We had our Bosch Greenstar combi condensing boiler installed a little over 2 years ago. Our previous home had a 40 gal indirect DHW tank and we never lacked for hot water. Even though this Bosch is rated at 4GPM DHW we are finding ourselves having to coordinatee washing and dishwasher use outside of shower and bath times. So we are thinking of either separate HW heater or adding a zone to the boiler and an indirect tank.

I had an idea of adding an electric water heater unit in series after the Bosch. So the DHW output of the Bosch would become the input to an electric water heater, effectively making the heater a storage cylinder. I am thinking it should use minimal electric since it is only maintaining water temp during periods of inactivity, such as overnight. Here on Long Island electric is very expensive where natural gas is relatively cheap, so energy use is a concern.

Anybody have an opinion on this? I'm all for more hot water but not if it adds $50/mo to the elec bill....

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Not a plumber by trade but a fierce DIYer

this is hard to calculate..
the more hot water you use then the less your electric bill will be.
if you go away for a weekend then the electric tank will cool off and you'll use electricity.
if you use alot of hot water the tank will supply your needs but the water entering the bottom of the tank will be hot so the elements should not work at all..

Yeah and that's what I've been struggling with. The longer the water sits in the tank the more it costs to keep it hot. Was wondering if anybody tried a set up like this and had some experience.

Secondary to this is that I want to replace my old lo-hanging gravity loop heating set up with a pex manifold configuration. If I had a separate HW heater I could take the boiler offline for a summer and take my time doing the heating work myself.

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Not a plumber by trade but a fierce DIYer

Has the boiler ever produced enough hot water for you? Has it been de-scaled annually? Is it set up for optimal DHW production?

The knock on combi boilers is that in most cases there's sacrifices that shouldn't be made to use them. Mod/Con boilers have to be matched to the heat loss of the structure to get that high efficiency. If the boiler is not condensing, you're getting the same efficiency as a cast iron boiler, low 80's percentage-wise. Contractor will put a combi boiler in that's twice the size of what's needed for heat loss. Then they'll tell you it's OK, the boiler modulates. In fact, it can only modulate down to the actual heat loss, and the boiler never condenses.


The best installation for any mod/con boiler is a high volume system, like a gravity conversion. That extra volume allows for cooler return temperatures, and will keep the boiler condensing longer. You're proposing removing that.

paul, while i do agree that the harder the mod-con boiler works, the happier it is... i do also think that removing big heavy (BTU eating) piping is always a good thing to do.
separating the house into multiple zones is always cost effective, not to mention more comfortable..
so if you lose some efficiency at the boiler, the fact that you are heating 10 gallons of water to heat the house has you way ahead of heating 30-40 gallons with 2 or 3 inch feed and returns.

Hot water has never really been satisfactory...the temp fluctuates and it will go cold for about a minute mid-shower then get hot again. It has done this from day 1, nothing seems to help, and a web search turns up plenty of folks asking the same question but nobody offering an answer. The installer added a filter on the cold input side of the DHW, service manual says that de-scaling is only necessary if there is a decrease in DHW output, which we haven't noticed.

If money was no object I would have a pro come in and do the heating re-pipe and at the same time abandon the DHW coil and install indirect DHW, but that's $$$...I can DIY an electric water heater no problem, and that would enable me to DIY the pex manifold, but I don't want to get killed with electric costs long term.

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Not a plumber by trade but a fierce DIYer

packy…....sorry.....some of that got lost in translation. A lot of that had to do with combi's not being sized correctly for the heat loss. Correctly done, if the house had a heat loss of say 80,000 btus, you'd put a stand-alone mod/con in, with a 10 to 1 turndown ratio and make sure your smallest zone is no smaller than 8,000 btus. If you wind up with some zones that are smaller, contractors will add a buffer tank. It adds volume and doesn't have to be wasted. It can be used to preheat domestic. I agree with the comfort aspect, but there's no sense in paying the premium for a high efficiency boiler, and not getting what you paid for. That said....some jobs are better suited to a cast iron boiler.


I understand you hesitating with the electric water heater. We have ridiculously high electric rates here also. I'd consider not wiring the electric, and using the thermostat on the electric water heater to run a circulator(poor mans indirect).

paul, they make a poor mans storage tank. it is called an aqua-booster..
if you used an electic tank as storage it would pipe like this..
how it works...
[www.vaughncorp.com]

I guess everyone has a different definition of poor....yikes!

Packy that's really interesting. So you think if I used a storage tank plumbed like this it would use less electric than firing a elements in the HW heater? Would you use a regular Taco circulator?



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Not a plumber by trade but a fierce DIYer



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taco 006-B4


NOTE.. the tank i show has an extra tapping but you could do the same thing by using the cold inlet at the top of the tank and cutting about a foot off the dip tube.
ALSO NOTE.. the check valve installed above the circulator so the incoming cold is directed into the coil not into the tank.

I wonder if it's possible to use an elec water heater and wire it so that the aqua stat could control either the recirc pump OR the electric elements?

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Not a plumber by trade but a fierce DIYer



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there are 2 elements on most electric water heaters. so there are also 2 thermostats.
the thermostats just set against the hot tankso i suppose you can add a third 110v thermostat quite easily..
just pretend the element is the 006B circulator..

"easily" would depend on how "easy" it would be to add a neutral wire for a 120v circuit.

he is not making a piece of artwork to display at the louvre..
he is essentially using the t-stat as a switch for a 1/25 HP motor.
mount a 4 x 4 electric box on the heater adjacent to a hole cut in the outer shell. cover the hole for the t-stat with a home made cover.
heck, the manufacturer covers the hole they make with something that looks like a cover to a half gallon of ice cream.

We'd need to know some more about the functions of his combi to integrate a storage tank/pump created with a water heater. It's not just a tankless coil on a boiler. There are switching functions for central heating vs. domestic hot water production. The route he seems to be going in his latest post suggests he is just putting the water heater in line and wiring it. Although that may not be the cheapest way, it is the easiest.

That's what I am thinking Paul. The Bosch combi is rated at 4 GPM continuous for DHW. My understanding is that the boiler has two sections, and the DHW side is not a coil per se but a high efficiency plate system like a tankless DHW uses (Rennai, etc.). We never run out of hot water, but the temp is inconsistent, so if you're in the shower you are constantly fiddling with the knob to keep the shower a constant temp. Also if you want to take a shower when the dishwasher is running, it struggles to keep the water temp up with 2 fixtures at once.

Here is the service/installation manual if you want to read more about the boiler: [www.bosch-climate.us]


I'm also thinking that if I have the HW heater wired for electric, then I could shut down the boiler over the summer and take my time replacing the gravity loop with a manifold system. I wouldn't have to change any plumbing, with the heating side drained the cold water can still flow through the DHW side of the boiler, it just won't be pre-heated and the HW heater eill do all of the DHW work.

I'm also thinking of plumbing in a bypass valve setup, so I can bypass the HW heater and just have on demand DHW from the boiler if I have a problem with the HW heater.

Here are the two leading choices right now, both are .93 UEF. The AO Smith says it has 3" of foam insulation, but a plastic drain valve:

[www.lowes.com]

The Rheem has a brass drain valve but doesn't say anything about insulation, what type or how thick:

[www.homedepot.com]

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Not a plumber by trade but a fierce DIYer



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True, but it appears he wants to power it with the heater's 240v system. To do that he MUST have neutral conductor which a water heater does NOT have, unless they "illegally" connect it to the ground wiring.

hj, there's no water heater and no wiring there now, so I could easily run 10/3 instead of 10/2. Most new 220v appliances like stoves, dryers, etc., require a neutral to power some of the lights and electronics so a 4-prong outlet is common now. Not that a water heater uses an outlet but you get the idea.,



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Not a plumber by trade but a fierce DIYer



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For quite a while, I ran my hot water through a tankless coil in the boiler(oil) to a gas water heater. Even in the summer, my gas bill was tiny. I realize you have a different scenario, but I'd expect your electric bill would not increase by much. The water heater will only kick on during periods that you would be exceeding the output of the combi. Set the tanks temperature with the combi shut off, at 10 degrees below the set point of the combi. Set it high, and use a mixing valve, off the tank.