Sizing of indirect for tankless combi boiler

sanjayp

I'm adding an indirect tank to my 'tankless' condensing boiler system. We're hitting the irritation of not being able to run 2 showers simultaneously. Replacing the boiler is not an option for me.

Advance apology for stupid questions:
If I add the tank, does that mean my endless 'on-demand' supply of hot water is no longer possible? This is one feature of the combi that is quite nice. I assume with the tank in place that the supply to the showers (and other inputs) will come from the tank. If the tank depletes, will the system go back to the default combi unit operation of on-demand (understanding that the output would be diminished again)?

My understanding is that is that the presence of the tank shouldn't adversely affect the heating system during winter. Or would it? For example, let's say enough appliances/showers are running that the tank is depleted in 30 minutes. How long does it take for the tank to fill back up (and therefore take priority over heating)?

Based on the last question, I'm trying to determine the size of tank. I want to go to 40 gallon max (due to taller tanks blocking a basement window)... maybe 30 gallon if there is an issue of the heating staying off to long. So this is related to my first question in that if the tank depletes, can the boiler go back to it's tankless on-demand mode and supply it's default tankless 2.5GPM.

Any recommendations on a good insulating tank to get?

Sanjay

kcopp

IF you add the indirect tank this will be a separate zone off the boiler.
The Combi part of the boiler will be abandoned.
You will have plenty of hot water.
How many baths and how many people?
What combi boiler?
Typically a 40 should be fine but if you have a lot of people (especially teenagers) you may want to step up to a 50 gallon.
There are many good options.
HTP has a stainless tank Superstor Ultra.
Bradford White has their PowerStor Indirect that is decent.

sanjayp

Thank you Kcopp. 2 bathrooms - 2 adults and 3 small kids. How will the addition of the tank affect the heating system? Concerned that the heating will go off for too long. Today, there is no noticeable affect when the combi goes into DHW priority for a 10 min shower. but I have no idea how filling a 40 to 50 gallon tank might affect the heating.

sanjayp

TriangleTube cc125. If I need to spend a little more for a higher end tank with faster recovery time, that's fine. I was looking at this line: http://www.triangletube.com/documents/2/SMART Literature.pdf

sanjayp

Great guidance Hatterasguy. i finally was able to track a designer and he indicated the boiler will want to use the ODR for heating. He wasn't sure how to pipe an indirect to get around the ODR. Have you heard of this? He said an electric tank used for storage might be better. I have seen reference to this approach on the board. I'm not concerned with getting ultra efficient as long as the heating and HW work. Any opinions on this? It sounds like indirect may be risky or if the indirect coil is of sufficient size, I should be OK. Again - no preference as long as things work.

sanjayp

Hatterasguy said:

Fire the designer.

Lol! Are you available for the job? (seriously!)

Firecontrol933

Another option to consider. You can use a standard 50 gallon electric water heater, a potable water circulator to add hot water capacity. This is how my setup at home is done after we found out our way of living wasn't compatible with our combi-boiler.

I installed the water heater in series with the output of the combi. I installed the circulator from the bottom of the water heater to the inlet of the potable water to the combi and also the cold water that used to go to the combi now goes to the tank bottom at the same location and added a check valve to keep flow in one direction only on the pipe from the circulator to the boiler. I wired the circulator to the lower thermostat on the water heater.

When the bottom of the tank cools down enough to close the thermostat (either because of natural cooling or you drawing hot water off the top of it) the circulator comes on and the boiler sees this as a hot water demand and fires up recharging the tank to whatever temp you have the thermostat set to.

To date we've never run out of hot water.

njtommy

If you use an electric hot water heater simply for storage it will not take much to keep the tank up to temp and it would use very little Electricity doing so. It would simply be piped in after the combi boiler in series.

sanjayp

This was recommended if anyone has an opinion http://www.thermo2000.com/content/en-US/s2_produits/optimizer.aspx I have never heard of this company.

Henry

I would go with a HTP SSU-45. It is of S/S construction. The Thermo 2000 are steel construction with copper coils. It will not outlast the HTP. There is also a drop in hot water pressure. They are made in a remote area of Quebec. Due to the exchange rate, they tend to be cheaper.

sanjayp

Thanks Henry.

Would be interested in electric tank approach vs indirect. It seems indirect is a little riskier in term of the installer messing it up.

Alan (California Radiant) Forbes

If you let us know where you live, one of the smart guys (or gals) here may be able to help you out if they are close to you.
Actually, other than Megan and Joanie, we have no women on HeatingHelp, no?

Henry

The boiler is designed to be adapted to an indirect. The only thing complicated, is the proper pump selection. The SSU-45 will give you 212 gallons of water at 140F and 295 at 115F which the normal shower temperature.

sanjayp

again thank you Henry. What did you mean by "There is also a drop in hot water pressure. " for the turbomax? How do you know?

gennady

I resolved these type of issues with indirect water heater piped as additional heating zone . I also left DHW heating on, in series with indirect water heater. Indirect aquastat is set at 140F, and cold water first enters water heater, then indirect water heater, then gets into mixing valve, where it downmixes to 125F to the house. The trick is to disable outdoor reset for indirect water heater operation. So I install additional relay to disconnect outdoor sensor and heating pumps to prevent overheating of the house and to provide 180F to heat indirect water heater on indirect water heater call.

sanjayp

Gennady, does that mean the outdoor reset is not available for the heating system anymore? Or are you saying it can be disabled temporarily? Right now with no indirect and using the regular tankless mode, the boiler obviously bypasses the outdoor reset in the summer and turns on. But with the indirect, I hadn't considered that the outdoor reset would prevent the boiler from heating. However per hattersaguy if this TTboiler can be configured for an Indirect, I would have expected an automatic bypass to be in place without doing anything special.

gennady

Outdoor reset is disabled only when indirect water heater aquastat is calling for heat. when call is off outdoor reset gets connected again. Any combi boiler has DHW priority. When no DHW flow is registered heating should work on outdoor reset. When weather will get mild, heating water will be not hot enough to provide indirect water heater heating. By disabling outdoor reset we send boiler to max heating water temperature.

Jason_13

I personally like the stone lined tanks. No anode, no water condition issues and a replaceable coil if need by and from the top not the bottom. The stone lining aids in recovery. I have not had any issues yet but only been using about 6 years.

bob eck

Have you considered installing two new 1.5 GPM shower heads?
Check the GPM your current shower heads are now delivering.
Do you have hard water? If so have a contractor flush out the heat exchanger that makes domestic hot water. Do you have a old system with cast iron radiators and black steel pipe. You could have iron in your system that has built up in the heat exchanger. Hire a good Heating professional and let him/her look at the system. You might install a indirect water heater and still not have enough domestic hot water because boiler heat exchanger might be getting plugged by iron. You could also have a cross connection in your plumbing system where cold water bleeds over into the domestic hot water side of the plumbing system cooling down your domestic hot water. Is the mixing valve on the TT CC125 working correctly? Many questions to answer. Where are you located at?

Alan (California Radiant) Forbes

For whatever reason, Mr. sanjayp doesn't seem to want to tell us where he's from. I give up on people who don't share even the most basic questions. Just say the Dakotas or Northern Saskatchewan or Long Island. You don't have to be embarrassed to tell us you're from the Hamptons.

Alan (California Radiant) Forbes

Sorry, I think it's Ms. sanjayp.

sanjayp

Closing this out.. hired a designer; Went with HTP as recommended earlier in the thread.

to Alan -- **** with the ridiculous replies? I had moved on with the selection before seeing bob's response today. It's a message board. Don't take a non-reply personally. Most folks don't. Stupid messages have no value to a board like this.