condensing mode vs non-condensing mode

Possetjohn

I suspect most boilers capable of weather compensation have a return flow temperature sensor, and so can adjust boiler output to suit. This means on cold startup, the boiler can go full blast, and then go to simmer when the house is warm, with condensing in both mode.

Hi Pincher,
I know my boiler is capable of this and I understand that the most modern ones are "self learning" Unfortunately at the time of getting the quotes for the boiler it was never mentioned and I was rather ignorant of them!

utomatic condensation optimisation eliminates user fiddling with the boiler, which is imprecise, and prone to forgetting.

Excuse me!!** I take great offence at this.
Unfortunately it is probably rather correct! sigh!

Canucklehead

Hi.

Agree with Pincher!


OP:
One of the things that caught my eye with the Viessmann boilers was their heat exchanger and the shape of it.

Laminar flow is the phrase you want to look up, where the shape of the tube improves the heat transfer. Circular shape creates a layer of slower moving water on the innner surface of the tube.

The 200 series also has Lambda control.

Some of the other brands are featuring this after some years of being left behind. Mostly because the market (punter) wasn't interested and the installers didn't know.

GSR.

grumpyoldsteve

liking this thread and looking forward to getting my new Vaillant system

Possetjohn

Hi Canucklehead,

An interesting post and picture. At the time of getting our new boiler it was a toss up between the Vailliant and Viessmann

Laminar flow is the phrase you want to look up, where the shape of the tube improves the heat transfer. Circular shape creates a layer of slower moving water on the innner surface of the tube.

This is rather interesting! In principle heat exchange is better when turbulent flow exists. Most plate heat exchanges are designed to give turbulent flow even if the Reynolds No for the flow is sub turbulent. If you think about it laminar or streamlined flow means you have a very slow layer of media close to the wall (boundary layer), and then progressively faster moving layers as you move towards the centre of the pipe/duct. The layers close to the centre travel further through the pipe/duct without being cooled as much because the transfer of heat is by conduction through the layers. With turbulent flow the fluid is constantly being mixed as it flows along the pipe/duct.

However the other thing Viessmann say is "The spiral wound tube,with rectangular profile is designed to ensure a laminar flow without a boundary layer for excellent heat transfer."

To me this implies the cross selectional shape of the heat exchanger gives closer to a plug flow which allows the fluid to be more uniformly cooled throughout the length of the heat exchanger.

Fascinating!

croninsfc

Pincher wrote: »

I suspect most boilers capable of weather compensation have a return flow temperature sensor, and so can adjust boiler output to suit. This means on cold startup, the boiler can go full blast, and then go to simmer when the house is warm, with condensing in both mode.

I had a Vaillant 618 system boiler fitted a few weeks ago in the attic and thankfully the control unit/thermostat downstairs (VRT 392f) means I never have to adjust the controls on the boiler. Although I don't have the weather compensation option, the system does continuously modulate down until the house is heated up and from the control panel I can monitor the target flow temperature from the downstairs controls, watching it decrease as the house quickly warms up. With a trip up the attic I can monitor the return flow temperature from the boiler front panel and it is nearly always below 55 degrees thanks to the boiler automatically modulating. The boiler prioritises hot water so rather than trying to heat the house (less efficiently) whilst the boiler gives a higher flow temperature to quickly heat the hot water, I have the hot water timed to come on when the heating is in set back mode at night (the boiler never comes on in this mode as the house temperature never falls to the default 15 degree setback temperature), with a quick boost mid morning after the heating is timed to go off until the afternoon. The only time the return flow temperature might rise higher than 55 degrees, is when all the TRV's shut off leaving only the small hall radiator where the thermostat/controls are located in the central hallway, but usually the target temperature of this (i.e the overall house target temperature) is reached by the time the other rooms are at their desired temperatures.
I might upgrade to the Vaillant weather compensation controls later on but I wanted to see how much gas I am now saving over a year compared to the old 28 year old dinosaur of a boiler and old hot water tank that I replaced, going from open vented to totally sealed system.

Pincher

Weather compensation is good for maintaining a steady state.

The traditional dumb thermostat waits till the room temperature drops by one or two degrees, clicks on and the boiler goes full blast until the set temperature is reached, so if you had a temperature chart, you will see a sawtooth pattern. It's a kind of hysteresis.

With weather compensation, you assume the house has a heat loss characteristic by choosing 1, 2, 3, 4 or 5. The idea is if the outside temperature suddenly drops by ten degrees, the boiler will flame up to PRE-compensate, so the room temperature won't drop much, and the temperature chart stays fairly flat.

So, strictly speaking, it's not about optimising condensation,
but the pre-compensation heat boost tends to be low flame, as opposed to the dumb thermostat, which tends to be full blast, so is less likely to condense.

Having the boiler output around 55 degrees is very satisfiying efficiency wise, but practically speaking the room heats up very slowly. With the old boiler, the radiators were 70 degrees or so, and the rooms heated up much quicker. So what I actually do is keep the temperature at 16 as default, but turn up the set temperature to 25 or even 30 if I want a quick boost. The weather compensation thinks there is a big differential, and so turns the flame up. The high temperature setting gets cancelled when the next program period starts, and the default goes back to 16.

So if it's a big hassle, e.g. finding a good spot for the outdoor sensor, I wouldn't bother, since you are already optimising for condensation.

croninsfc

Pincher wrote: »

Having the boiler output around 55 degrees is very satisfiying efficiency wise, but practically speaking the room heats up very slowly.

So if it's a big hassle, e.g. finding a good spot for the outdoor sensor, I wouldn't bother, since you are already optimising for condensation.

Hi Pincher, thanks for the points you've made about weather compensation. I suspect the cost of the replacement controls may not pay for themselves for quite a long time with additional savings on the bill, especially as the controls I did get with the new boiler, as well as the new boiler itself seem to be both be intelligent pieces of kit anyway.
Just one point about the temperatures mentioned in my last post, the boiler return temperature quoted as never going above 55 degrees; I should have also said that from a cold start the boiler target flow temperature is initially 90 degrees, so the house does warm up very quickly and then then this target flow reduces to around 60 to 65 as the system ticks over once the house is warm, and regardless of the target flow automatically set by the system, return flow rarely goes above 55, so as you say I guess the system is automatically optimising for condensing mode.

croninsfc

For anybody reading this whilst considering a replacement boiler, one thing that became apparent when I was getting quotes is that many companies over specified the boiler rating. I got 6 quotes and only 3 companies specified the correct sized system for my house (even though my old boiler was perfectly sized to heat the house and hot water, albeit inefficiently with it being G rated). When looking at boiler specifications, if the MINIMUM output that the boiler can give is too high for your property, then when the system is ticking over, it will not be able to modulate down far enough to maintain condensing mode whilst ticking over. In fact one company specified a boiler whose minimum output was the same as the output recommended using the "whole house" method of specifying boiler size. The maximum output was DOUBLE the output of my old boiler! What did they think I needed all that extra potential power for I wonder?
I did get an boiler with an extra 3kW of potential maximum output (as recommended by 2 of the 6 companies), which enables it to very quickly heat the hot water (in a bigger replacement sealed hot water tank) and the radiators and also allows for the conservatory radiator which wasn't there when the house was built (which one company did not to take into consideration and specified the same output as my old boiler). The new boilers minimum output means that when ticking over the return temperature can drop to 55 degrees or below and maintain condensing mode.
One reason that I did not move to a combination boiler is that these boiler are by definition specified to optimise hot water flow rates from the hot taps, which isn't necessarily going to give the most efficient (condensing mode) heating - again because of the boilers minimum modulated output.

Pincher

To combi or not to combi, the eternal question.

The deal breaker is always mains water pressure.

My Gas Safe guy talked me into what he knew:

1. £1,100 just to buy a Megaflo 250 litre cylinder, plus complicated plumbing, timer, zone valve and the boiler room is cramped.

If only I knew what I was doing:

2. £700 Rinnai Infinity 16i 35kW Gas Water Heater. Instant hot water, simple plumbing, empty boiler room. No combi diverter valve!


If ever I got Thames Water to change the external stopcock, so I get 4 bars or more, the Infinity 26i is 54kW! I could get an 18kW GCH boiler, and be condensing all the time.

Cardew

Pincher wrote: »

If only I knew what I was doing:

2. £700 Rinnai Infinity 16i 35kW Gas Water Heater. Instant hot water, simple plumbing, empty boiler room. No combi diverter valve!


If ever I got Thames Water to change the external stopcock, so I get 4 bars or more, the Infinity 26i is 54kW! I could get an 18kW GCH boiler, and be condensing all the time.

Using these for gas supply?