Replacing outdoor wood boiler with LP Gas Boiler - Control stategies

farrellg

I currently have a Central Boiler 5648 that is around 20 years old and has served me well. It services a home and shop and is located between the two. The home manifold is a Caleffi Hydrolink with zones controlled by pumps. Water is constantly circulated to the manifold from the wood boiler. The shop is a concrete floor installation and only circulates when there is a call for heat. I am looking at installing an LP gas boiler in the same location as the wood boiler. This would be in a small building that would only house the boiler. I am trying to come up with a boiler control strategy as there are no thermostat runs to the boiler location. My design calls for another Caleffi Hyrolink at the boiler location servicing the two buildings. My thoughts are to only circulate the house on a call for heat from the zones. This would require controlling the boiler based on water flow in the zone or aquastat. Looking for thoughts, input, experience regarding boiler control with out access to the building thermostats. Boiler is fought 500’ from both buildings.

hot_rod

Are you keeping both boilers connected? Or deleting the Central? It could trigger with a flow switch. Or some wifi control thermostat maybe?
Is it protected from freezing out there?

GroundUp

What type of underground lines do you have? 1000ft of even the best quality lines will have a significant amount of heat loss, and will cost a pretty penny in LP usage just for distribution loss so it may very well be more cost effective to simply add a new boiler in each building, especially after the cost of the intermediate shed.

If you're stuck on the centralized LP boiler, it's as simple as having your gas boiler set up as a "hot start" where it's always hot, and then the circ in each building is controlled by its respective thermostat to draw heated water from the boiler to the zone- not unlike what it's currently doing. I do these all the time in different configurations, including at my own home.

farrellg

I plan on removing the wood boiler. My burried lines are thermopex corrugated pipe filled with foam. It is a commercial product from the central boiler supplier at the time. Hot start may be an option. Also burying thermostat wire from both buildings. One design issue I am working with right now is that the loop feeding the house ran 100% of the time to keep hot water at the house manifold. Thermostats controlled just the house zone pumps. In the new gas boiler config, I am wrestling with only activating the house loop on a call for heat, but that would require each thermostat to trigger a zone pump and the over all house loop pump at the same time. Not sure my current controller has that capability. This may cause me to use the hot start option and leave this circulator on all the time.

farrellg

Not protected from freezing other than the shed will be highly insulated and is only about 5x7. Two boilers mounted in each building is an option, but provides no economy of scale as building materials for the shed are already available.

hot_rod

No way to get the boiler in the building where the load is? As @GroundUp mentioned, a bit, maybe quite a bit of your LP will be heating the great outdoors. That is not a huge deal with "free" wood. But with expensive LP that is not a great use of energy.

GroundUp

Thermopex is a great product, but there is still a significant amount of heat loss. I run the Logstor variety of line to my house and Thermopex to my garage, both from a centralized boiler (wood primarily, but an LP boiler is plumbed in parallel for backup to my backup) located in my shop which is halfway between the house and garage and heats all 3. I'm a geek so I did the math on this 3 winters in a row now, and the underground loss is approximately 25-30 million BTU. So 300-350 gallons of LP would need to be burned JUST to get the BTUs to their destination. At $2/gal, that adds up to 2 separate boilers pretty quickly. If I were not in this line of work and hadn't gotten this boiler for next to nothing, I never would have done it this way. My recommendation as a hydronic heating professional who also happens to have personal experience doing what you're proposing, install two separate LP boilers. You will be money ahead in the long run, and will be much happier as well.

farrellg

Putting a boiler in the house is possible and probably the easiest and most cost effective. Putting one in the shop, creates some issues regarding having a burner in the space, a second LP tank etc. Not impossible to over come, but presents its own issues. I do understand the loss in the thermo pex and have considered this in the overall scope. Like anything, there are trade offs with all decisions. Wood was the way to go, but not for my golden years. So need to come up with something that works without breaking the bank or redesigning the whole system. I think we have decided to bury thermostat cable from the house and shop to call for heat from the boiler. Right now I intend to constantly circulate the house loop to the distribution manifold in the house as was done with the wood burner. I just need to come up with a firing strategy for the boiler, ie boiler primary loop temp (warm start) or zone call for heat (cold start). In my case I am leaning to warm start.

hot_rod

What are your current lp prices, that could help with the decision
The small wall hung mod con boilers are sealed combustion, no open flame. If that matters.

Bury a condiut if you decide to run wire. Then you could put multiple wires in it without need special direct bury wire.

farrellg

LP prices in our area are running in the $1.79 range. It does fluctuate based on time of year. My shop is 38 x 30 hydronic in the floor. Super insulated, limited windows and only one door. So the load on the wood boiler honestly was not noticeable. Not sure I ever did a heat loss calc on that space as we weren’t going to change the wood boiler install at the time any way. The house I have heat loss calcs. I am currently looking at Laars FT wall hound heat only boilers. I already have an indirect water heater in the house alone with electric backup for summer if I didn’t want to burn the wood boiler. I am running cost numbers for both separate boilers mounted in the two spaces .vs. one shared out at the old wood boiler location. Not considering the shed at the wood boiler location as we already have all the materials on hand, the cost for a shared boiler is significantly less. However, there is heat loss heat in transport to the buildings. Also, boiler installed in both locations is simpler as manifolds already exist in both locations. Make up water in the shop will be an issue if I decouple it from house as there is no water in the shop. But I think I can overcome that with periodic topping off with a hose.