help with dual-coil indirect tank and condensing boiler

b027

Hi everyone, first post here, hoping to get feedback on our system setup and understand if a change in configuration or operation can help us use our indirect DHW tank more effectively. I have searched and could not find information specific enough to our situation.

Our system was installed around 2012, by previous homeowner in a two-story, single-family home. We have a dual-coil stainless indirect DHW tank. Original owner's intent was to add solar heating to the bottom coil, but this was never done. So the tank is only connected through its top coil to a condensing Viessmann Vitodens 100-W BIHE boiler (recently installed as replacement for the previous Vitodens boiler at end of its service life). The boilers DHW temp sensor is connected to the top port in the DHW tank, located at about 75% of the way up from tank bottom. There is a second sensor port near the bottom, at around 30-40% of tank height, that is unused.

Before the new boiler was installed, one of the companies that came to give an estimate indicated that the DHW tank should be connected using the bottom coil. Four other companies also reviewed the system but said nothing about the DHW connection (we were not aware at the time and did not ask directly). This question prompted me to start paying more attention to how the DHW is performing, and ultimately, to this post.

We heat the tank to 130F and get enough water for one shower at a time, and the boiler does a good-enough job replenishing hot water once it kicks on. The home also has concrete radiant slab downstairs and wall-mounted radiators upstairs. Home heat is nice and even using outdoor reset and a heating curve. We're very happy with the Viessmann boiler for home heat.

My objectives are to understand if we can change the system to have more hot water available to support 2 showers (most of the time with only brief overlap), and to understand if the current system setup is good or would show improvements from being re-plumbed in some way, or possibly the boiler configuration changed somehow.

specific questions:

1. Should the boilers' DHW temp sensor be installed in the upper DHW tank port? I suspect it may have been there when we purchased the house, and that a technician changed it at some point when we had the tank serviced, for unknown reasons. However it's possible I'm misremembering and it was always installed in the upper port.
2. The temperature of the DHW tank's lower port is a lot colder; when I briefly placed the DHW sensor there it read around 63-65 F. This seems to indicate much more stratification than I expected. Is this normal? I have long thought the tank does not deliver as much hot water as expected — I previously assumed the whole tank equilibrated with the bottom being only mildly cooler than the top, but I'm seeing a 65F degree difference. Stratification seems to explain the limited amount of hot water we get from a heated tank. I didn't leave the sensor in the bottom position as I expect this will cause overheating at the top and we don't have thermal protection on the DHW output.
3. Would it be advantageous to replumb the boiler to use the bottom coil, as suggested by the one installer? Would it be advantageous to plumb them in series, e.g. with input going to the top first, then the bottom, then returning to the boiler?
4. The boiler is using default settings for DHW temperature limits (I believe it heats at 5F below set point and shuts off at 5F above) — Would changes to these settings be advantageous? I expect reducing the differentials could make it more responsive to reheat faster on use, but we also want to avoid short cycling when the tank is idle.
5. Is there another recommended way to use the bottom coil? It doesn't seem that solar is very well regarded presently, and in any case our roofline faces east.

Thank you in advance for any feedback and suggestions! I can answer any questions that may be helpful.

Jamie Hall

Nothing fundamentally wrong with the hardware — but if you change your piping so that you are using the bottom coil to heat, not the top one, you'll be a lot happier… move the sensor down there, too.

Hot water tanks stratify — hot water on the top, cooler at the bottom — so if you are heating at the bottom, the whole tank will get nice and hot (the water circulates up slowly as it is heated). If you heat at the top, the bottom is going to be cool — if not outright cold — and will stay that way.

I would venture that you will wind up with at least twice if not three times as much hot water with that simple change.

hot_rod

Is it a Viessmann tank? It it is any tank with large diameter coils, low pressure drop, then tie them together. This may allow you to drop the boiler temperature down, get the boiler efficiency up. Possibly even get into condensing temperatures. 130 SWT, 120 return or lower.

May as well use both coils.

You have two options for flow direction. Hot boiler into the top gets you hot water quickly. Hot into the bottom first puts the boiler water against the coldest temperature, for best efficiency.

Is there a recirc system on the DHW?

b027

Thank you for the quick replies!

It sounds promising to switch, one way or another, to use the bottom coil. And it sounds like we should keep system config as-is until we're ready to make a plumbing change.

Tank is Heat-Flo 60 gallon, I don't have any specs on the coils. It may not be much help, but I found this on the web: "The Heat-flo Multi-Energy Indirect Water Heaters features large, dual coils with a smooth-wall design". Also, no recirculation system.

hot_rod

HeatFlo actually ties together the two coils in their hi performance tanks, inside the tank. They show outputs down to 150°SWT even with small output residential boilers.

I've just connected them in series on the outside of the tank, since the upper and lower are often different length coils. They could be parallel piped as HeatFlo does if the coils are equal length.

Heat exchange is always about the surface area. Can't have too much with a mod con/ indirect combination.

Here is an example of a dual coil solar tank with the coils connected with a piece of flex tube. Set up as a drainback solar tank. The coils were large enough to leave the air space in the upper coil, no drainback tank needed :)

hot_rod

You probably have this tank with top connections? It is super simple to connect the two coils on this tank. A couple ells and FIP adapters. With a press tool it can be done in an hour or less.

b027

Yes, I think you're right, the tank looks very similar and all the connections are all on top.

Thanks for the examples and diagram! I can see that it should very straightforward to do. Interesting also to see that HeatFlo does this already for higher output — which makes sense.

Looking back, it's ironic that we (second owners) had the capability for better performance and just weren't aware of the gap in setup, and that the original build, while intending to provide a higher efficiency option, may have caused more frustration that it was worth all these years…

Are dual coil setups with solar still of interest in new installations today? From the little I've read, it seems solar water isn't perceived to be as advantageous as it once was… though I'm sure that depends on regional climate. We're in a cold area (Chicago latitude). I wondered a few years ago if it might make sense to follow-through on the solar option but for us it seemed to add significant complexity with pumps and coolant etc.

hot_rod

It just depends on the motivation for doing solar thermal DHW. Dollar-wise, unless you build your own, install it yourself and get wholesale price on components, I don't see a payback for a residential application that uses so little DHW.

Some people add solar because they want some independence from the utilities. Some weave through the rebates and make it pencil out in their minds that way. Some buy it because it makes them feel good, like driving a Tesla.

When ST is in Vogue it is usually, due to generous state, federal and utility rebates. High energy costs, an administration that promotes RE options.

Pretty much anywhere in the country you can get a 50% solar fraction with a few collectors for DHW. Wisconsin and New Jersey have always been on the top of the list for solar installations.

It doesn't need to be complicated, a pump is the only moving part. Control, tank and collectors. Far less technology than your furnace or boiler. Repair parts will always be available. They are not hard to troubleshoot.

The big, daily, continuous load jobs penciled out. Year around water parks, with hotels, laundry, restaurants, etc.

One job in Michigan ST pre-heated the water for the Zamboni. Zambonis use hot water, 250 gallon or more typically. The ice is being resurfaced daily, depending on who uses it, it could be several times a day. So a predictable every day load to shoot for.

Several hundred collectors going on a water park in Dubai, possibly for heating and cooling the pools?

TAG

I have solar in SA .. there was always some debate on where to pipe the solar when using a single primarly duel tank. Most people connect to the bottom because the solar in most cases is enough to heat the tank and the second source is just there to help when there is a problem — like no sun.

These tanks are not used any more — or at least I have not seen them in quite a while. My place uses direct solar and the holding tank has electric for back up. The older setup used a small "on damand" electric.

My guess is the previous owner wanted solar DHW —- or thought it was a good idea and had it piped before he undersood the issues