Boiler replacement

Steve8403

I am having a disagreement with a wholesaler and I believe we are being upsold a lot. The attached picture is what we currently have and the old boilers (2) were 300,000 btu boilers that cycled after so many hours. The wholesaler is wanting us to install 2 b2ha Vitodens 200’s with a rack system and gigantic low loss header. I’ll try and attach a picture of the material list as well. We would be going from a 4” pipe on the low loss header to a  1 1/4” copper on the system. It does seem right. Thoughts ?  

Hot_water_fan

You should size the boiler(s) to the heat loss. If 600kbtu (input or output?) kept the building warm on the coldest day, that’s the ceiling for sizing. It won’t necessarily make a huge dent in the quote, but it’s better not to oversize. With multiple modulating boilers feeding a huge in-floor system, you might get runtimes in the months! Generally, 600,000/12,000 sqft is a high btu/sqft ratio for in-floor heating and would imply that the floor surface is 95 degrees or so on the coldest day. Yikes! What’s the nature of the building? 

EdTheHeaterMan

In your Illustration of the existing boiler, you are showing 3/4" with an arrow pointing to something. But that something is vague. Is the 3/4" the pipe to the radiant tube manifold? is the 3/4" the tubing size?

I'm guessing that the wholesaler is taking the size needed from the existing equipment. This is not the best way to size a new boiler. A Load calculation of the heated space is the only BEST PRACTICE way to come up with the proper size.

I get from your description that only one of the boilers is needed to heat the space on the coldest day of the year. There is some type of control that switches from Boiler A to Boiler B at some predetermined interval. Is this the case, or is there a chance that both boilers are necessary at some very cold outdoor temperature? Based on the information the wholesaler understood, they selected the equipment. Once the equipment is selected then the ancillary accessories are selected.

If the info you gave the estimator was misunderstood, then you may actually need a smaller heating source and therefore smaller accessaries. The estimator will never use smaller equipment that the existing equipment. They don't want the responsibility of the equipment being too small. If You want to specify that smaller equipment and are sure it will perform adequately, then the estimator will give you pricing on what YOU specify, without being responsible for specification.

It all depends on how you asked for the QUOTE.

hot_rod

A condensing boiler with a mixing valve? Seems odd

EdTheHeaterMan

Based on conventional sizing methods the 1-1/4" boiler piping can only handle 140,000 BTUh of heating capacity at a 20° temperature difference between supply and return at a flow rate of 14 GPM. Now, if the temperature difference is greater or the flow rate is greater, or both, then the amount of heat that a 1-1/4" pipe can carry can exceed 200,000 BTUh. But I can't see that being much more than 200k. So My guess is that you probably only need 2 boilers that are rated at 105,000 BTUh AHRI NET. That might be a 150,000 BTU input (+-) Boiler depending on the efficiency of the equipment. You may even be able to use boilers that are smaller than that and have them both fire during extreme cold. There are controls that will stage two boilers as needed. Only one boiler will operate for most of the time. And the primary boiler can switch from A to B in order to keep both boilers operating equal time.

There is more mathematics that need to be done here. Load Calc, Pump size, Flow rate (GPM), and more... before you make a selection that is going to be there for a long time.

I agree that 2 boilers rated at almost 400,000 BTU input connected to 1-1/4" system piping is OVERKILL times TWO

GGross

As others have mentioned let's start with the basics. What is the load of the system? Including space heating needs, DHW, snow melt etc. You absolutely need to know the heat loss of the space or it is all guess.

Something else to note. Your drawing shows a mixing valve on the original system. This implies the old equipment is cast iron 80%. That puts the output of each boiler at around 240,000 btu/h If this is the case that means the new boilers ( output rated at 375,000) are very oversized.

Viessmann offers a B2HA-285 boiler as well (output 260,000) that could come in the same configuration as the 399 rack they quoted you. No matter what, the quoted system is too big