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Old 11-05-2011, 08:54 PM   #1
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electric hot water heater in line with DHW coil from boiler


My wife complains that lately when she showers the shower will become "freezing cold" after about 15 minutes. I suspect that it's because both heating zones are on the at the same time. Just after she complained about I checked the boiler and I see that it was down to 180*F.

This is our first winter at this house... we use hot water like a chinese laundry...

My hot water line exiting the DHW coil is a 1/2" copper pipe which runs 10' or so before branching between bathroom and kitchen. My thought is to cut that line along the 10' part, then plumb the boiler side into the cold inlet of an electric tank and the hot output of the tank into the house side of existing hot line?

The main point would be to provide consistent hot water temp, the secondary point would be that I could shut the boiler down in the summer.

Any issues with this? Is there anything special I'd need to do to the water heater? I'm thinking a 30G tank would be a sufficient buffer...

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Old 11-06-2011, 08:21 AM   #2
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electric hot water heater in line with DHW coil from boiler


Usually the domestic hot water coil works better when at least one zone in the house is calling for heat. This "churns" the water in the boiler so the DHW coil is constantly exposed to "new", hot, boiler water.

There should be a thermostat (aquastat) immersed in the boiler near the DHW coil that acts as an additional zone so the DHW temperature is more consistent in summer or when no regular zones are calling for heat.

Putting an ordinary water heater in line (in series) with the DHW coil does not work unless the "last" one on the line is "turned on" all year 'round. If the ordinary water heater is put in parallel with (bypassing) the furnace coil then you can shut off the furnace in summer and shut off the ordinary heater in winter.

Tankless furnace coils also work better if the incoming (cold) water first goes through a few dozen feet of finned pipe (dissected out of baseboard radiators and) mouinted on the wall or ceiling where air can circulate around it and also no one will bump his head or arm into it.

It is possible you may need a replacement furnace coil because the one you have is caked up with sediment inside or outside.

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Last edited by AllanJ; 11-06-2011 at 08:33 AM.
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Old 11-06-2011, 10:29 AM   #3
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electric hot water heater in line with DHW coil from boiler


Instead of a 30 gallon electric tank, which doesn't have that great a recovery time, you might considering removing the tankless and putting in an indirect fired water heater.

It's plumbed as a separate zone off your boiler. Some of them can easily make over 120 gallons an hour of hot water. Depending upon your existing system, you could set it as a "priority" zone.

It's more efficient than a tankless since you don't have to keep the boiler "hot" all the time and the tanks are very well insulated.

Just a thought.
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Old 11-06-2011, 12:43 PM   #4
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electric hot water heater in line with DHW coil from boiler


Quote:
Originally Posted by AandPDan View Post
Instead of a 30 gallon electric tank, which doesn't have that great a recovery time, you might considering removing the tankless and putting in an indirect fired water heater.

It's plumbed as a separate zone off your boiler. Some of them can easily make over 120 gallons an hour of hot water. Depending upon your existing system, you could set it as a "priority" zone.

It's more efficient than a tankless since you don't have to keep the boiler "hot" all the time and the tanks are very well insulated.
.
I agree with everything you say, but money is an issue. It's ~$300 for an electric tank WH; and ~$3000 for an indirect setup.
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Old 11-06-2011, 01:07 PM   #5
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electric hot water heater in line with DHW coil from boiler


I understand about cost.

Check out fixing the tankless. While you're doing that, change out your shower heads to something with lesser flow.

Good luck.
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Old 11-06-2011, 01:07 PM   #6
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electric hot water heater in line with DHW coil from boiler


Quote:
Originally Posted by AllanJ View Post
Usually the domestic hot water coil works better when at least one zone in the house is calling for heat. This "churns" the water in the boiler so the DHW coil is constantly exposed to "new", hot, boiler water.

There should be a thermostat (aquastat) immersed in the boiler near the DHW coil that acts as an additional zone so the DHW temperature is more consistent in summer or when no regular zones are calling for heat.
My aquastat is set with a low limit of 180*F, a high limit of 200*F and a differential of 15*F.

What is happening is that when both zones are calling for heat the boiler fires and the temp drops to about 185-190* while it's firing. The tankless coil simply is not picking up enough heat as the water travels through it.

It's as you say, the coil is getting caked w/ sediment which is not only impeding flow but also insulating the water flowing thru the coil from the heat around it.

Quote:
Originally Posted by AllanJ View Post
Putting an ordinary water heater in line (in series) with the DHW coil does not work unless the "last" one on the line is "turned on" all year 'round. If the ordinary water heater is put in parallel with (bypassing) the furnace coil then you can shut off the furnace in summer and shut off the ordinary heater in winter.
My plan was that the electric tank WH would be "last". When the boiler is running during heating season the water flowing into the tank WH would be already basically at temperature, so the tank would act as a buffer for higher demand periods when the coil can't provide hot enough water, and it would also act as a temperature maintainer for the water it is storing. The summer, when the boiler is off, the tank WH would receive cold water which would first pass thru the cold coil. The tank would be required to both bring the water up to temp, and maintain it at temp. I estimate we spend about $50-75/mo in oil during the summer, that's roughly equal to what the electric tank WH would cost according to the energy sticker (and factoring the higher cost per kW for my local power).


Quote:
Originally Posted by AllanJ View Post
Tankless furnace coils also work better if the incoming (cold) water first goes through a few dozen feet of finned pipe (dissected out of baseboard radiators and) mouinted on the wall or ceiling where air can circulate around it and also no one will bump his head or arm into it.

It is possible you may need a replacement furnace coil because the one you have is caked up with sediment inside or outside.
Finning it is an interesting option, unfortunately I'm not sure if it's really possible for me because I had plumbing work done when we bought the house, the cold pipe into the coil is PEX except for the last few feet.

I think I do need a replacement coil, but when it comes to the boiler I feel that I don't have enough knowledge to do the repairs myself. Last year when I was quoted the $3000 for the indirect setup I was told that a replacement for the coil usually runs about $500. The plumber mentioned to me "the coils usually work great for 2-3 years, but year 5 you're noticing a problem. This one is about 8 years old, you're going to notice it". We just have one bathroom in the house, so he told me if I'm planning to add a 2nd bathroom, budget the $3000 for upgrading to indirect, otherwise just replace the coil when it becomes a problem.

In this case I feel like the electric tank WH is a viable option because although it will INCREASE my costs in the winter (because I will be paying roughly the same amt for oil, but now also for electric to maintain temp in the tank), but in the summer I will be able to stop using oil, which is currently $4/G at present.
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Old 11-06-2011, 01:13 PM   #7
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electric hot water heater in line with DHW coil from boiler


How about bypass the tankless completely, change the boiler over to cold start, and then put in a bigger, appropriately sized, electric water heater.

Use the $50-60 you're not spending in the summer to do that.
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Old 04-08-2012, 09:53 PM   #8
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electric hot water heater in line with DHW coil from boiler


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Old 04-08-2012, 09:55 PM   #9
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Old 04-09-2012, 10:43 AM   #10
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electric hot water heater in line with DHW coil from boiler


Quote:
Originally Posted by AandPDan View Post
How about bypass the tankless completely, change the boiler over to cold start, and then put in a bigger, appropriately sized, electric water heater.

Use the $50-60 you're not spending in the summer to do that.
" I second that get rid of the indirect system reset your boiler control to eliminate the hot water cycle. And follow A and Pan."

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