Condensing boiler - temperatures

[daverobev]

Despite being on a fixed rate for natural gas for the next year and a bit... I'd still like to 'do my bit' and reduce our gas usage this winter where possible.

I'm in France, with hot water radiators and hot water for washing both coming from this giant machine, a DeDietrich EGC 25. There is no separate hot water tank. I have two settings without going into menus, for the temperature of these two things - heating hot water and tap hot water.

I don't want legionella. I've turned the tap water down to 53 (celsius) but I'm guessing I should probably leave it at 55. This is plenty hot enough for us. We currently use 5-6 kWh a day of gas, comprising of hot water and a gas hob.

But what about the radiators? Some of the radiator pipes run in the basement. I've put insulation round them where I can but it didn't make a noticeable difference last year. I'm assuming the radiator hot water in no way mixes with the tap hot water - so can I run that bit at a lower temperature (for longer) - is it going to be more efficient doing that? Say down to 45 rather than 55?

We've changed three large patio doors to double glazing and we're getting new loft insulation which will hopefully make a decent difference already, but if I can push a few buttons and save a few more %... Last winter we used about 80-90 kWh a day of gas (cooking, water, heating combined), which is manageable at 5 euro cents a kWh but will be very painful if we end up paying the current spot price here - closer to 30c/kWh! €25 a day for 3 months!!

[Bartlebooth]

I believe you can run your hot water radiators at any temperature.  At some point they will not put out heat quickly enough to keep your house comfortable.

In the north central US I run at about 120F in the fall and spring, and ~160F for most of the winter.  When there are cold snaps of -20F outside and especially if windy, I bump up to 180F.  My 1890s house has no wall insulation.

What I am thinking as I make these adjustments:

• Efficiency: a lower water temperature seems like it would be more efficient, as it will harvest more energy out of the flame/exhaust (drops it to a lower temperature).  I'll allow that a modulating boiler (Buderus in my case, GB142 I believe) may not actually burn as efficiently at low flames...IDK.  Less efficient combustion but you extract more of the heat?  Or you may get more on/off cycling with a non-moderating flame.
• Exhaust temp: with 180F water, you are probably exhausting 200-220F temperature exhaust.  My boiler has PVC intake and exhaust piping.  This is pretty hot exhaust for that pipe, and may cause problems over time.  So I imagine that I am extending the life of something by running cooler when I can.
• A 180F radiator is pretty hot, more difficult to enjoy sitting on it or resting your hand on it.  Somewhat risky for young children too.

In the end, I suspect more gains can be made by running the room thermostat itself at a lower set point, yet there is still some to be gained by running the water temperature at a lower temp.

[JLee]

I believe you can run your hot water radiators at any temperature.  At some point they will not put out heat quickly enough to keep your house comfortable.

In the north central US I run at about 120F in the fall and spring, and ~160F for most of the winter.  When there are cold snaps of -20F outside and especially if windy, I bump up to 180F.  My 1890s house has no wall insulation.

What I am thinking as I make these adjustments:

• Efficiency: a lower water temperature seems like it would be more efficient, as it will harvest more energy out of the flame/exhaust (drops it to a lower temperature).  I'll allow that a modulating boiler (Buderus in my case, GB142 I believe) may not actually burn as efficiently at low flames...IDK.  Less efficient combustion but you extract more of the heat?  Or you may get more on/off cycling with a non-moderating flame.
• Exhaust temp: with 180F water, you are probably exhausting 200-220F temperature exhaust.  My boiler has PVC intake and exhaust piping.  This is pretty hot exhaust for that pipe, and may cause problems over time. So I imagine that I am extending the life of something by running cooler when I can.
• A 180F radiator is pretty hot, more difficult to enjoy sitting on it or resting your hand on it.  Somewhat risky for young children too.

In the end, I suspect more gains can be made by running the room thermostat itself at a lower set point, yet there is still some to be gained by running the water temperature at a lower temp.

Are you sure about that?

https://propane.com/2018/09/17/8-facts-to-know-about-venting-tankless-water-heaters-2/

5. With a condensing tankless water heater, you don’t need metal venting. Non-condensing tankless water heaters typically transfer to the water only about 80 percent of the heat they generate. The remaining heat creates a hot exhaust gas that requires metal venting, typically stainless steel or thick aluminum. Condensing units, on the other hand, are typically about 95 percent efficient, so the temperature of the exhaust gas is lower — around 110 to 120 degrees. That means they can be vented with a less expensive plastic, generally PVC or polypropylene. The price difference in the venting can even offset the cost of the higher-efficiency unit, Olsen says. “The overall installed cost of a high-efficiency unit is typically equal to or lower than that of a mid-efficiency product, so it’s an easy up-sell at that point.”

I suspect building codes would not allow for exhaust piping that would exceed safe temperature tolerances.

Also...no wall insulation!? Holy crap, fix that!

[daverobev]

By moderating flame you're talking about how much gas is being burned? Mine can go through has 3-4 levels of power it seems.

It's a new condensing thing, it has a heat exchanger so I don't think the exhaust is anywhere near that hot! IIRC it's 'up to 109% efficient'.. this is it:

https://www.dedietrich-heating.com/products/product_ranges/floor_standing_boilers/gas_condensing_boiler_twineo/twineo_egc_25_v100sl

It rarely goes much below freezing here. We don't have wall insulation either - but the walls are 3 feet thick concrete, so it takes a while to heat and cool.

We also have a wood burner... with the new double glazing in that room I'm really hoping the occasional fire will keep us toasty for a lot longer.

All right, so 120F is just under 50 degrees C. I guess I'll turn the rad bit down a little.

Edit - actually the US govt says 120F is pretty much safe for the hot water part https://www.energy.gov/energysaver/do-it-yourself-savings-project-lower-water-heating-temperature

So maybe I'll leave the hot water at... 53 deg C, and turn the rads down to 50 as well. And then perhaps set a reminder to increase the temperature to 60 for an hour for the hot water every so often.

[zolotiyeruki]

Also...no wall insulation!? Holy crap, fix that!

You can't simply add insulation to a 140-year-old house.  You have to be *very* careful about how you do that, because you could end up trapping moisture in the walls and end up rotting out the structure.  Those houses were built to tolerate water by breathing it out, and insulation can impede it.

[JLee]

Also...no wall insulation!? Holy crap, fix that!

You can't simply add insulation to a 140-year-old house.  You have to be *very* careful about how you do that, because you could end up trapping moisture in the walls and end up rotting out the structure.  Those houses were built to tolerate water by breathing it out, and insulation can impede it.

Surely modern technology has devised a solution capable of handling such complications.

[Bartlebooth]

I believe you can run your hot water radiators at any temperature.  At some point they will not put out heat quickly enough to keep your house comfortable.

In the north central US I run at about 120F in the fall and spring, and ~160F for most of the winter.  When there are cold snaps of -20F outside and especially if windy, I bump up to 180F.  My 1890s house has no wall insulation.

What I am thinking as I make these adjustments:

• Efficiency: a lower water temperature seems like it would be more efficient, as it will harvest more energy out of the flame/exhaust (drops it to a lower temperature).  I'll allow that a modulating boiler (Buderus in my case, GB142 I believe) may not actually burn as efficiently at low flames...IDK.  Less efficient combustion but you extract more of the heat?  Or you may get more on/off cycling with a non-moderating flame.
• Exhaust temp: with 180F water, you are probably exhausting 200-220F temperature exhaust.  My boiler has PVC intake and exhaust piping.  This is pretty hot exhaust for that pipe, and may cause problems over time. So I imagine that I am extending the life of something by running cooler when I can.
• A 180F radiator is pretty hot, more difficult to enjoy sitting on it or resting your hand on it.  Somewhat risky for young children too.

In the end, I suspect more gains can be made by running the room thermostat itself at a lower set point, yet there is still some to be gained by running the water temperature at a lower temp.

Are you sure about that?

https://propane.com/2018/09/17/8-facts-to-know-about-venting-tankless-water-heaters-2/

5. With a condensing tankless water heater, you don’t need metal venting. Non-condensing tankless water heaters typically transfer to the water only about 80 percent of the heat they generate. The remaining heat creates a hot exhaust gas that requires metal venting, typically stainless steel or thick aluminum. Condensing units, on the other hand, are typically about 95 percent efficient, so the temperature of the exhaust gas is lower — around 110 to 120 degrees. That means they can be vented with a less expensive plastic, generally PVC or polypropylene. The price difference in the venting can even offset the cost of the higher-efficiency unit, Olsen says. “The overall installed cost of a high-efficiency unit is typically equal to or lower than that of a mid-efficiency product, so it’s an easy up-sell at that point.”

I suspect building codes would not allow for exhaust piping that would exceed safe temperature tolerances.

Also...no wall insulation!? Holy crap, fix that!

Yeah that is for a hot water heater, heating to 120F or so.  Exhaust would be just a bit higher for a pretty efficient tank unit, or even lower for said tankless unit as incoming cold water can be used to scavenge the last bit of available heat.

Now think about heating water for a heating system to 180F.  Exhaust temp cannot be lower than return water, which is probably 165-170, and exhaust temps will realistically be 10-20 degrees higher than that by my understanding.  So you are looking at 180-200 degree exhaust in your PVC pipe.  Sketchy.

I read a really good piece once...wish I could find it.  Pretty authoritative industry expert, said it is risky, said you will find no pipe manufacturer recommending their product for exhaust use, etc.  some other reading: https://scotthomeinspection.com/cellular-core-piping-exhaust-systems/

Obviously there are millions of homes done like this...but I bet someday it broadly won't be allowed.

Edit: found the link: https://www.phcppros.com/articles/1911-is-pvc-an-acceptable-vent-material-for-flue-gases

[JLee]

I believe you can run your hot water radiators at any temperature.  At some point they will not put out heat quickly enough to keep your house comfortable.

In the north central US I run at about 120F in the fall and spring, and ~160F for most of the winter.  When there are cold snaps of -20F outside and especially if windy, I bump up to 180F.  My 1890s house has no wall insulation.

What I am thinking as I make these adjustments:

• Efficiency: a lower water temperature seems like it would be more efficient, as it will harvest more energy out of the flame/exhaust (drops it to a lower temperature).  I'll allow that a modulating boiler (Buderus in my case, GB142 I believe) may not actually burn as efficiently at low flames...IDK.  Less efficient combustion but you extract more of the heat?  Or you may get more on/off cycling with a non-moderating flame.
• Exhaust temp: with 180F water, you are probably exhausting 200-220F temperature exhaust.  My boiler has PVC intake and exhaust piping.  This is pretty hot exhaust for that pipe, and may cause problems over time. So I imagine that I am extending the life of something by running cooler when I can.
• A 180F radiator is pretty hot, more difficult to enjoy sitting on it or resting your hand on it.  Somewhat risky for young children too.

In the end, I suspect more gains can be made by running the room thermostat itself at a lower set point, yet there is still some to be gained by running the water temperature at a lower temp.

Are you sure about that?

https://propane.com/2018/09/17/8-facts-to-know-about-venting-tankless-water-heaters-2/

5. With a condensing tankless water heater, you don’t need metal venting. Non-condensing tankless water heaters typically transfer to the water only about 80 percent of the heat they generate. The remaining heat creates a hot exhaust gas that requires metal venting, typically stainless steel or thick aluminum. Condensing units, on the other hand, are typically about 95 percent efficient, so the temperature of the exhaust gas is lower — around 110 to 120 degrees. That means they can be vented with a less expensive plastic, generally PVC or polypropylene. The price difference in the venting can even offset the cost of the higher-efficiency unit, Olsen says. “The overall installed cost of a high-efficiency unit is typically equal to or lower than that of a mid-efficiency product, so it’s an easy up-sell at that point.”

I suspect building codes would not allow for exhaust piping that would exceed safe temperature tolerances.

Also...no wall insulation!? Holy crap, fix that!

Yeah that is for a hot water heater, heating to 120F or so.  Exhaust would be just a bit higher for a pretty efficient unit.  Now think about heating water for a heating system to 180F.  Exhaust temp cannot be lower than return water, which is probably 165-170, and exhaust temps will realistically be 10-20 degrees higher than that by my understanding.  So you are looking at 180-200 degree exhaust in your PVC pipe.  Sketchy.

I read a really good piece once...wish I could find it.  Pretty authoritative industry expert, said it is risky, said you will find no pipe manufacturer recommending their product for exhaust use, etc.  some other reading: https://scotthomeinspection.com/cellular-core-piping-exhaust-systems/

Obviously there are millions of homes done like this...but I bet someday it broadly won't be allowed.

Hm maybe?  I run my Navien at 140f. 180 is way past the efficiency zone for my boiler, and given I have modern insulation/air sealing in my house I don't need to crank it way up.

[bill1827]

Legionella is a very low risk in domestic properties, virtually zero risk with instantaneous water heaters.

There is a very low risk if you store hot water as legionella multiplies at temperatures between about 20C and 45 C, which why it is recommended that stored hot water has a disinfection cycle to 60C every few days. Personally I keep my stored water at about 47C and don't do a disinfection cycle.

If you use a public water supply it should have been disinfected and have very low levels of legionella and will likely be well below 20C when delivered to your house, so no opportunity for legionella to multiply. It will be heated to usable temperatures very quickly, so no time to grow before use - no risk. Just set the water temperature to one that you find comfortable.

To kepp the boiler in condensing mode the importnat factor is the return temperature. You want to kee that as low as possible. It is generally recommended that there is about a 20C difference between flow and return temperatures. E.g. a flow of 60C and a return of 40C giving an average emitter temperature of 50C. To acheive that you need to be able to adjust the flow rate (variable speed pumps are common these days) and you need to balance the radiators carefully to ensure that they all get the right flow to achieve the intended temperature. Depending on the design of the system this may or may not be adequate to heat your house. Unfortunately heat output from radiators drops dramatically if the temperature is reduced.

Another effective control is a weather compensator. This monitors the outside temperature and reduces the system flow temperature to match the heat requirement of the house. Unfortunately this needs a boiler with temperature which is controllable by a weather compensator which tends to restrict you to the boiler manufacturers controls. They are also quite tricky to set uo correctly.

[daverobev]

Legionella is a very low risk in domestic properties, virtually zero risk with instantaneous water heaters.

There is a very low risk if you store hot water as legionella multiplies at temperatures between about 20C and 45 C, which why it is recommended that stored hot water has a disinfection cycle to 60C every few days. Personally I keep my stored water at about 47C and don't do a disinfection cycle.

If you use a public water supply it should have been disinfected and have very low levels of legionella and will likely be well below 20C when delivered to your house, so no opportunity for legionella to multiply. It will be heated to usable temperatures very quickly, so no time to grow before use - no risk. Just set the water temperature to one that you find comfortable.

To kepp the boiler in condensing mode the importnat factor is the return temperature. You want to kee that as low as possible. It is generally recommended that there is about a 20C difference between flow and return temperatures. E.g. a flow of 60C and a return of 40C giving an average emitter temperature of 50C. To acheive that you need to be able to adjust the flow rate (variable speed pumps are common these days) and you need to balance the radiators carefully to ensure that they all get the right flow to achieve the intended temperature. Depending on the design of the system this may or may not be adequate to heat your house. Unfortunately heat output from radiators drops dramatically if the temperature is reduced.

Another effective control is a weather compensator. This monitors the outside temperature and reduces the system flow temperature to match the heat requirement of the house. Unfortunately this needs a boiler with temperature which is controllable by a weather compensator which tends to restrict you to the boiler manufacturers controls. They are also quite tricky to set uo correctly.

Ok, good to know the risks are low.

I read a bit about the departure and return temperatures but I don't quite get it. The colder the house is, the more heat will be disbursed by the radiators, so the lower the return temp. I don't understand the 'average emitter temperature'. Right... so I want to have the rad thermostats open? Upstairs we have thermostats but downstairs they are just valves. I want to dump the heat out that's being circulated, because I want the return water to be cold(er). Correct?

I think the rad circuit was set to 60 or even higher before and I have gradually turned it down. The heating isn't on yet. Now the rad circuit is set to 50. Does that seem a reasonable place to leave it? I know the boiler has the ability to accept an outdoor sensor but there isn't one installed.

Let's say in the winter, in February, it generally hits but doesn't go much below freezing, and gets up to 10-15 during the day. Well - I'll try it and if it isn't hot enough I'll have to turn it back up I suppose. But in terms of the condensing bit?

[Malossi792]

I'm new to this, too.
Here's my plan:
Start with a low temp, 40-50 degrees C. In the autumn when you just need a little bit of heat, that will do. A day will come when it's heating all day at that low temp but the house is still colder than it should be, that's when I'm planning on raising the outflow temp. Rinse, repeat. Note the outside temperature at each change, and do the same thing in reverse order in the spring.
Or just install an OTS and rely on it.
Decisions, decisions...
There are 2 'power' ratings on my boiler, at 80 degree C outflow temp it puts out a certain amount of heat, at 50 degrees the number is 10 percent higher, same amount of fuel used. That's the efficiency gain of condensing.

[BNgarden]

A different use case, but when our condo corporation was considering condensing boilers, we were told by the consulting engineering firm our radiators could not run at the higher temperatures condensing boilers would run at to maintain efficiency, so we would be setting the temps lower and getting efficiency only in line with the atmospheric boilers.  It was a counter-intuitive result for us lay folk.

For this reason, I'm unsure you would save on gas by lowering temperatures.  I'm sure there are more informed forum posters than me though!!

So this seemed unwise in our case (southern AB, Canada).  (We still had a choice of which boiler type to choose for replacements before 2025--although Trump / Project 2025 may have targeted such measures for repeal already, or it could be a California driven manufacturing choice...)