carbon saving and modulating controls on gas boilers

[ALEC MORROW]

Load compensation or modulating controls such as weather compensation and room sensing all contribute to reduce carbon emissions, by forcing the boiler to run at the lowest temperature possible, according to inside or outside temperature or both.

Of course it takes the same amount of energy to heat a body under all circumstances, but its how you gain the heat from the source thats important. With this in mind modulating controls use temperature profiling, to match the demand to the need very accurately.


Condensing boilers are designed to condense  being more efficient at lower temperatures , but with on-off controls a condensing boiler is only marginally more efficient than a non condensing boiler.

The reason is obvious,  the flow and return temperature are just too high  well above55c to burn gas efficiently, and a lot of heat that goes out the flue. Then there is standing losses when valves shut and useful heat sits in the boiler or pipes serving no useful purpose, and of course the mandatory by pass that just sends hot water back to the boiler...very wasteful.


Modulating controls avoid all this wastage and customers who have these controls installed on specific boilers benefit from more comfort and lower gas bills

Are architects and specifiers missing a trick by ignoring this technology?

[john cantor]

It is interesting to note that I fitted a new Vaillant modulating boiler recently, but since I have not got the single room control, it actually doesn't modulate.   The only way it modulates is when it is configured with the Vaillant inside room controller.  I have weather compensation, and have taken to programming the parameter in the engineers log to limit the output. Presently its fixed at 10kW. (Its an 18kW boiler).
Whilst I am disappointed and surprised that it wont modulate, I don't think the energy reduction is that great. It condenses well, and it normally runs around 40C.   

Any tips on setting up this boiler would be welcome.

I'm convinced that few systems are set up well.  e.g. I think default DHW output for my boiler is 18kW!  I have a 3.2mē coil so temp.diff. is close, but even so, I'm sure its far better with a reduced output. Again, it would not be difficult for Vaillant to modulate this... but they don't.  I have fixed this to 9kW.

I hope to get some monitoring on it soon so that I know exactly what it is doing.

[Peter Bayer]

Mr Morrow

Given a reasonably insulated new building, a design flow temperature below 55c is easily attainable .
Weather compensation can be useful with low water content boilers at high flow temperatures where a high proportion of the central heating return is coming via the by-pass as the TRVs start to close , in these cases the return from the open radiators may well be cool enough for condensing but not once mixed with the by-pass flow .
With weather compensation the closer match between heat supply and demand means that the TRVs remain open more of the time thus reducing the amount of time water flows through the by-pass.

Peter

[ALEC MORROW]

John

I frequently install ecotecs with weather compensation, and yes it does modulate between the lowest output, probably 5kws and the 10 kw you have rated the boiler at. Theres no way as far as I know you can see that...other than watching the gas meter!

The flow temperature of 40c is clear example that it is operating efficiently.

Do you have the VRC 400 or 430?

either of these can be located out of the boiler and act as a modulating room sensor, which gives an added level of intelligence...however note that the out door sensor overrides this if the outside temperature drops....also the out door sensor also averages the outside temperature, which is why it works so well..

a 3.2m2 coil is huge... part L cylinders are a fraction of that...you need to use a cylinder sensor (available with the VRC65 control center to make the boiler modulate.

I agree Peter, but by not using the controls supplied by the manufacturer the benefits of condensing technology isn't really realised...and certainly TRVS and  external by passes aren't strictly necessary as long as the radiators are correctly sized...

The more heat you gain at the lower flow temperature the higher efficiency of the boiler....

[john cantor]

I guess we should not discuss the finer details of my boiler on this forum, but it is the VRC430 room controller.
I cannot use it as a room sensor within it since:- a) the main room has a woodstove, b) we have over 10 rooms. I am fitting thermal actuators and programmers to each room.
I cannot see a way of setting it up so the boiler modulates, so I am manually changing the fixed output over the seasons (if i remember).
The DHW has the digital sensor, but this doesnt modulate the boiler. Having said that, I'm not sure that modulating the dhw would help.
My gripe with this, and many heat pump controller is that unless you install a very standard system into a standard house, you can be stumped.   

Its good to network on this topic because I bet there are plenty of systems (including mine) that could be improved by a few parameter changes.

[ALEC MORROW]

why go to all this expense? If several rooms are shut down the rooms below above or next too just loose heat to them....

if all the house is regularly used then you are better off with getting the system running properly...I have lots of systems with no indoor sensor and lots of very happy clients...

The boiler does modulate, how do you know that it is not...

one way of knowing is counting the revolutions on the gas meter, the other is whether the length and size of the plume...

contrary to most peoples understanding a plume is a sure sign that it is wasting energy, you should be seeing gentle whisps of steam, some of the time, more so in hot water mode and sometimes nothing...

In terms of being stumped the issue is that the manufactuers are very bad at getting infromation out...especially when all they need to do is comply to part L of the building regulations which doesnt accomodate this type of technology!

[Nick Grant]

To put a cat among the pidgeons

On the Passivhaus designer course we were just taught that in a Passivhaus, it is important to override the weather compensation outdoor sensor by replacing with a resistor !!!

All the insulation decouples indoor and outdoor and so the time constant is very large making the temperature at any moment less relevant.

I'm hoping John and others can correct me is my explanation is wrong but I'd be wary of questioning the advice.

Nick

[ALEC MORROW]

This is correct. All modern weather compensation kits that use the boilers intelligence to manage heat use a resistor to measure current according to temperature.

Confusion arises, even with professionals who confuse the terms thermostat (on-off: inherently inefficient) with sensor (continuous, varying the current to allow the flame size to reduce according to match the heating requirements)


In the trades its not much better though....

The point is that if you are installing a gas condensing boiler it cant get any more efficient than the good ones being installed nowadays!

[ALEC MORROW]

Its also true to say that the greater the insulation the less important the out door temperature, because you have slowed the transfer of heat right down.

Out door temperature sensing is also useful when there is a thermal lag as in underfloor heating, or even large cast iron radiators...

To ensure that you dont get temperature over shoot a resistor measures the temperature and determines how much heat is needed in the building, so preventing a boiler using more gas than necessary.

Thermostats are inherently wasteful, as the boiler doesn't know how much gas to burn as it works to a fixed temperature.

[Nick Grant]

Alec

For that very reason I wouldn't use normal underfloor heating in a genuinely low energy building as the controls would need to know how many kWh need to be put into the slab. If underfloor heat is used in Passivhaus (eg to keep heat pump temp differential really low) then I have heard of floor running just above room temperature so that it self regulates. Control would be floor temperature and room follows that less a couple of degrees.

Again for Passivhaus it seems fine to use a dumb thermostat if heating with rads or air. Any slight overshoot gets mopped up by the fabric which has far more heat capacity than the air being heated.

Interested what others think but I'm very keen on really simple controls that don't rely on correct commissioning and ongoing user understanding.

Nick

[ALEC MORROW]

What do you mean by normal ufh?

The key is the mixing valve that gets programmed according to the floor and the building characteristics. Indeed the boiler I retrofit on ufh systems enable me to:

1) match kw output to heating load at peak demand (design temperature)
2) ramp the flow temperature  up and down according to the outside temperature
3) cap the flow temperature at as appropriate (this is another way of capping kws!

Indeed the flow temp is just above room temperature and it does self regulate. In some I install internal sensors in some I dont..

Yes dumb thermostats will work, but excess heat is excess heat and increased carbon emissions...either you chase negawatts or you dont....

Modern society is based on things people dont understand... why is a heating system so different especially if the benefit is higher user comfort and lower emissions....

[Peter Bayer]

Nick

Must admit to liking simple solutions even when  extra effort is required in order to eliminate the need for sophisticated controls , your example of running floor heating near to room temp certainly appeals .
with a little bit of attention to detail ( baffles in tanks , careful pipe routing etc. ) a simple system can be at least partially self regulating leaving only basic controls necessary .
It is often easier to use the manufacturers solutions even if more complex  as the quest for simplicity is met with incredulity, why design a system to use essentially interchangeable generic components when "X" manufacturer can sell you an  electronic box  to save you the effort?
Maybe I'm just eccentric , but I have been left in the lurch having to change a control system because a compatible sensor was no longer available.

Peter

[Nick Grant]

Alec

I think you have misread my posts. I'm saying that for a seriously low energy house, weather compensation is counterproductive and if it is built into the boiler as standard it should be overridden.  

i.e. I meant resistor not thermistor.

This is what I have thought for some time so it was good to have it confirmed by someone who has designed over 50 Passivhaus buildings (Inc non domestic).

The beauty of a very low heat loss building is that the temperature stays very stable. We experience that with our own sub passivhaus. It feels like thermal mass but as David Olivier has pointed out before it is the combination of insulation and thermal mass that leads to the really long thermal time constant. Like in electronics big capacitor and small resistor (massive) or small cap and big resistor (light but insulated) can have same time constant.

I really don't know about heating systems in not very well insulated buildings but can see that weather comp could be really good idea.

Nick  

[ALEC MORROW]

I'm  not sure where the complications are coming from, its all very subjective. Generally though heating controls are conflictive because they rarely get used...even simple on-off ones cause grief. The best controller is probably the one you dont need to touch, which is what these controls amount too.

My organisation "Ecotechncians" has a significant number of these systems installed as retrofits  with people of all ages in the London area. Indeed the people who are usually classified as problematic (over 60s) take to them really well, simply because they need very limited intervention. In terms of simplicity of installation these system occupy less space and take less time, but they do have to be commissioned correctly (average time 5 minutes!)

One of our recent projects has been to replace condensing boilers in flats with UFH. The solution has been installing a dual temperature combi boiler to replace the existing combi,  a  programable internal sensor  and an external sensor. The heat curve  and range rating facility is in most  condensing modern boilers and the boilers have there output lowered to match the likely heating loss of the flats. The boiler pumps are man enough to run the entire system, so a pump is removed saving energy . All zone controls are removed, saving even more energy, and ensuring that there is a continuous open path between the boiler and the floor. Heat output of the boiler is calculated using the outdoor temperature (averaged over time in some cases) and moved to the floor zones with no valves. Heat ouput of each room can be varied by reducing the flow of water, by closing a valve. The average flow temperature is around 27c (this is wooden flooring rather than screed) and levels of comfort are very high.

The 5 year old  condensing boiler removed were quite simply mis specified with an excess number of controls  with no consideration for condensing technology or ultra low temperatures heating system design. Standing losses were huge and wasteful because of the use of mixing valves and actuators on  the UFH manifold.

The most efficient way of heating a space must be to match the heat output of the heat generator to the heat loss of the building varying the flow temperature, which is a way of varying the kilowatt output of the boiler.


For UFH to be self regulating the flow temperature does have to vary, as the building heat loss will vary with outside temperature. Again the greater the insulation the less importance the outdoor sensor, but what has to be avoided is ramping up flow temperatures unnecessarily forcing the heat generator to be in efficient.


If one is going to chase the Negawatts, then one cant exclude the control system...merely because  it requires someone somewhere to understand it!

[ALEC MORROW]

nick I think our posts overlapped!