How does a combi boiler handle substantially higher or substantially lower DHW input water

john123

How does a combi boiler manage its gas (turn down?) and its DHW temperature output when the temperature of the incoming water increases or decreases substantially.

It looks like you can set the temperature high limit of the DHW output for the combi side of the boiler. In addition, it looks like there is a fixed maximum limit to provide a scalding protection as well (?140F). When there is a demand for DHW, I understand the valve sends all the boiler water to the flat plate heat exchanger for the DHW side and cuts off completely that flow of boiler water to the space heating side. The boiler water returning from the heat exchanger is plugged in to the boiler stream just before the fire box. Is this correct? Do all combi boilers work this way??

So when the temperature of the incoming water rises so much that the boiler water could exceed the set point or even the scalding limit, what happens? Does the boiler modulate down in terms of its fire capacity--no longer using the full capacity of the boiler?

When the temperature of the incoming water drops to near freezing (say 35 F) and the boiler is running at full capacity: what happens? Lower gpm? Lower temp?

GGross

It depends on the boiler, some are really just guessing, going to high fire or close trying to maintain a setpoint boiler temperature, others are modulating the burner to reach the desired output DHW temperature, almost all are monitoring flowrates. Some have slightly more advance programming that controls boiler setpoint temperature to more accurately be able to get the desired output DHW temperatures

john123 said:

When the temperature of the incoming water drops to near freezing (say 35 F) and the boiler is running at full capacity: what happens? Lower gpm? Lower temp?

Depends, if the boiler can maintain the load nothing really happens. Generally if the boiler cannot maintain the load the output temperature would just drop, some brands may limit flowrate in some way but I don't see that personally



If this question is coming up because you think you want a combi, any good brand will have a chart showing achievable flowrates at a given temperature rise. Well water can vary greatly from one place to another, but generally from the same source it won't deviate that much. If you know the temperature of your well water, and the temperature you want out of the unit, then you can get the temp rise needed and plot it against the chart to see the possible flowrates. Personally I recommend a boiler with indirect, or standalone water heater if you have the room for it

mikeg2015

Brazed plate heat exchangers don't have much mass, so they change temperature pretty quick. However, they don't always respond fast enough which is why here by code, a mixing valve much be installed.


We only recommend combis for small condos and homes witt 1 or 1-1/2 bathrooms or maybe 2 baths and slightly larger home with in floor radiant or cast iron radiators... very high mas systems.

With the very cold winter water here and altitude derate (except lochinvar) they can struggle on anything but a small home.

Jamie Hall

@GGross 's explanation is excellent. However, let me reiterate one point: one way or another, the boiler at maximum fire will have a very specific relationship of temperature rise for the domestic hot water and flow rate. If you exceed the flow rate -- and the boiler doesn't limit it (I'm not sure that any do) -- will get a nice cool or cold shower or whatever.

hot_rod

An ASSE 1070 mixing valve can respond in seconds and maintain a fixed output within a few degrees. That is part of the listing requirement. If you use a mix valve look for a brand and model with that listing and fast response.
Newer shower valves also have temperature regulation, some do it by temperature some by pressure and top line will be pressure balance and temperature control.

The tankless/ combi manufacturers got together and developed a few new listings. They claim their products can maintain temperature as accurately as an ASSE 1070 valve, so no mixed should be required to meet code.

You will also find some tankless, maybe combis too, that are adding a small buffer tank to help even out the temperature fluctuations and "cold sandwich" effect. It also helps a recirculation system work better.

4 gpm or more is possible with mid and large size tankless. That should cover most all residential.
With the exception or 15 gpm body spray showers.

Also remember if the tankless is putting out 4 gpm of 120F water, you will blend some cold at the shower, so the actual flow and mix of cold needs to be considered.

john123

Thanks for all your comments. Are/is there any combi boilers that treat the DHW as a zone and send the unused hot boiler water into the system loop for the space heating?

Mad Dog_2

They should all have "priority" logic.  Even Indirect Tanks and Relays work like this.  On a Call for Domestic HW, all hands are on deck and run to meet the demand.  Circulators for the  Heating zones shut off, until HW Demand is satisfied.   Mad Dog 🐕 

mattmia2

Mad Dog_2 said:

They should all have "priority" logic.  Even Indirect Tanks and Relays work like this.  On a Call for Domestic HW, all hands are on deck and run to meet the demand.  Circulators for the  Heating zones shut off, until HW Demand is satisfied.   Mad Dog 🐕 

It doesn't have to be that way but that is how most systems are designed. You could use an indirect with more storage capacity to let the indirect use whatever is left over from the heating zones.

mattmia2

john123 said:

When the temperature of the incoming water drops to near freezing (say 35 F) and the boiler is running at full capacity: what happens? Lower gpm? Lower temp?

It is up to the engineer that specd the boiler to make sure it has enough capacity for that condition. In cold climates where you have municipal water supplies that are just above freezing in the winter that would mean cascaded or commercial boilers to get over 3 gpm or so. An indirect is almost always the more practical solution unless it is a commercial installation that has a continuous dhw demand.

john123

Thanks all for your comments.

Suppose you have a combi boiler that, from the specs, can provide maximum 3 gallons per minute of 140F DHW at 77F delta and the specs say 3 gallons per minute is also the max DHW flow rate. Is it likely or necessary that the maximum DHW flow rate would be designed to be the same?

Suppose the incoming water temp is 63F and you turn on a kitchen tap a little bit to rinse dishes at full hot to kill the germs. You are only getting a dribble---perhaps 1 gallon per minute but at 140F, I guess. Can this be done any way without modulating the boiler? (I suppose a small buffer tank will be helpful in the short run.) I have heard it said that when the the DHW is called for, the boiler runs at full throttle all the time?? Would that be cycling--on and off --on and off---on and off etc.??

Suppose the incoming water temp is 63F and you turn on the washing machine on hot, I guess you will get 3 gallons per min at 140F.

Suppose the incoming water is 83F and you turn on the washing machine on hot--well for sure you are going to get the max 3 gallons per minute at 140F, but can this be done in any other way without modulating the boiler?

Suppose the incoming water is at 33F and you turn on the washing machine on hot. Perhaps there is a valve controlling the gpm and you get something like 2 gallons per minute at 140F and the balance of 1 gallon per minute at 33F??

Thanks in advance for your interest. I am just trying to get a handle on how this combi thing works.

Jamie Hall

It may be a little easier to realise that what is really limiting is how much heating power -- BTU per hour -- the boiler can transfer to the domestic water.

If a boiler is rated, as you note, at 3 gpm with a 77 degree F temperature rise, that means that the boiler is rated to transfer 1917 BTU per minute (about 115,000 BTUh) to the domestic water. If you have less flow, the potential delta T will be greater (although the boiler should either modulate or turn itself on and off to maintain the maximum temperature). If you have more flow, the delta T will be less. Some boilers may have a maximum flow rate throttle build into them, though I would say it is unlikely. Any combination of flow rate and delta T which works out to 640 BTU per minute will work. For example, if you have cold incoming water at, say 33, then you could either have a maximum of 2 gpm at 140 or 3 gpm at 110 -- or somewhere in between.

john123

Thanks Jamie for your comment. Pertaining to your last sentence above-- "For example, if you have cold incoming water at say 33, then you either have a maximum of 2 gpm at 14O or 3 gpm at 110...": However if you only open a tap to dribble hot water, 1 gallon per minute, you will get 140F with the boiler either modulating or cycling quickly --ok but how would you find out which. It doesn't appear in the specs that I can find. I think the cycling would be hard on the boiler and the only reasonable design, is to modulate the boiler. Is there any other way besides cycling quickly?

hot_rod

I doubt you would want 140F at the faucets?
A shower typically 95- 105. So the output should be based on 120. you can set the tankless to 120F.
The hotter you run a water heater the more mineral scaling you will get. then output really drops off

Hot_water_fan

However if you only open a tap to dribble hot water, 1 gallon per minute, you will get 140F with the boiler either modulating or cycling quickly --ok but how would you find out which. It doesn't appear in the specs that I can find. 

It depends on the minimum boiler capacity: many combis can do a 10:1 ratio, so say a max of 200,000btus and a minimum of 20,000btus.

So as long as the output required is above 20,000btus, it won’t cycle. It’s pretty forgiving,  1 gpm at 40 delta T is about 20,000btu. 

hot_rod

They do cycle a lot in DWH mode.

My mother in law would rinse every dish by turning on the faucet, rinse, turn off the faucet  pick up another dish, rinse, turn on and off for every dish.  Then they went into the dishwasher. 

The combi must have cycled a 100 times after every family dinner. It lasted 16 years however with all those cycles

Mad Dog_2

I do that when I shave! When I first became a Plumbing Apprentice, I also became A Water Conservation Nazi. I would yell at my older brother Bart 6'1 245 College Middle Linebacker to not take 20 minute showers...Not a happy camper. In 1987, I worked in the Infamous, Notorious NYC Rikers Island Jail providing MORE hot water for LONGER showers...inmates were complaining! There was an outdoor water fountain in the middle of nowhere probably 200 yards from its source. The Laborers would rig the handle, so it was always Ice cold. They'd even leave it in over night. Really irked me. I'd shut it off, they'd rig it up. Showdown...The guy looked and acted like he should have been IN Rikers..

"What the #%%#$...your the #$#$% that keeps shutting it off!!" Me "Yup...you're wasting thousands of gallons of water, Bro!"

Him : "what the $#### to you care...you ain't paying for it!!! Why don't you move up in to the #$%$## mountains!!!" They stopped...mission accomplished.  Mad Dog 🐕 🤣