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Add a electric hot water tank in combination with a gas combi boiler on the same circuit

StartledP

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Jan 24, 2020
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Hi All,

I currently have a 3 year old gas combi boiler which provides hot water and central heating for our apartment. However Im looking to add a 100l AC electric hot water boiler into the hot water circuit in order to better use more of my solar generation and reduce my gas dependency. Central heating would still be performed by the gas boiler (for now), and the gas combi would still remain connected to the hot water circuit, but I would turn the water temp on the boiler to 0C so that doesnt heat.

This could be done in one of two ways, either pre or post the gas combi, I cant really see much difference in the two however but is that really the case? Its seems straightforward to plumb the electric tank into the hot water circuit with a three way valve, which would isolate the electric tank if necessary. Is there something important I'm missing here though?

I can easily do all the work myself, Im reasonably experienced with plumbing (including installing hot water tanks).

This is far as I can see is very rarely done, at least here in Europe, which considering its an excellent method for storing energy from my solar panels I was surprised about. It will save me at least 300 euros a year, plus less GHG emissions.
 
Combi boilers can be terrible. My sister had one and the furnace ran all summer the same time she ran air conditioning. The laundry room was like a sauna. Ditch the combi entirely unless you are heating for several apartments. For solar tall tanks with upper and lower element are better. These are not common in Europe. Water easily stratifies and the top can quickly heat giving the water you need. Excess is stored lower down. The lower temp water has far less heat loss which is 1-2KWH a day for an entire tank. Why wait to heat an entire tank just to have it lost.
 
Combi boilers can be terrible. My sister had one and the furnace ran all summer the same time she ran air conditioning. The laundry room was like a sauna. Ditch the combi entirely unless you are heating for several apartments. For solar tall tanks with upper and lower element are better. These are not common in Europe. Water easily stratifies and the top can quickly heat giving the water you need. Excess is stored lower down. The lower temp water has far less heat loss which is 1-2KWH a day for an entire tank. Why wait to heat an entire tank just to have it lost.
Thanks for the reply but I'm not ditching my (very reliable) combi right now, it isnt even possible at this moment in time.

Not looking to use a solar water heater, these are more trouble than they are worth. Ill just be using the standard AC water boiler and not worrying about solar water heaters leaking all over the place or the big inefficiencies of MPPT DC units which are so expensive they will never give a decent ROI. I have a 220v 25 amp connection to my house so instant hot water heating a complete no go.

An extra electric tank fits my needs and the limitations of my apartment well enough, at the moment in time.
 
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I would plumb the second electric water heater with a bypass circuit, like people do for water softeners so they can work on the water softener without cutting all water in the house. Uses three ball valves (and only use ball valves as they last longer than the gate/pad types).

You may have a challenge in the combo unit if you want to heat the house but use electric water heating. Perhaps you'll want an electric combo system you can switch back and forth. If you push hot water from the electric heater into the combo you may prematurely wear out the combo unit as it's likely not designed to accept hot water at its inlet.

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I would plumb the second electric water heater with a bypass circuit, like people do for water softeners so they can work on the water softener without cutting all water in the house. Uses three ball valves (and only use ball valves as they last longer than the gate/pad types).

You may have a challenge in the combo unit if you want to heat the house but use electric water heating. Perhaps you'll want an electric combo system you can switch back and forth. If you push hot water from the electric heater into the combo you may prematurely wear out the combo unit as it's likely not designed to accept hot water at its inlet.


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Thank you, that layout was what I was intending to do although using a three way tap to simplify the plumbing. On your advice I wont now and will instead choose for the more reliable option. 20 minutes plumbing is a lot less work than cleaning up after a busted tap.

You may have a challenge in the combo unit if you want to heat the house but use electric water heating. Perhaps you'll want an electric combo system you can switch back and forth. If you push hot water from the electric heater into the combo you may prematurely wear out the combo unit as it's likely not designed to accept hot water at its inlet.

That makes sense, although I could get round that by bypassing the hot water circuit of the combi completely. Any ideas on what the downside might be? This thread here would appear to confirm what youve written:

 
Thanks for the reply but I'm not ditching my (very reliable) combi right now, it isnt even possible at this moment in time.

Not looking to use a solar water heater, these are more trouble than they are worth. Ill just be using the standard AC water boiler and not worrying about solar water heaters leaking all over the place or the big inefficiencies of MPPT DC units which are so expensive they will never give a decent ROI. I have a 220v 25 amp connection to my house so instant hot water heating a complete no go.

An extra electric tank fits my needs and the limitations of my apartment well enough, at the moment in time.
Grouping me with solar thermal people was a low blow. I heat all my water with PV. Tapping excess power to heat water is low hanging fruit. For me it is dirt cheap, with efficient MPPT conversion and I have three tanks that I can heat. Each has its own priority level. A 15L tank heats quickly as it is already hot and that serves immediate needs. A preheat tank allows fast recovery of a small tank. Laundry has its own tank which can recover every other day with the excess of the excess. No need to ditch the combi. Just don't run it when it isn't needed. I'm all for automation. You will just have to wait. Solar world is still in the dark ages when it comes to heating water. China in a couple years will be popping out controllers for $60.
 
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