30,000Btu heat load oil boiler

DD_5

I've come across a low temp, 2-zone, in-floor radiant system that used to run on an oversized Brad-White thermal mass type oil burning tank. Its probably been short cycling for years and it began leaking from the tank. The owner wanted it removed and now I'm considering what to replace it with. The heat loss calc we came up with shows 30, 000Btu heat load with a design temp of 28F (Seattle area). That's a very low load and I'm wondering if anybody has ideas on an efficient oil boiler design for that small demand....The piping is existing although I can change it inside the mechanical room. I was thinking a conventional Buderus 115 and piping it primary / secondary except won't this be oversizing the boiler once again? How do I manage such a small load?

SpeyFitter

Buffer Tank

I suggest you use a buffer tank. Lochinvar just came out with some really nice stainless buffer tanks that act as a hydraulic seperator/low loss header as well (plus, due to the size of the tank, they probably do a great job in dirt seperation and an admirable job at air removal/seperation as well). They have a supply & return tapping (1 1/4") for the boiler side and a supply & return tapping (1 1/4") for the system side (heat-flo also makes a similar type of tank). They have models from 30 gallons all the way up to I believe 120 gallons if I'm not mistaken. I would put a 4 way mixing valve off of the boiler to heat the buffer tank (so the buffer tank only sees lower temps). 

You might also want to consider putting a time delay relay on the call for heat. Have zone valves on the system side with a Grundfos Alpha plugged into the wall. When the call for heat is on, the zone valves open, the Groundfos alpha moves water out of the tank into the emitter(s) and at the same time the end switches close into time delay relay. The time delay relay will act as a further buffer allowing some of the built up heat in the buffer tank to move out into the floors to potentially mitigate the call for heat without having the boiler having to fire up. Then when the relay snaps shut after it's duration, the boiler should have a nice long burn cycle to have to heat the tank back up to it's setpoint (which presumably would be determined by an outdoor reset sensor).

If you decide to get one of these tanks to buffer the system, if you are going to use an indirect, I strongly suggest you pipe the pump for the indirect directly from the boiler into the indirect - that way you aren' t heating the entire buffer tank up to 160-180 degrees everytime you need to heat the indirect.

meplumber

Thermal Storage/Buffer Tank

DD,

Buffer tank is the only way to go. 

Is gas or LP an option?  If so, look into the Phoenix (could have the brand name wrong) by Heat Transfer Products.  It is a modulating burner with DHW and Buffer built into one unit.  I have not personally used one but have seen them in action.  Sounds like an option for these small load places.

DD_5

Still thinkin Brad White Oil

LP gas is an option but the HO is resisting it. I've seen the Phoenix in use as well and its impressive.

How bout that Brad White oil Combicor? I've read varying opinions on it but saw one thread that says their qualitiy's improved with improved coil etc

meplumber

Haven't seen the updates to the Combicor

When they initially hit the market there were some problems.  Up here in Maine we don't struggle with the small load issues that you guys do in other parts of the country.  -10 deg design temp.

However, as houses seem to be getting smaller and hyper insulated, I have seen my heat loss numbers starting to fall considerably.  I have been looking at the Phoenix for another job and I will look in to the Brad White. 

Let us know what you decide to use.  What is the HO's hang up with LP?