Oil Boiler, Hot Water/Radiator Problem

Discussion in 'HVAC Heating & Cooling' started by RiftPoint, Jun 3, 2010.

  1. RiftPoint

    RiftPoint New Member

    Jun 3, 2010
    Waltham, MA
    My Wife and I just recently moved into one half of a rented duplex that has an old oil boiler for heating and hot water. It seems as though if we set the aquastat high enough to have comfortable hot water (135/140F ish) for a shower, all the radiators begin to heat up whenever the boiler switches on the keep the water warm, and in the morning it takes a very long time to have any hot water if we leave the boiler on overnight (if we turn it on in the morning about 30minutes before our showers we get enough hot water). Originally thinking that the thermostat was the problem, we had it replaced, and it seemed to work, but they had also turned the aquastat back down to 120F and once we turned it back up the radiators seemed to come back on again. Is it the aquastat, or something else with the boiler? Any thoughts?
  2. Dana

    Dana In the trades

    Jan 14, 2009
    This is unlikely to be an issue with the aquastat, but may be inherent in the system design, or something may actually be wrong. Is this a thermosiphoning hydronic heating system- hot water, not steam, but no pumps to move the water? Steam? Circulating hot water with pumps & valves? A thermosiphon could be happening even in a pumped system if it doesn't have proper checkvalves, but the effect would be weak at 120F, but gaining considerable flow at 140F. Systems designed to thermosiphon will have a more noticable effect at 120F, and a VERY significant flow at 140F & up. If it's steam, there could be other issues. I'm assuming the hot water is from an embedded coil in the boiler, not an indirect-fired tank operated as a zone?

    But a prime suspect would be an issue with the wiring between the thermostat and the boiler's controls( rather than the thermostat itself). A short circuit between the wires would be sensed by the boiler as a call for heat. If it sorta worked for awhile after the T-stat was replaced, it could very well be that wiggling the wires during installation disconnected the short, but vibration has re-connected it.

    Can we assume you're turning the boiler on/off with the main power to the system, not with the T-stat, not with the aquastat?

    BTW: 120F is too cold a temp for an old oil boiler. At 120F the combustion exhaust will condense suphuric acid & water onto the flue & heat exchangers damaging them. 140F is the minimum most oil equipment should be run to protect the heat exchangers, and even then flue-condensation can still be an issue. As a tenant, set it for 150F and switch it on/off with the power switch while you're figuring this out to avoid potential liablilty when the flues begin to crumble or boiler begins to leak. Keeping it at 150F all summer WILL use a considerable amount of oil, and turning it on/off will have it running cold & condensing for a significant fraction of the time, but setting it at 140F & cold-starting every day means it's running MOST of the burn time in cold & condensing mode. Set to 150F it'll be condensing maybe half the the time on a cold start, and not at all during subsequent cycles.

    If there's a gas service to your apartment, you might check to see if there are incentives offered by the utilities for retrofitting a gas burner. Gas in most of MA is currently much cheaper than oil per BTU delivered, and it's exhaust condensate while still sligthtly acidic, is much more benign. You can run cast iron boilers on gas down to ~130F without condensing on the heat exchangers, and the condensate is only about as acidic as red wine, not the near-battery-acid you get with oil exhaust condensate.
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  4. RiftPoint

    RiftPoint New Member

    Jun 3, 2010
    Waltham, MA
    Hmmm, I'm actually not entirely sure whether it's steam or hot water, or pumps/no pumps. Is there any way to determine this just by looking at the boiler system? It's an old coal fired system that was converted to oil. It does seem as if the heating water comes from an internal coil, there is a small ~10gal tank which is heated by the boiler as well so I'm not sure.
    The main issue when the new thermostat was connected is that they turned the aquastat down to 120 when they did it (changing too many experimental variables!), which made the problem "disappear," and when I put it back to 140 the radiators came alive again.

    Correct any time I've "shut the system down" was with the main power switch we have.
    It could very well be a short, the wiring to the thermostat is very old and is actually a woven jacket type, rather than today's plastic coated types! Not entirely sure what we'll be able to do if that's the case...

    I wish we could just upgrade, but there's no gas hookup (my grandfather was a fireman, and hated gas, and this was the duplex where he lived). To change over the oil furnaces even for new ones would be an expensive proposition since beneath the "new" exterior that you can see is an asbestos insulation...

    So, if it is a thermosiphoning issue is there a solution?
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    If there are no pumps (and those should be visible), and you don't already have a valve, you'd have to add a valve to stop the flow. Depending on the piping and space, this may be easy or hard.
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