Hot water efficiency.

The owner has a Burnham oil burner (hot water heat, single zone) with an attached tankless hot water coil . The outlet from the coil goes to a gas fired hot water tank. There is a Honeywell aquastat that controls the temperature of the heat loop. I don't think that the Honeywell aquastat controls the temp of the coil.

I want to eliminate the coil & use the gas hot water heater or at least re-pipe it with other valves so there is a choice. I think that it's more efficient without the coil. I read that those coils are not efficient. Some people disagree. The gas bill is about $100 per year. Oil costs much more.

 

Is this a home that you rent rather than own? If yes, then you need the owner's written permission BEFORE even thinking of making any changes to the system. Not having permission could open the door to a world of hurt for you if the owner decided to get nasty.

You are correct that the "tankless coil" is a poor method of heating domestic water. It does take a bit more than simply bypassing the coil however to actually save on fuel oil as any boiler with a tankless coil has controls to maintain a minimum boiler temperature year-round.

 

Neither

The only aquastat controls the water for the heating loop. It doesn't seem to affect the domestic loop. The owner shuts the boiler in the summer & lets the gas hot water tank heater control the domestic water loop. There are two separate cold water feeds, one to the coil & one to the heating loop.

 

There are multiple issues here.

During winter heating season oil boiler tankless coil is more efficient cost wise than separately fired heater. The coil also preheats water to the tank increasing hot water capacity.

During summer separately fired winter heat is more efficient cost wise.

Have system valved so either heat source can be used or isolated.

Found that over the years, rust particles in city water had eroded Taco 007 plastic impeller down to a stub. Added whole house water filter with clear plastic housing to the hot water feed line. Was surprised to see how much rust accumulated every few months.

 

I don't understand how/why the effeciency of one over the other would change from one season to another.

 

Your heating boiler is oil fired, but your stand-alone domestic hot water heater is natural gas? Why don't you use gas for both?

 

The house will probably be sold in the next 3 years so a conversion is not going to happen. If the house weren't going to be sold, I would agree.

 

Stack temperature is a basic reference to efficiency. An oil burner stack of 400F is close to 84%. A 600F stack is 79%.

The common round water heaters with a single pass design are not as efficient heat exchangers as multi pass boilers. Those oil fired water heater stack temps are usually in the 500F+ range.

I have the same model Beckett AFG burner (both firing at .80 GPH) on my boiler and water heater. Boiler stack is 350F and water heater 550F. Those are measured temps. Subtract 70F room temperate when looking at chart of Efficiency VS Net Stack Temp.

The boiler has a much higher latent heat loss than the water heater. It used 60 gallons in past summers Over the summer it cost less to run a lower efficiency oil fired water heater. Which used 20 gallons this year a 66% saving.

 

Thanks for that explanation. I still feel that it's worth it to re-pipe the system so either the coil or the hot water tank can be used. The cost is minimal.

 

Another way to have the best of both worlds is to set it up as a pump-storage system so in winter higher efficiency boiler is heating tap water. In winter water heater heat source or set for lower temp only acts as backup.

Connect the tankless coil "out" to water heater "inlet". Connect water heater "outlet" to tankless coil "in" Place a check valve so hot tap water draws only from tank and not tankless coil.

Install a pump (Taco 007 is fine). Also put a check valve in cold water feed from meter to tankless coil.

To activate pump I use a $15 PID controller set at 113F in the winter with 1 degree Delta T. In summer set delta T to 4F.

 

How will the system get water from the city service, if I pipe it the way you described?

 

Your system now has water from meter feeding tankless coil. Put a check valve in that line then a tee to bring water from water heater to tankless coil.

If there is any chance that water from tankless coil could go directly to taps, install an automatic tempering valve to avoid scalding.

What state are you in? Here in New York oil is cheaper than gas.

 

It's not that way yet but part of the plan is to send then water from the coil directly to the taps.

Oil maybe cheaper at the moment but the efficiency of the boiler vs the efficiency of the gas hot water heater has to be included in the equation. Old fashioned coil vs newer hot water heater.

 

My point was getting the “best of both worlds”. Gas efficiency is 80%, oil can go to 88%. Keep the old more efficient boiler/tankless coil and optimize the separate hot tap water system.

Condensing hot tap water systems are 92% + efficient but not when used on hydronic baseboard/convector systems where water temp's of 140F to 180F are needed.

When gas is cheaper than oil I pull the Beckett oil burners and plug in gas conversion burner units.

 

Can that be done on a Burnham RSA series boiler? What unit would you recommend?

 

For oil burner fired boilers with 4" air tube there are several types of gas conversion units with 4" tubes that just slip in.

In 1979 when oil went over $100 per barrel bought my first one from Sears for $150. It had gas pilot light and no blower. Newer models have electronic ignition and blowers.

Gas nozzle in conversion unit came with no hole. Had to select drill size according to BTU needed and drill hole.

Here is a typical one of many on eBay Another search brought up this collection:

 

Thanks, I'll look into that.
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The RSA oil boiler is certified for oil fire only. Check you I&O manual.

 

Ok, thanks. The owner decided that she wasn't going to do that anyway.