Micro-load Dilemma: Add Storage or Nuclear Option

BigRob

Background:



Five residences are heated by a common heating system. Each residence has a small area below the main level. The small area is heated by a 24"x24" radiator in all the residences. The small radiator emits about 2000 btu/hr. The heating boiler sends 33.2k btu/hr net at minimum fire. Heated water travels through 1" PEX to a Rehau manifold with zone valves controlled by thermostats in the small areas. Radiators are fed by 3/4" PEX. When the small radiators call for heat the boiler cycles a lot. There is room by the manifold for a 40+ gallon storage tank.



Do I:



1. Leave it alone and have a cocktail

2. Add storage

3. Nuke the radiators and put in Myson convectors



Let me know your thoughts.



-R

hot_rod

How often

does the small zone call by itself?



How often does the boiler fire, and how long does it run when that load calls.

Zman

Buffer

If you know your system water volume and mass and the on/off differential on the boiler you can make an accurate estimate on the buffer tanks impact on boiler cycles. A quick look says they will get about 8x longer with a 40 gal tank.



The cocktails are also a good idea.



I don't think the average tech is quite ready for nuclear yet... 

BigRob

Yes

The radiators do call for heat individually and are fed by an Alpha. It's about 60ft each way from the manifold to the boiler. The radiators vary from 5ft to 60ft each way from the manifold.



The first call for heat maybe lasts for 5-10 minutes. After that the cycling is pretty rapid.

BigRob

Ok

It's about 11 gallons including boiler piping and one radiator. We try to run from 120 to 100, so that is about 3 minutes of maintenance burn for one radiator. This assumes the firing rate is always 33200 btu/hr, which doesn't always happen. The boiler fires to whatever to get up to set point, so it's probably shorter than that. I guess I can set the ramp rate to stay at low fire for 10 minutes or so, but that only gets me a 3 minute maintenance burn. The off time isn't that long.



I was thinking about adding a 40 gallon tank in series with the return line to the boiler. I cannot add a buffer tank due to piping limitations.

Zman

Buffer

It looks like putting your 40 gallon tank in series will make your on cycles about 16 minutes.

BigRob

Ok

So the convector route gets me 20 minute burns, with off times dependent primarily on the heat loss of the room. The average price of the convectors will be about $700 each.



The 40 gallon storage tank gets me 16 minute burns, with very long off times. I can probably beast in the electric storage tank, Hot Rod style, for $500. If there is no more demand I leave about 0.35 therms in the tank. Let's assume I waste 20 full tanks a season at a total of 7 therms. Yeah, I should just put in the tank and have a cocktail.



Now, could I use the tank to buffer my thin slab zones with a three way Valve at the manifold? Maybe I'm getting greedy.

SWEI

Maybe I'm missing something here

but what are the other loads served by this boiler?  Is this a mod/con boiler?



Is there a separate thermostat in each main level and each lower level?  If so, do the lower level zones call for heat when none of the upper level zones are?  I'm thinking TRVs on the lower level rads might make sense.

BigRob

There are..

but often the micro-loads are called individually. I was thinking about using TRV's, but would I always have to turn on the pump based on an outdoor temperature sensor? If that is the case, I wonder if the pump will turn on based on outside temp a decent amount of time while people don't adjust the TRVs, which would waste energy. The spaces served by the radiators aren't used very much. In the other case, if people do reasonably adjust the TRVs down when not in use, maybe I am back in the case where 1 radiator is effectively calling for heat. What do you think?

Gordy

Regarding TRV control

It would be best to use constant circulation. TRVs control space temp ODR controls boiler/ water temp. Use a ecm circulator to curve power consumption.