Indirect with cold start boiler

spd1980

Hi,

Does it make sense to use cold start with an indirect? Without enough storage and boiler capacity, I imagine the recovery rate wouldn’t be very good so it would probably make more sense to have a low limit. I guess more generally what factors would determine whether or not cold start should be used with an indirect?

Thanks!

STEVEusaPA

It's fine, do it all the time, even at my house. But it depends on the details. What boiler, what indirect-model and size for both?

In effect with an indirect it's a warm start. If you go all day and don't use any domestic hot water it will probably fire 3 times for 10 minutes.
When using domestic hot water, say for a shower, your boiler will be firing to replenish the tank before you even know it, especially true if you set your tank aquastat to 140° and you have a mixing valve.

Now if you have some crazy hot water usage, like 2 people come home and take showers at the same time while starting laundry and filling a bath tub...

EBEBRATT-Ed

your low limit is the indirect control

Robert O'Brien

Eliminating boiler standby loss is the key to efficiency

spd1980

STEVEusaPA said:

It's fine, do it all the time, even at my house. But it depends on the details. What boiler, what indirect-model and size for both? In effect with an indirect it's a warm start. If you go all day and don't use any domestic hot water it will probably fire 3 times for 10 minutes. When using domestic hot water, say for a shower, your boiler will be firing to replenish the tank before you even know it, especially true if you set your tank aquastat to 140° and you have a mixing valve. Now if you have some crazy hot water usage, like 2 people come home and take showers at the same time while starting laundry and filling a bath tub...

I’ve got a Utica Trifire 5100 oil boiler with 123 MBH capacity and a Superstor SSU-45 indirect. I am adding a mixing valve so I can set the indirect to 140 and the typical hot water peak load would be 2 showers or 1 shower plus laundry. With this setup I wonder if I should turn off the low limit? If not then what would be a good low limit value? Also, I wonder how condensation factors in. If my hydrostat holds off the circulators until the boiler water comes up to a certain temp then would that be enough to prevent condensation issues? Thanks for the info, it has been really interesting to learn more about these various systems.

spd1980

EBEBRATT-Ed said:

your low limit is the indirect control

That’s interesting, would you mind expanding a bit on this? Are you saying that the indirect control would call for heat frequently enough that the temperature of the boiler water wouldn’t have time to fully drop to ambient temperature?

STEVEusaPA

It depends on how your boiler is piped and wired. Do you have 2 circulators, or one circulator and 2 zone valves, or are you using your tankless coil and piping that to the indirect.
I don't think the low limit is controlling the indirect. Low limit does typically control a tankless coil.

Alan (California Radiant) Forbes

I've got a 50,000 BTU Munchkin and a 50 gallon indirect and don't have any problems with recovery just so long as the showers are less than 10 minutes if there's heavy usage.

If money grew on trees, I'd upgrade to a larger indirect. But really, in this day and age, just pace the DHW usage. If you run out of hot water, wait.

EBEBRATT-Ed

@spd1980

What I meant was normally with an indirect you don't use a low limit on the boiler. No call for DHW the boiler is allowed to go cold.

The control in the indirect fires the boiler on a call for DHW so that becomes the low limit as far as dhw goes

TAG

My cast Iron Buderus boilers hold the circulator until the boiler gets up to some preset temp. Never changed them from whatever Buderus uses. Only then does it start to charge the indirect. The circulator is direct from the boiler. Another interesting thing the boiler does is stop the indirect from recharging at a preset time later in the evening. The tank uses its stored hot water for any use after ... it timed to restart and have the indirect up to temp when you want hot water in the AM. This also has the boiler hot should the indirect need to provide additional heavy hot water usage. When I did my first one 30 years ago this timed off period worried me .. but, I can count on one hand the times I have hit the override on the control. These are 112k boilers and matching 40g indirects. The older Buderus indirects with heavy steel walls and glass lining had more mass and often never even needed a recharge in the AM even with usage .. The new SS tanks seem to need the AM boost.

A typical 40g gas water heater has something like a 30k BTU burner ... so your boiler is much more. Most indirects are well insulated and don't drop very much.

MikeAmann

I just added a Hydrostat to my boiler. The Hydrostat will not let the heating circulator (C1 C2) operate until 125 F. And if the IWH is wired through ZC ZR, the burner will immediately fire and cut off C1 C2 until the temp reaches 170 F. Then the water is hot enough to operate both circs at the same time. I also have a mixing valve and set the thermostat on the IWH to 140 F. That tstat has an 8 degree differential, so I set the LO limit to 132 F.

spd1980

STEVEusaPA said:

It depends on how your boiler is piped and wired. Do you have 2 circulators, or one circulator and 2 zone valves, or are you using your tankless coil and piping that to the indirect. I don't think the low limit is controlling the indirect. Low limit does typically control a tankless coil.

There’s two zones (1 for heat and 1 for hot water) and each zone has a circulator. The boiler hydrostat is set to 140 for the low limit and set to 190 for the high limit. The water heater has an aquastat set to 130 that activates the circulator for the hot water zone.

My current understanding is that the boiler low limit keeps the boiler water at a minimum temperature which should help the indirect start to recover faster during peak hot water demand. If I turn the boiler low limit off and the boiler water drops down to ambient temperature (outside of heating season for example) then it would take longer for the boiler water to heat up and take longer for the indirect to start to recover during peak hot water demand. Assuming my understanding is correct, it made me question whether or not I should keep my boiler water at a minimum temperature.

spd1980

TAG said:

My cast Iron Buderus boilers hold the circulator until the boiler gets up to some preset temp. Never changed them from whatever Buderus uses. Only then does it start to charge the indirect. The circulator is direct from the boiler. Another interesting thing the boiler does is stop the indirect from recharging at a preset time later in the evening. The tank uses its stored hot water for any use after ... it timed to restart and have the indirect up to temp when you want hot water in the AM. This also has the boiler hot should the indirect need to provide additional heavy hot water usage. When I did my first one 30 years ago this timed off period worried me .. but, I can count on one hand the times I have hit the override on the control. These are 112k boilers and matching 40g indirects. The older Buderus indirects with heavy steel walls and glass lining had more mass and often never even needed a recharge in the AM even with usage .. The new SS tanks seem to need the AM boost. A typical 40g gas water heater has something like a 30k BTU burner ... so your boiler is much more. Most indirects are well insulated and don't drop very much.

Sounds like a nice setup especially not heating the indirect overnight. 

spd1980

MikeAmann said:

I just added a Hydrostat to my boiler. The Hydrostat will not let the heating circulator (C1 C2) operate until 125 F. And if the IWH is wired through ZC ZR, the burner will immediately fire and cut off C1 C2 until the temp reaches 170 F. Then the water is hot enough to operate both circs at the same time. I also have a mixing valve and set the thermostat on the IWH to 140 F. That tstat has an 8 degree differential, so I set the LO limit to 132 F.

It sounds like you have a similar setup so maybe 132 would work well for me too. 

mattmia2

Alan (California Radiant) Forbes said:

I've got a 50,000 BTU Munchkin and a 50 gallon indirect and don't have any problems with recovery just so long as the showers are less than 10 minutes if there's heavy usage.

If money grew on trees, I'd upgrade to a larger indirect. But really, in this day and age, just pace the DHW usage. If you run out of hot water, wait.

I'm guessing in CA you also don't see 35 f incoming water temps.