Looking for users with condensing boiler with DHW experience

[randy]

We have a 20 year old boiler/indirect water heater combination. I’m thinking about replacing it with a condensing boiler with DHW built in. When producing hot water, this is essentially a tankless water heater. I’m looking for information from someone who has one of these, specifically about hot water rise time. Tankless water heaters can generally produce hot water in 15-20 seconds. When this feature is in a combination unit, how long to produce hot water? Production volume is not a concern as much as the time it takes to get hot water. I don’t think this will vary much between brands, we are currently looking at a Noritz

[ServiceXp]

The best way to get that information is to contact the manufacturer. Some have heat rise charts in their litt. but otherwise give them a call.

Never heard of Noritz, we install Rinnai and have had great success with them, customers love them.

We used to sell Takagi, (it's been about 8 years) but we had problems with them.

[randy]

I've read what the specs are - somewhere between 15 and 30 seconds to produce hot water, depending on the type of heat exchanger. The flow rate is between 4 and 8 gpm, depending on the boiler. The reason I was looking at the Noritz was the higher sustained production rate of 8.3gpm, as compared to 4.2 on the Rinnai. I was looking for more subjective experiences from actual users. Does it seem like forever for hot water to flow or is it reasonably quick? We don't have a recirculating system, so it takes about 10-15 seconds before the water comes out hot in the bathrooms and kitchen. Adding another 10-15 seconds wouldn't bother us, but is that what it will be? I can just replace the boiler and keep my indirect DHW system, but if these combination boilers can make hot water quickly enough, i would be inclined to go with one of them.

[ServiceXp]

I've read what the specs are - somewhere between 15 and 30 seconds to produce hot water, depending on the type of heat exchanger. The flow rate is between 4 and 8 gpm, depending on the boiler. The reason I was looking at the Noritz was the higher sustained production rate of 8.3gpm, as compared to 4.2 on the Rinnai. I was looking for more subjective experiences from actual users. Does it seem like forever for hot water to flow or is it reasonably quick? We don't have a recirculating system, so it takes about 10-15 seconds before the water comes out hot in the bathrooms and kitchen. Adding another 10-15 seconds wouldn't bother us, but is that what it will be? I can just replace the boiler and keep my indirect DHW system, but if these combination boilers can make hot water quickly enough, i would be inclined to go with one of them.

Wow, 8.3 for DHW (assuming your time rise is 70-80F). That is CRAZY good for a Resi Tankless Combi unit.

[randy]

Wow, 8.3 for DHW (assuming your time rise is 70-80F). That is CRAZY good for a Resi Tankless Combi unit.

Digging deeper and looking at how hot we like our water and the groundwater temperature, we would need up to 80 degree temperature rise. On the Noritz that is going to be more like 4.5-5.5gpm, almost identical to a similar sized Rinnai. Regardless, I’ll decide on brand next, but first I want to decide if I am going to retain an indirect water heater or if a combination boiler would be a better choice.

[racerfern]

My comments are on the wait time nothing more. I used to have to wait about 20 seconds with a conventional gas fired 40gal tank being a ranch house with a long run to the other side. Then I installed a small pump with a z-wave control and a dedicated return line. Being creatures of habit, it was easy to create an Easytrigger schedule to accommodate our life style. I now have to wait less than 5 seconds for hot water even at 3 in the morning because of Xiaomi motion sensors in the bathrooms.

It's been bulletproof until the z-wave switch failed. That very morning, the WAF took a nose dive. It's amazing how quickly we come to accept the new norm when it's a nice improvement. Then all of a sudden it's expected. I told Karen to calm down which of course started another battle. 😆

We're back to less than 5 seconds and I can call my wife by her real name once again. What really started if for me was watching all that water going down the drain.

[ServiceXp]

This is what we recommend for those who don't want to wait and don't mind the extra energy costs.

Sent from my LM-G820 using Tapatalk

[ServiceXp]

Digging deeper and looking at how hot we like our water and the groundwater temperature, we would need up to 80 degree temperature rise. On the Noritz that is going to be more like 4.5-5.5gpm, almost identical to a similar sized Rinnai. Regardless, I’ll decide on brand next, but first I want to decide if I am going to retain an indirect water heater or if a combination boiler would be a better choice.

Ok, that makes way more sense.

Sent from my LM-G820 using Tapatalk

[racerfern]

ServiceXp I'm not sure I could tell the difference in the gas bill, though it's not nothing. Cycling relatively warm water back into the tank on a regular basis might be about the same as bringing in cold water. Hard to tell. No wasted water though.

[mminehan]

Take a look at OpenTherm. It's a non-proprietary communication protocol for boilers and thermostats. You can even get MQTT OpenTherm gateways. OpenTherm is available from a wide range of boiler manufactuers. A bit of Googling and you'll soon find the relevant information.

We have a condensing combi boiler (central heating and domestic hot water). The central heating (CH) is via hydronic underfloor heating in the concrete slab. Domestic hot water (DHW) is via a storage cylinder and heat exchange coil from the boiler. And like racerfern we have a reticulation pump/loop from the showers controlled by a zwave really that is on a timer (push a button and wait 2 minutes then the showers are instantly hot, and no need for a continually running loop or wasting water waiting for the hot water to arrive).

With OpenTherm you can control setpoint for both DHW and CH as well as modulation (condensing boilers are most efficient when not running at 100%) and get fault codes. All this is visible and controllable from HS via the MQTT OpenTherm gateway (see images attached). In our setup the modulation level is adjusted based on the CH demand (number of running zones). That way the CH is not rapidly turning on/off while the CH zone valves slowly open and close (see the graph). Modulation can be quickly raised when there is DHW demand.

Let me know if you want more info.

Marty

[randy]

I appreciate all the answers and suggestions, but at this point I am only trying to find out if anyone has any real world experience with any of the new condensing boilers with built in continuous hot water. I want to know how long it actually takes to generate hot water from a cold start. These boilers do not have any DHW reserves, they heat in demand. A recirculating pump defeats the purpose. Besides we have a small house and the longest run for hot water is only about 12 feet. Once I have decided whether to maintain my indirect DHW tank or ditch it in favor of continuous hot water, I can then work on the automation. Even my 20 year old Vailant boiler has benefitted from automation, I have written about it in detail. A condensing boiler will take completely different logic, but I’ll cross that bridge when I get to it.

[billt]

I wouldn't install one from choice.
We've recently bought a small flat which has a fairly modern modulating condensing combi boiler (hot water on demand). The delay in starting is irritating but that's not the only issue (never timed it, but it's too long). It needs a certain flow to operate, so if you only want a gentle flow the boiler turns off and the water goes cold. The set temperature is too high; when you mix with cold water the low flow issue gets worse.
If the whole plumbing system is not properly designed you can get interaction with the other water consumers in the house. If you're using a shower and someone else runs water elsewhere the flow drops. If the system is mains pressure opening a cold tap elsewhere can drop the flow to the boiler and raise temporarily the hot water temperature.
Some of these issues can be alleviated by proper system design; maybe the temperature control issues aren't a problem with a well designed boiler, but I've never been in a house with a combi system that I would be happy to use. (In the UK they are most often installed in small, cheap properties as they take up less space and are significantly cheaper to install than stored hot water systems, so the standard of installation is probably commensurately poor!)