Why

Constantin

... hot water and steam systems are very interesting animals, both have their own characteristics. I'll do my best to help you but hopefully the luminaries here can step in and help as well, as I doubt that my response will explain it all (nor be 100% correct).

IIRC, the theory behind sizing steam systems to the connected radiator+pickup load is that steam systems are seized thus because otherwise the system will not fill completely with steam, leaving you with cold spots. However, some experts like Boilerpro, Noel, etc. have also successfully staged residential steam boilers, allowing them to modulate up or down as the connected load changes due to the weather conditions.

Two-pipe and one-pipe steam systems can further be differentiated as the two-pipe systems lend themselves to orifice plates or other metering devices, allowing you to reduce the apparent attached load of a radiator. One pipe steam systems have to have the valve 100% open or closed, though a TRV can modulate (on-off) the output of a one-pipe radiator as well.

Reducing the output of steam radiators may be quite desirable in homes where there is a heating system that was installed in the days where insulation was frowned upon, fuel was cheap, and systems were sized to function well enough to allow windows to be kept open in the winter (I kid you not). Renovated structures can have much lower heat losses than they originally did.

My home's calculated fuel needs went down 4x, for example. Consequently, left unchanged, any radiators in my home would now be 4x oversized (if they were sized correctly in the first place). In a steam system, no modifications could lead to short cycling as the steam system will quickly satisfy all the heating needs in all the rooms in record time. Unfortunately, that'll also cost you a lot in efficiency and it may lead to some heat balancing problems, cold spots at the ends of the mains.

A hot water system does not rely on steam condensing in the radiators and hence can be sized to the load, not the radiators. The size of the radiator simply dictates the maximum supply temperature that will be needed on a design day to satisfy the heat loss out of the structure. Very large, high mass radiators can run at very low temperatures, just ask Mike T. in Swampeast, for example.

I cannot comment on what floors will do to steam systems but would wager that the pickup factor ought to be calculated on the basis of the length & the insulation of the steam mains, regardless of whether they're horizontal or vertical. That also comes into play for getting the main vents seized properly.

I also wager that a lot has to do with where the pipes are located. Pickup factors for uninsulated pipes in the exterior wall of a building are likely to be higher than ones buried behind insulation in the center, regardless of whether its a steam or hot water pipe. If there are uninsulated pipes in the exterior envelope, I suggest you somehow account for the heat loss imposed by such a less-than-desirable installation location.

Several contractors here size their hot-water systems to the heat loss of a structure and then use the DOE output of the boiler as a match, not the IBR rating, which is discounted a further 15% from the DOE rating. The IBR rating seems to be reserved for situations where supply pipes are running through unconditioned space (i.e. outside the thermal envelope of the home). Ideally, that should not happen (i.e. the pipes always ought to be inside the thermal envelope).

Anyway, that's it for now from this homeowner. Hope it helped.

John L

Why?

Why are there two distinct methods of boiler sizing-steam using the connected radiation load, and hot water using the Manual J system. There have been some recent posts on boiler sizing, but no explanation of why each system requires a different method of sizing. I have been using the connected radiation load method for both steam and hot water in my ignorance. What problems can/will arise if using the connected load method for hot water systems. I have been told that oversizing a hot water boiler will create more problems than oversizing a steamer. I would think that oversizing is not good period. I have also been told to add on 15% per floor for piping pick-up.Is this true fot both steam and hot water. What happens if you have 4,5 or 6 storey building. I know that I am asking a lot of questions on this post but I respect the knowledge of the people on this forum and their willingness to help people like me. Just trying to learn more about both systems. Thanks for all the help.