to the boiler. If there is a call for heat the zones are heated with the wood boiler outlet hot water. I time my last burn so that the boiler is out of wood just about the time I hit the high storage temp or just let it go out at some temp below high set point. Eventually the wood boiler cools down to the point where the jacket temp drops below 140 F and the oil burner lockout relay closes allowing the oil boiler to run normally.

The controls are via relay logic and I have the system set up to fail so that if the wood boiler is powered off, it becomes a oil fired system that any standard burner tech could work on. I can also shut three valves and the wood boiler and storage is completely isolated to work on or be removed. I used relay logic which is lot less flexible than a PLC, The PLC could add some flexibility to operation of the system but few techs will go near a PLC and I personally don't like using a small PLC as inevitably at some point the software goes out of date and not readily accessible.

The DHW zone is actually not connected to the wood boiler controls, it could be but I decided I don't want to heat my DHW off my storage. Since I am running my wood boiler just about daily, I just turn on the hot water maker when the wood boiler is running and the storage return temp is high. I have the thermostat on the hot water maker set quite high and since I have a SHW system set up to preheat the water going to the hot water maker, that 30 gallon hot water maker lasts a long time with my usage.

My system works, I keep getting tempted to make changes especially after taking a heating course from an industry expert but I burn at most 4 cords a year that I cut and split myself so the cost to do the upgrades far exceeds any potential return. The biggest issue is that even though I have flow checks on my zones, I can get "ghost flows" during operation which means some heat may be going to zones even if not called for. If I switched to common pump and hooked up zone valves in place of the pumps I would solve that issue.

 

[Mike Fromme]

The tank cooled off below the setpoint this morning while the house was calling for heat, so I went down and felt the pipes to see where the heat was going (very briefly). The line headed to my baseboards was definitely hotter than the line going to the tank, but the tank was definitely receiving heat as well. I think this elaborate plumbing scheme helps to direct the water to the best place. All of which has nothing to do with my original query, but I thought it was of interest.View attachment 190254

Can you put a zone valve between the tank and oil boiler that closes when the tank drops too low?


And +1 on the HPWH...just added one myself.

 

[Nofossil]

I went through a bunch of iterations of relay logic to solve this kind of problem. Relays are surprisingly powerful, but tough to troubleshoot. I finally ended up building a digital controller, which also ended up solving the problem of having any free time ;-)