Water boiler with tankless coil not enough for DHW

Mikerandazzo78

TLDR: can I add a wall switch to tell the boiler in taking a shower and to bring it to its high temp.
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have a brand new boiler with tankless coil. hydrostat is now set to low of 150 with diff of 10.

So when the boiler hits 140, the temp will rise to 150 even I don’t need any hot water.

This produces decent water around 110 water temp on the cold side and hotter as the boiler gets hot.

A few issues. I feel this is a waste of oil to keep the boiler temp at 150 for the “chance” I might take a shower.

I know cold start doesn’t work either, but I’d like to keep the boiler around 120 low limit when not in use and have a way to tell the boiler to “kick on” and get up to temp for a DHW demand.

Assuming I can do this with a light switch and relays, but wanted to get thoughts on this of am I just crazy to think this up.

Was also thinking of a flow detector on the hot water output side of the boiler to automate this, but a switch is more flexible.

Thanks for reading. You made it to the end. Appreciate it.

EdTheHeaterMan

You have a system with a tankless water heater in your boiler. As a result of this choice you have chosen to leave your boiler maintain a minimum temperature all year long (just like a tank type water heater that heats water with gas or electricity or oil all year long). There is nothing wrong with that. A tank type water heater will also operate every so many hours for a short time the maintain hot water even what you are not using it.

In order to get hotter water you will need to increase the minimum boiler temperature from 140° to 150° by setting the low limit to 160°

Larry Weingarten

Hi, Boiler controls are not my thing, but others here will have things to say. I'm certain 😎 … I'm thinking a simple on-off switch could cause problems, but might a timer switch limit the time other controls are overridden? 🤔

Yours, Larry

EdTheHeaterMan

Regarding your query to install a switch near  your sink or shower to turn off the maintain boiler temperature until you want to take a shower, that is doable. you will need to set up your boiler to be a cold start boiler if you have an electronic aquastat like the HydroStat 3200s or Honeywell/Resideo L7124.  Then you will need to add a low limit to maintain temperature that will also operate the burner thru a manual switch.  If you have the older L8124, then all you need is a switch.

I can draw you a diagram if you wish. What controls the burner. Open the cover and take a picture of how the wires are connected.

Mad Dog_2

I hope you have a thermostatic mixing valve on the coil to prevent scalding. Can you show us a picture? Mad Dog

Mikerandazzo78

thank you Ed/Maddog for the quick responses!!!

Some pics attached…using Hydrostatic 3250

Right now with low limit set to 150 the shower is decent. Shower Flow is barely 2GPM so I’m okay with the pressure. When the heat is running boiler goes to high and if you’re in the shower then, you get a much hotter shower. We will add cold as needed in the shower as we are used to this game.

I feel like I’m wasting oil by keeping the boiler so hot (150) for the chance I “might” need hot water. Also in the summer when the boiler is only used for DHW this can get expensive.

Was looking to drop the LowLimit to 120 and then have a “light switch” turn on to kick the boiler on to its high temp. As for scalding, we are safe with this since we have to deal with it when the heat is also callling

I’m in Long Island, NY. Is this high value for low limit?

Was hoping

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EdTheHeaterMan

@Mikerandazzo78 said: I feel like I’m wasting oil by keeping the boiler so hot (150) for the chance I “might” need hot water. 

Would you feel this way if you had an electric tank type water heater? Gas tank type water heater? Oil fired tank type water heater? If you want to use hot water like Americans like to use hot water, then you want it when you want it. Not 20 minutes after you turn on a switch. Europeans have higher energy costs so they have adapted to Gas on demand tankless water heaters along with other energy saving technologies like indirect water heater tanks. Your heating boiler is just a funny shaped hot water tank with a heat exchanger inside it.

That said, you do have the ability to make the burner operate with a switch about 10 minutes before you take a shower. Let me look at the stuff you will need and draw you a diagram

HVACNUT

These are the drawbacks of having a tankless coil. I should know, I have one. An indirect wasn't an option?

If nothing is connected to C1,C2 in the Hydrostat, then you can move Z/I to I, and switch ZR,ZC. Switch off, Cold Start, or low setting. Switch on, burner runs to high limit.

I don't recommend it, but you can do it.

EdTheHeaterMan

I am not sure if this will work, but I should.

1. Set the high limit to 160 or above (If you don't get enough heat on the coldest winter days then set higher)
2. Set Low limit to lowest temperature or OFF
3. Add a switch to connect L1 with ZR on the Hydrostat 3250 control
4. Set the Indirect / zone switch to I
5. When the switch is turned ON the burner will operate to maintain 160° and the circulator will be turned off for up to 45 minutes. after 45 minutes the DHW priority will time out and the circulator will operate for space heating. This is a feature you want to have in the event you forget to turn off the hot water manual switch.

This is the simplest, most energy saving way to operate your system using the water heater on manual demand. When the switch is off, your boiler will operate like a cold start boiler. That is the most efficient way to operate a boiler that does not have a tankless coil. In the winter when your boiler is operating on and off as the thermostat calls for heat, you will get warm water from your tankless coil for hand washing and other short uses.

Let me know how this works out. I'm no longer able to go into basements and test this out for myself.

Mikerandazzo78

thanks everyone. I will get a local electrician to make sure this is code and I can always disconnect if it doesn’t work.

@EdTheHeaterMan any recommendation on a switch? Regular light switch?

Shower is about 50 feet from hydrostat so was thinking of a light switch near shower.

Attached under “circulator contacts” are I think the amp values for Zr but I’m not sure.

Thanks for all the help.

EdTheHeaterMan

I would do a test before getting an electrician involved. Use a regular 15 AMP light switch from home depot and connect it near the boiler. after you are sure that it will work, then you can get an electrician to do a nice job running a concealed wire from the bathroom to the boiler room. Make sure it is located in a place where someone that is not used to it will not just turn it on to see what it does. As far as they will know, nothing will happen and the switch may stay on and your savings will go out the chimney until you find it and turn it back off. Perhaps a red cover and a label that states Hot Water Demand. Perhaps a 30 minute 120 VAC Timer Switch would also be a good idea.

There may be a 10 minute wait time from switching on before you get hot water, in some cases.

mikedo

put in an indirect water heater and you wont have to maintian the boiler tempature it will only run on a call for heat or hot water

Mikerandazzo78

@EdTheHeaterMan thanks. Just seeing if a 15a light switch is good enough. It’s just connected to 120v hot so there should be no draw.

Thanks again.

EdTheHeaterMan

5 amp is enough. the circuit from 1 to ZC is 120 VAC less than 5 amps.

Mikerandazzo78

thanks @EdTheHeaterMan

I will test with a basic 3 screw light switch and my DMM.

Assuming no grounding needed since it’s grounded at the emergency shutoff.

Does it matter which screw (power/load)

of the light switch goes to L1 and which one to Zr?

EdTheHeaterMan

EXACTLY THE SWITCH I WAS THINKING OF

Does it matter which screw (power/load)?

NO the switch will work in both directions.

Kaos

I hope you don't take this the wrong way but whoever installed a new boiler with a tankless coils did you no favours. Those were a bad idea 30 years ago when fuel was cheap, they are even a worse idea now as cooling season is longer.

Practically anything is better than a tankless coil. Best option is to replace with either a resistance heater or a HPWH. If you want to keep the tankless coil for wintertime heat, you can plumb it in before the new tank. This way if the boiler is running in the winter time, it will pre-heat the water goin to the tank. No need for any new controls or even keeping the boiler hot all the time.

This will let you shut the boiler down during the summer which will significantly reduce your summer time fuel use never mind the reduced cooling load.