Keeping boiler in condensing mode

Filikin

Hi,
I am getting a new oil condensing boiler installed next week (I hope).
A friend mentioned that the boiler will only be efficient when it is in condensing mode and that only occurs when the difference between the send and return flow is >20DegC.

While I am also getting a new hot water tank which I assume will be efficient enough to extract the heat, the rads are 20years old and I am reluctant to replace them.

Is there any way to ensure the boiler stays in condensing mode, or would if be enough to replace the radiator in the hall (largest area to heat)

heinbloed

The effiziency increases with the temperature reduction of the outgoing flue gases. The colder they are the less heat is wasted.
The temperature of the returning circulated water - the water which comes back from the radiators and goes into the boiler to be heated again- passes along the flue gas heatexchanger.
As long as this heatexchanger is 'cold' condensation will happen, the boiler running on an effizient modus.
So keep the return temperature at or below 50 degrees Celsius. This can be set at the boiler.

The colder the return temperature is the more condensation will be seen.

The " Delta T " is the temperature difference between flow and return. So you could run the flow theoretically (!) at 100 degrees Celsius and the return at 30 degrees. And still making use of the condensing modus.

Even if not running in the condensing modus (for example with a return temperature of more than 50 degrees Celsius) the condensing boilers are more efficient than non-condensing boilers. Simply because the heatexchangers are larger, the flue gas temperature lower.

Read the installer's as well as the owner's manual to see how the optimum efficiency can be reached. Before purchasing the boiler. You have to understand them. Badly designed manuals are very likely resulting in a non-satified user.

If the minimum output of the boiler (kW) is to much to be taken by the radiators and needs to be buffered in a thermal storage tank better go for a smaller output boiler.
Cycling of the boiler (going on and off) should be avoided as far as possible. With the correctly sized boiler this shouldn't be a problem.
Frequently boilers are far oversized. Costing more money to purchase and to fuel than would be necessary if sized correctly.

Ask for an A-rated circulation pump to be installed with the boiler.

About the radiators: size them as large as possible, the larger the heat emitting surface is the lower the boiler temperature can be set. If you have the choice go for UFH or wall heating.

Filikin

thanks heinbloed, I told the installer I wanted an A rated pump - he first said they were very expensive (€500+). when I said I would source one myself, he went and checked the catalogue - suddenly the pump was only an extra €80 I want to avoid replacing rads unless there is no other option. I'll see how the boiler fares with the rads I have already.

dathi

make sure he power flushes the system to get all the crud out of your old radiators and you should be fine

Filikin

dathi wrote: »

make sure he power flushes the system to get all the crud out of your old radiators and you should be fine

yes that's on the quote. thanks.

oxygenman

heinbloed wrote: »

The effiziency increases with the temperature reduction of the outgoing flue gases. The colder they are the less heat is wasted.
The temperature of the returning circulated water - the water which comes back from the radiators and goes into the boiler to be heated again- passes along the flue gas heatexchanger.
As long as this heatexchanger is 'cold' condensation will happen, the boiler running on an effizient modus.
So keep the return temperature at or below 50 degrees Celsius. This can be set at the boiler.

The colder the return temperature is the more condensation will be seen.

The " Delta T " is the temperature difference between flow and return. So you could run the flow theoretically (!) at 100 degrees Celsius and the return at 30 degrees. And still making use of the condensing modus.

Even if not running in the condensing modus (for example with a return temperature of more than 50 degrees Celsius) the condensing boilers are more efficient than non-condensing boilers. Simply because the heatexchangers are larger, the flue gas temperature lower.

Read the installer's as well as the owner's manual to see how the optimum efficiency can be reached. Before purchasing the boiler. You have to understand them. Badly designed manuals are very likely resulting in a non-satified user.

If the minimum output of the boiler (kW) is to much to be taken by the radiators and needs to be buffered in a thermal storage tank better go for a smaller output boiler.
Cycling of the boiler (going on and off) should be avoided as far as possible. With the correctly sized boiler this shouldn't be a problem.
Frequently boilers are far oversized. Costing more money to purchase and to fuel than would be necessary if sized correctly.

Ask for an A-rated circulation pump to be installed with the boiler.

About the radiators: size them as large as possible, the larger the heat emitting surface is the lower the boiler temperature can be set. If you have the choice go for UFH or wall heating.

Hiya,
Hope you don't mind me asking but how can you set the temp of the return at the boiler? I always thought that the Temp control was only for the Flow pipe at the boiler panel?

yoshytoshy

People should keep in mind that radiators are sized with a temperature of around 60oC.

If you set your boiler below this temp ,your not going to be heating your home properly. You really need bigger radiators to compensate for the lower temperature.

oxygenman

Shouldn't your heating circuit be sized so that there is a lower temp returning to the condensing boiler? When you see the amount of insulation being put into houses these days,it's hard to get that difference in temp. Seem's to me that the only time the boiler will condense is when it starts up after it hasn't been used for a while.

Is it true that some people undersize their condensing boiler to get that difference in the Flow and Return temp on an existing circuit?

yoshytoshy

It would be silly to put in an undersized boiler ,you need a certain output for your heat requirements.

Oversizing radiators would be the best solution and leave the boiler at a lower temperature.
Boilers like bosch gas boilers are very good at maintaining set temperatures ,it's possible to run the boilers at 50oC ,by turning the temperature control down.
If the boiler is not outputting high temps ,the temps coming back will be a lot cooler in comparison.

oxygenman

yoshytoshy wrote: »

It would be silly to put in an undersized boiler ,you need a certain output for your heat requirements.

Oversizing radiators would be the best solution and leave the boiler at a lower temperature.
Boilers like bosch gas boilers are very good at maintaining set temperatures ,it's possible to run the boilers at 50oC ,by turning the temperature control down.
If the boiler is not outputting high temps ,the temps coming back will be a lot cooler in comparison.

Hiya,

Would 50oC be hot enough though to heat your hot water cylinder?

yoshytoshy

oxygenman wrote: »

Hiya,

Would 50oC be hot enough though to heat your hot water cylinder?

Thats the problem alright. Water takes much longer to heat when the heating is at a lower temp.

Ideally though ,solar hot water is where its at and having a seperate system for heating.

I still don't think a smaller boiler is the answer though ,if you do want more heat in really cold weather ,the output is just not there.

Micky Dolenz

Yes, it probably would be. However it is recommended you bring the temperature up to 55 - 60C max atleast once a week to see off Legionaries Disease. This is very rare in domestic systems.

Depending on your boiler, some stats only have a range between 60 - 80C. Good ldea to have stat on cylinder also.

oxygenman

I agree completly about boiler size, just heard it from other plumbers I know. I'm very fond of the solar system, haven't got it in my own house yet but I will get it though.

yoshytoshy

I haven't got it or installed any yet myself:o

It's all still expensive though as far as I know ,even simple fittings seem to be overpriced. When things settle down a bit more ,I can see more people going for it.

I would be happy with a properly designed and functioning traditional rad system, as Heinbloed posted modern condensing boilers are still very efficient even when they are not condensing and that would do me.

oxygenman

yoshytoshy wrote: »

I haven't got it or installed any yet myself:o

It's all still expensive though as far as I know ,even simple fittings seem to be overpriced. When things settle down a bit more ,I can see more people going for it.

The flexi pipework that they recommend is totally overpriced, some plumbers recommend s/s cylinders for solar, Myself personally will go for a copper cylinder and half inch copper pipe with cuproteched fittings. Way cheaper than the flexi pipe.

oxygenman

oxygenman wrote: »

Hiya,

Would 50oC be hot enough though to heat your hot water cylinder?

With some gas boilers the flows are split at the boiler sending 80c to the coil and what ever you want around the heating circuit.

oxygenman

gary71 wrote: »

With some gas boilers the flows are split at the boiler sending 80c to the coil and what ever you want around the heating circuit.

Hiya,
I didn't know that, handy to know now, don't suppose you know what models?

yoshytoshy

I think johnnieK installs viessman boilers and they are capable of this.

(AS ABOVE)


Viessmann http://www.precisionheating.ie/product_catalog/Viessmann_Boiler_Range,188,0.html
hot water priority works well with the bigger coils in the unvented 80c+ running round a 26/27 kw coil with a temperature sensor wired straight back to the PCB to make wiring easy.

Micky Dolenz

gary71 wrote: »

(AS ABOVE)


Viessmann http://www.precisionheating.ie/product_catalog/Viessmann_Boiler_Range,188,0.html
hot water priority works well with the bigger coils in the unvented 80c+ running round a 26/27 kw coil with a temperature sensor wired straight back to the PCB to make wiring easy.

Clever boiler alright. The wiring option is very useful, saving moving in primary circuit motorised valve.

The most important thing about this boiler is user knowledge on the product. Set water to come on an hour before heating as hot water takes priority. Heating won't come on until cylinder stat is satisfied. With correct use, they are a great boiler.

yoshytoshy

Micky Dolenz wrote: »

Clever boiler alright. The wiring option is very useful, saving moving in primary circuit motorised valve.

The most important thing about this boiler is user knowledge on the product. Set water to come on an hour before heating as hot water takes priority. Heating won't come on until cylinder stat is satisfied. With correct use, they are a great boiler.

I was wondering how the boiler worked ,I presumed there was mixing valves on the flows and returns to reduce temperature going out to/from each circuit.
There wouldn't be any condensing on a single hot water ciruit on it's own.

Micky Dolenz

yoshytoshy wrote: »

I was wondering how the boiler worked ,I presumed there was mixing valves on the flows and returns to reduce temperature going out to/from each circuit.
There wouldn't be any condensing on a single hot water ciruit on it's own.

Two separate circuits, so four pipes from boiler, primary circuit to cylinder and secondary circuit to space heater.

EDIT: In hot water mode, it won't condense but it should very little time in high temp mode. It should always condense in heating mode.

yoshytoshy

Micky Dolenz wrote: »

Two separate circuits, so four pipes from boiler, primary circuit to cylinder and secondary circuit to space heater.

I got that ,but theres no electronic control by the boiler ,mixing both circuits to obtain condensing by the exchanger when the water and heating are on demand together.

Edit : Got above ,usually takes about 30mins or so to heat a cylinder.

Micky Dolenz

yoshytoshy wrote: »

I got that ,but theres no electronic control by the boiler ,mixing both circuits to obtain condensing by the exchanger when the water and heating are on demand together.

The can demand together but can't operate together. I assume there is a three way divert valve in boiler that will only allow one of them to operate depending on what the PCB is telling it. Hot water will always have priority when demanded by stat.

yoshytoshy

Micky Dolenz wrote: »

The can demand together but can't operate together. I assume there is a three way divert valve in boiler that will only allow one of them to operate depending on what the PCB is telling it. Hot water will always have priority when demanded by stat.

I don't think a lot of people would appreciate this type of system ,it seems a bit of a waste having the boiler running and only one circuit is in operation.

For everyday you have to set aside time for hot water firing.

It's probably great for people with underfloor heating and no solar. But for people with radiators ,I wouldn't think it's ideal.

Micky Dolenz

yoshytoshy wrote: »

I don't think a lot of people would appreciate this type of system ,it seems a bit of a waste having the boiler running and only one circuit is in operation.

For everyday you have to set aside time for hot water firing.

It's probably great for people with underfloor heating and no solar. But for people with radiators ,I wouldn't think it's ideal.

It is all down to end users education of the boiler and system. We aren't used to this system but stick it on timer, with hot water to come on before heating and it's sorted.

yoshytoshy

Combi boiler with bigger rads is my opinion for irish houses.
I've installed a few combis with pumps and fed them from the cold water storage tanks in the attic.
No problems operating thermostatic showers:)

weefarmer

a slightly undersized condensing boiler is the best idea and make the boiler work to make up the heat, a handy way to keep the temp difference of 21 degrees would be to put a thermostat on the return pipe and if it rises above 50 then it will change a diverting valve and send the warmer water back into the flow pipe, this way the boiler will always be condensing and you will be getting maximum efficiency around 107% if the boiler is running and the return temp difference is not 20% you will be getting mid 90's efficiency. thats the way i fit them and customers always seem to be happy with the job. the correct sized rads will do the right job too without oversizing

yoshytoshy

weefarmer wrote: »

a slightly undersized condensing boiler is the best idea and make the boiler work to make up the heat, a handy way to keep the temp difference of 21 degrees would be to put a thermostat on the return pipe and if it rises above 50 then it will change a diverting valve and send the warmer water back into the flow pipe, this way the boiler will always be condensing and you will be getting maximum efficiency around 107% if the boiler is running and the return temp difference is not 20% you will be getting mid 90's efficiency. thats the way i fit them and customers always seem to be happy with the job. the correct sized rads will do the right job too without oversizing

Diverter valve ? What your talking about is a bypass valve and the boiler would heat up even quicker with one

weefarmer

weefarmer wrote: »

a slightly undersized condensing boiler is the best idea and make the boiler work to make up the heat, a handy way to keep the temp difference of 21 degrees would be to put a thermostat on the return pipe and if it rises above 50 then it will change a diverting valve and send the warmer water back into the flow pipe, this way the boiler will always be condensing and you will be getting maximum efficiency around 107% if the boiler is running and the return temp difference is not 20% you will be getting mid 90's efficiency. thats the way i fit them and customers always seem to be happy with the job. the correct sized rads will do the right job too without oversizing

You can effect the warranty with a system design like that, if you were to do it with one of my boilers your on your own.

A house would have a heat requirement, that requirement would be found by doing a calculation, you can then fit normal sized rads to give that heat requirement or larger sized rads to give the same output and then you size the boiler to meet the need, to me under sizing is just as bad as over sizing, the boiler should match the required heat output anything else is a guess in my opinion.