Propane Tankless Boiler as Backup - Return Protection Required?

[Medman]

I am planning the replumb of my system this fall to incorporate storage and install a propane backup boiler. My system will be identical to the Simplest Pressurized Storage sticky at the top of the forum.

I am looking at a Noritz tankless boiler as my backup heat source, using the direct-vent configuration. My question is, does this boiler require return thermal protection like the wood boiler does, or can it work with input temps less than 140*?

Also, with all the recent discussion on variable delta-t circs and outdoor reset, how can these devices be incorporated into the system to increase efficiency?

My design delta-t was 20* F and I am seeing anywhere from 15-25 degrees difference between supply and return depending on the outside temperature. Currently, the water flows continuously through the loop and DHW exchanger once the output reaches 145*F. Once storage is installed the water in the loop will only flow when there is a call for heat. Otherwise it will flow to storage. I am looking to maximize my efficiency using variable speed circs and outdoor reset, in order to get the most out of my storage.

Any thoughts?

Ryan

 

[rkusek]

Propane backup boiler? If you're home is forced air wouldn't it be better to just to have a propane/electric/heat pump furnace and let the propane/electric water heater handle the DHW?

 

[Medman]

Home is forced air but I have an air handler only, not a furnace. Also, the system heats 2 separate spaces now - I want backup heat for both spaces from one unit. I`m not concerned about DHW or storage when the backup is running and the control system would disable those zones when backup is in use. My question is does a tankless boiler require return water temp protection?

 

[Medman]

I found the installation instructions for the boiler I am considering. It does not need return protection. I'm still wondering about variable speed circs with outdoor reset though.

 

[Donl]

Medman, are you on time of use electric? I was going to go propane, however I looked into an electric boiler and making use of storage with TOU billing. Store heat to storage during the night and use it during the day when electric rates are higher. This would likely be cheaper than propane or oil. I have not installed this as backup yet, but I hope to next year. The down side of electric is that during a power failure it won't be available, however with a small generator I can keep the wood boiler running.

 

[Medman]

Yes, I am on TOU billing. Thanks for the idea, I hadn't considered that option. Electric boilers are a little cheaper, if I recall. I may run into an issue with capacity in my shop panel though - only 40 amps total.

 

[Donl]

40 amps probably would be too small unless you have a low btu requirement. The most 40 amps would provide about 32000 btu with nothing left for other requirements. Might be ok though if you are just looking for freeze protection or low inside temps.

I'm looking at between 62k and 68k btu's which will require approx 75 to 85 amps. For back up this is looking pretty good to me because it will hardly ever be used, little maintenance, no truck delivery and with the use of my storage I think I can operate it cheaper than propane or oil. I'm going to give it a try unless someone can point out something I have missed.

 

[maple1]

I'm looking at between 62k and 68k btu's which will require approx 75 to 85 amps. For back up this is looking pretty good to me because it will hardly ever be used, little maintenance, no truck delivery and with the use of my storage I think I can operate it cheaper than propane or oil. I'm going to give it a try unless someone can point out something I have missed.

That's why I went with an electric boiler for backup.

Found an 18kw one for $400 in kijiji with 2 years use. Came with a circ pump. Cost me a couple hundred for the electrician to hook it up (100a breakers & that super fat wire isn't cheap). Mounts on wall so takes up next to no room & allowed me to get rid of the oil all together. Ours only got used one day last winter, that was on the second day of being away for two days. Storage got the house through the first day. You do need a decent entrance - ours is 200a so we were OK. The boiler specd 82 amps, so we had to go with the 100a breaker to meet code - even though it only drew around 75 when tested with all elements heating. If you can find one that specs less than 80 you'll be able to use a smaller & cheaper breaker. I'm pretty sure if we had a smaller one it would work fine for backup & not cycle as much.