Boiler supply temps below 130

Matigi

Hi there,

After learning lots about how boilers should run thanks to this site, i have noticed that my grandmothers boiler doesn't get up to a happy running temps. Seems that calls for heat last 5-10 mins in the winter, but supply temps don't get above 130.

The house is 780sqft. Boiler is an Olsen 72000 btu, grossly oversized.

I noticed the pump seems oversized, being a taco 007, its a 1 story home with 8 cast iron rads.

i have ordered a Taco outdoor temp reset with relay, hoping to use the minimum boiler temp function to delay the circ pump until up to operating temp, but i'm wondering if there are other options i should consider. I have tried restricting the return flow, and this does bump up the temps. Is this a safe alternative? I do have a spare Grundfos 15-42 pump, which should decrease GPM substantially at an estimated 6-7 ft head.

Any other options to improve the situation?

Thanks all!

HVACNUT

You dont mention what the operating controls are or the limit setting.
It seems as if the thermostat is satisfying on 5 to 10 minute cycles. Or is the house not getting up to the set temperature?

EBEBRATT-Ed

You need a boiler bypass pipe to bypass some of the supply water into the return to keep the return temperature up above 130.

If the boiler is a cold start, it is normal to return cold water for a period of time but the boiler should come up to temp. Having an over sized boiler is an issue

Matigi

Sorry for the lack of detail. the boiler has a Honeywell L8148E Aquastat, set to 180. Its a 2 pipe system. The boiler doesn't reach operating temp before satisfying the thermostat. I have adjusted the thermostat to increase the differential to allow longer heating periods, however doesn't seem to help much. boiler runs longer but temps remain low.

Will using the ODR with minimum boiler temp help with the issue? Turning off the Circulator to allow the boiler to get up to temp?

I can install a bypass pipe between the supply and return with a ball valve, or should i use a thermostatic valve? definitely going to be a PITA, having to take that route.

Jamie Hall

Does that thermostat have a cycles per hour setting? If so, set it to 1 or 2 cycles per hour. It sounds from your description as though it may be set for 6 -- the standard for hot air.

And that probably isn't enough to do it; it sounds as though the boiler isn't just oversize, but insanely oversized. @EBEBRATT-Ed 's suggestion of a bypass will help a great deal, and you may be able to get away with a fixed setting with a ball valve. It may not cure the problem, though, and you may have to resort to a control strategy similar to what is sometimes used in steam systems with equally badly oversized boilers: a timer circuit which controls the maximum off and on times of the boiler once it is allowed to come to operating temperature with the bypass. The objective being to have the two times balanced so that the boiler's average output is more like what is required -- but the off time is sufficiently short (perhaps only two or three minutes at most) that it never cools off.

A bit hard to explain. And a bit hard to implement, but it can be done.