Contender

ratrace

I have one contractor (A) telling me i can run my new MC99 Heater as a single primary space heating zone with no secondary and another guy (B) is telling me i cant, it only can be used for a Primary loop, and i need a seperate circulator for the a seperate "zone". The manual says it cannot be used for a space heating zone, it requires a second circulator for a seperate zone. Contractor A says he gas done at least 5 installas with no second circulator with great success and it is a little cheaper. It appears to me that if i dont use a second circulator or creating a seperate "zone", i would be imposing major thermal shock on the boiler.



Any help would be greatly appreciated

icesailor

Contender:

Just because he says it can be done, in spite of the manufacturers explicit instructions, doesn't mean he is right.

And maybe he has done it "lots of times" before and gotten away with it. Maybe you could be the first one that doesn't get away with it.

All the small gas boiler piping schematics I have looked at recently, ALL show a primary loop WITH THE CIRCULATOR ON THE RETURN SO IT IS PUMPING INTO THE BOILER. Many show an indirect, piped as part of the primary loop.

Veissmann shows one example like you have with no Primary/Secondary. A boiler with ONLY an indirect and NO heat zones. Or, ONE heat zone and no indirect.

You are smarter than Contender "A".  



There's never enough time to do it over but there's always time to do it over.

It's amazing how much money you can save when you do it wrong the first time.

Jean-David Beyer

There's never enough time to do it over but there's always time to do it over.

We haven't time to stop for gas. We are late already!

Gordan

TT Prestige, also

As other "fire tube" exchanger boilers come out, I suspect that they too will specifically mention the option of "pumping through" due to the low water side pressure drop of these heat exchangers. Nevertheless, you still have to make sure that 1) flow through the heat exchanger is within the acceptable range at all times and 2) the overall pressure drop is within the capabilities of whatever circulator you're using. Those are the criteria in any situation, they're just harder to meet with some boilers (and some systems) than others, whereas hydraulic separation is always an option.

NRT_Rob

in a single zone system

the only reason you would ever need primary/secondary piping is because of pressure drop through the heat exchanger.



New HTP diagrams are starting to show primary piped systems with pressure bypasses instead of primary/secondary... we've been doing that for years, of course, even though they weren't in the manuals.



it's not magic. If you have enough flow through a mod/con boiler to extract the heat, it will be happy. mod/cons don't have "thermal shock" issues. MFGs specify primary/secondary because most installers don't do math, not because it's really necessary in all situations.



the contender mC99 can pump 8 GPM through the boiler and lose 5 feet of head. If your primary pump contractor is prepared to calculate your flow requirements and frictional loss of your heating system, I would say he's a good bit smarter than the one doing primary/secondary on single zone systems.

NRT_Rob

also

the presence or absence of an indirect has nothing to do with the need for primary/secondary piping. it's just a second primary loop (DHW pump is an either-or run vs the Boiler pump, assuming priority is used).

Jean-David Beyer

primary piped systems with pressure bypasses

It seems to me piping in two closely spaced Ts is no more complicated than piping in a pressure bypass. And possibly more reliable too.



I do not know what contractors pay for the extra circulator compared to a pressure bypass valve; on pexsupply, the difference is only about $20. Is the decision to use one instead of the other done on the basis of initial cost of materials? The differential labor cost? Or what?

NRT_Rob

the difference

is in energy usage, not first cost. also, it should be better for return temps too... bypass flow is much more controllable than primary, unrestricted flow of a pump in 10 feet of circular copper.



Also, you don't need a bypass if your smallest zone can pass the minimum flow of the boiler (typically min fire at a 20 degree dt but specs may vary by MFG). bypass is only necessary in small zone situations. completely not applicable in the single zone situation of the original poster.