Primary-Secondary... Tricking the Boiler???

Jason Quinn_2

A co-worker of mine suggests that one of the true benefits of primary-secondary is tricking the cast iron or copper fin tube boiler(non mod-con) into believing the load has been satisfied; his thought is to oversize or generously size the primary piping in order to reduce the run time of the unit. He also looks to increase pipe sizes in the distribution system to increase the system water content to act as a system buffer without actually installing a buffer tank. Is this logic correct?
Depending on the application,my logic sways more towards smaller pipe sizes- leveraging delta t within acceptable ranges with non mod con's- and adding a buffer tank when appropriate. Thoughts??

Brad White_2

Interesting...

Never really thought of it that way.

When I do think about it, this is where I go:

The boiler should still be sized for the heat loss of the building, regardless of connected radiation (which one hopes is larger than the heat load by a fair degree by the way).

Say the boiler input is 75 MBH and the net output is 82% of that, 61.5 MBH. Say the boiler-side flow is 6.0 GPM, roughly a 20.5 degree rise across the boiler.

Say too that your water volume is, just a guess, 20 gallons in a cast iron boiler, maybe two gallons in a copper fin boiler. Different beasts indeed.

In the copper fin boiler, your through-put is about 20 seconds, the entire volume of boiler water is replaced in that amount of time. Without P/S or a guarantee of a minimum flow rate, your boiler would lock out on high temperature and may well flash into steam in some points. That underscores the real need for P/S right there.

In the cast iron boiler, your through-put is over three minutes, enough time for the boiler to have a minimum firing rate that will at least approach a stable operating condition on the combustion side.

A buffer tank would do the same things, but for such non-condensing boilers, I would put them on the outlet side so as to not prolong a cold start and condensation time. (It will happen with all boilers at some time but not in a prolonged fashion. One has to allow it to dry out.)

But the notion of tricking the boiler to think that the load is satisfied? I cannot get my head around that. To do so would shut down the burner (fine if heating needs are met but defeats the system purpose otherwise). Maybe I am missing something in the analogy.

Bill Clinton_5

buffer, buffer, who's got the buffer

This gets into an area that's a pet peeve of mine. Here are some realities of life:
1. Micro loads are gonna happen. I at least can't get around it. My customers like lots of zones. At night, they may have just the Master Bath heated. Perhaps they would like just a towel warmer. I don't want to tell them they can't have it: So I do it. Instant micro load. Boiler makers don't like to talk about it, but this is a problem every low mass boiler on the market.

2. Low flow rates are gonna happen. Again, micro loads are the reason. Large mass boilers are not terriblly troubled by it, but low mass ones are. This is the biggest reason for primary/secondary pumping.

3. Condensation is gonna to happen. Not a problem for a condensing boiler, but serious for boilers meant to be non-condensing. If your radiant tube is in concrete, gyp, mortar, or such, and a large area has to warm up from cold, the boiler is going to see some frigid return water. Yes, you can add on themostatic valves, injection mixing, an other tricks: They are something of a problem, but it can be done. Extra money and extra complexity we don't especially need.

Buffer tanks, twenty gallons and more, plumbed into the primary loop can be used to good effect in these situations. I keep coming to the question of why aren't there condensing boilers with thermal mass appropriate to their output? The reason of course is the cost of building a large mass condensing boiler would raise the price-point above the competition. Never mind that installing a buffer tank will cancel out the money savings.

I've always favored the solution of installing a water heater: a stainless steel, condensing water heater of around 50 gallons.

There have been several contenders in this arena: Polaris has gone through numerous revisions, but in my experience has not been all that reliable. Voyager was a disaster. Reincarnated as the Phoenix, however, they appear to be a lot better. I especially like installing them in combination systems using a heat exchanger to separate potable water from the water in the heating system.

It's an expensive solution, but usually no more than a good wall hung, stainless steel, condensing boiler WITH buffer tank.

The nice things: No minimum flow rate; no minimum temperature; micro zones welcome.

Works good for me.

Bill

Wayco Wayne_2

Good stuff.

I dont like the idea of tricking the boiler since it would increase the cycles of the boiler. Causing lower efficiencies and parts worn out sooner. I'm a highways per gallon guy. Long cycles close to the heat loss of the house are best. There are boilers that have a P/S built in to protect the boiler. (Lochinvar solution boiler) There are thermostatic bypass valves that protect the boiler from cool return temps. Or my favorite, Jump up to a mod con and get your money back in savings. I have tried both Polaris and Voyager and had my troubles. I havent ventured into Phoenix land yet. Perhaps a little gun shy. Maybe I'll let someone else do the field testing this time. WW

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