Issue With Hot Water Heat And Domestic Hot Water, Tankless Coil On Boiler - HVAC - DIY Chatroom Home Improvement Forum


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Old 12-05-2013, 09:45 AM   #1
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issue with hot water heat and domestic hot water, tankless coil on boiler


I'm having an issue with my hot water boiler and tankless coil. I purchased the house in late spring, so I never really put the heat to the test until recently. When we first moved in, we were having trouble providing hot enough water to supply the bath needs of our young children and even the shower needs for ourselves.

I adjusted the settings on the aquastat and was able to resolve this. Now that its winter in the northeast and the heat is running often, I'm noticing that hot water is sporadic at best. After a little investigation, it seems that when I try to fill our tub with hot water while the heat (circ pump) is running, I get luke warm water at best. When I turn the temp down on the tstat (ie turn the heat off) I am able to supply domestic hot water.

I checked the temp of the DHW at the boiler just by touch on the output pipe and its as luke warm (when the heat is running) as it is when it reaches the faucet, so Im ruling out the idea of drastic heat loss along the way. The temp in the boiler reads fine, usually 150-160 (aquastat set at 160)

My gut feeling on this is that the boiler simply does not have the BTUs to bring circulating HW and DHW up to temperature at the same time. I dont know much about the internals of boilers in general, but Im guessing the HW coil is heated first then the tankless coil for DHW is heated with whats left. The boiler is in good shape, but its certainly not new, I'd guess its 15-20 years old.

Does my analysis seem plausible or is there something I am missing here?

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Last edited by JustinNY; 12-05-2013 at 09:47 AM.
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Old 12-05-2013, 10:02 AM   #2
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issue with hot water heat and domestic hot water, tankless coil on boiler


It is possible. You should get the boiler properly serviced by a oil tech who is familiar with that system. Not all are. Some only do forced air furnaces. Main reason is safety and efficiency. If it is not burning cleanly or properly your fuel bill will be huge and oil can produce CO carbon monoxide same as a gas furnace. They should use a combustion analyzer to set it up properly. The tech should be able to sort out the domestic water problem at the same time. Make sure you describe the system so they send the right tech.

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Old 12-05-2013, 11:18 AM   #3
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issue with hot water heat and domestic hot water, tankless coil on boiler


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Originally Posted by yuri View Post
It is possible. You should get the boiler properly serviced by a oil tech who is familiar with that system. Not all are. Some only do forced air furnaces. Main reason is safety and efficiency. If it is not burning cleanly or properly your fuel bill will be huge and oil can produce CO carbon monoxide same as a gas furnace. They should use a combustion analyzer to set it up properly. The tech should be able to sort out the domestic water problem at the same time. Make sure you describe the system so they send the right tech.
I have a service contract through my oil provider and the boiler's been cleaned and serviced every year according to the tag. I dont know whats involved with the service they do though.
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Old 12-05-2013, 11:25 AM   #4
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issue with hot water heat and domestic hot water, tankless coil on boiler


with a hot water system there is an expansion tank (looks like a torpedo) hanging in the ceiling unless you have a bladder tank at the boiler. it needs draining occasionally and the chimney checked and the boiler setup with a combustion analyzer. get a hold of them and see if they do that. there may be other guys here who can help but it still needs checking if it has not been done lately or since you moved in. your nozzle may be partially plugged and underfirng the boiler and that can foul up the boiler and chimney with soot in no time and be a huge problem. the last tech may have been in a bad mood and not done his job properly, you never know. don't want to find out on Xmas morning when it stops completely.
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Last edited by yuri; 12-05-2013 at 11:29 AM.
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Old 12-05-2013, 05:37 PM   #5
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issue with hot water heat and domestic hot water, tankless coil on boiler


Part of the problem is that now that its cold outside. the water entering your house is colder also, so it takes more BTUs to bring it up to temp.

Also a good chance that the coil is coated inside which insulates it, and it needs a chemical cleaning.

Tankless coils are a waste, very inefficient. An electric water heater would work better, and save you money. Your oil company will not agree though, imagine that. LOL
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Old 12-05-2013, 07:57 PM   #6
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issue with hot water heat and domestic hot water, tankless coil on boiler


Quote:
The temp in the boiler reads fine, usually 150-160 (aquastat set at 160)
Ayuh,... I also use a tankless coil for DHW,...

Anytime a zone is callin' for heat, Yes, it takes away from the DHW, Btu wise,...

With mine, I found I needed to put a restrictor in the line goin' into the coil, 'n knocked the flow down to 3 gpm,...
That made the hot water last longer, 'n even out the temps somewhat,...
'course, bein' on a well, the heat varies with the pump pressure, as well as other calls for heat,...

Another thing that jumps out at me is yer temps,....
Kick it up to 180,...

When was the boiler last cleaned,..??
Soot is a Great insulation,.....
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Old 12-06-2013, 09:48 AM   #7
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issue with hot water heat and domestic hot water, tankless coil on boiler


like bondo said see if you have a flow restrictor

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