Boiler efficiency query

[XTiffy]

Thanks for the update Deema.
Hopefully the heat-exchanger of our Vaillant will be replaced on Wednesday as this was the part affected by its fault code of F769 Overheat).
Unfortunately having a physio appointment at the same time I am unlikely to be able to quiz the Vaillant engineer. My wife is resolutely non-techie and is not likely to ask qustions (or understand the answer!). I might try to print out your conclusions to give to the engineer to see if there is a response. I also have a gas safe engineer friend (too far away) that I can ask the same questions. All in all good information.

 

[Spectric]

What I’d clearly missed out considering is WHAT the boiler is designed to operate at.

That is why an oversized boiler is not efficient, a smaller one working harder delivers more efficiency but you still get installers fitting larger boilers "just in case" .

The return temperature to your boiler is simply the main flow temperature minus all the heating outputs plus some loss in the pipework. The low return temperature allows the water in the flue gases to condense on the secondary heat exchanger and we recover some heat whilst the condensate now runs down the condensate pipe. In exchange for this efficiency we have reduced the lifespan of our boilers though.

 

[XTiffy]

Edit fault code was F76 (don't know where that "9" came from).

 

[Norman Boyes]

The boiler is a Worcester 40 CDi classic Regular ErP setup for LPG. It’s the only bit I didn’t change in the heating system as I expect to swap it for air source heat pumps at some stage in the near future. The boiler has only one heat setting which is used for both the hot water and the heating side, so setting it too low affects hot water temperature / creates opportunity for legionnaires to breed. Now, I have also installed a programmable timer for the emersion heaters on the hot water tank so I could drop the boiler temperature down and top up the heat with the emersion heaters as an option. I have tried to cover the bases!

The boiler is powerful it’s max output is 43.3Kw (50/30 C) with a 92% efficiency A rated, so I didn’t see a reason to swap it immediately. Its minimum output again at 50/30 is 15.5Kw, or c36%. So, I need at least c16KW of constant demand to use it efficiently (As I understand it and stop it short cycling) that to me suggests I need to be heating circa 1/3 of the house whenever it’s on! If I don’t, if my understanding is correct the boiler will short cycle and a lot of heat will be going out of the flue as it won’t be condensing……which it isn’t!

My thoughts are, that when the heating kicks on and doesn’t demand much energy, I’m actually wasting a lot of heat. It would be far more efficient to heat up bits of the house to be too warm rather than dumping it outside / short cycling?

This issue must affect a lot of other people, as it would seem reasonable that everyone’s boilers have a similar range of heat output, and they may well be better off looking at what is actually heated when the boiler switches on.

I’d really like to get my head around this as again, it’s my understanding that air source heat pumps need to be run rather than stop start to be efficient. I’d like to work out how to run the heating of the house to achieve the minimum heat load what ever heating system I have going forward.

You are on a hiding to nothing searching how to get this boiler operating anywhere near the efficiency of a modern condensing boiler. In a nutshell - it is too old, too basic and far too overpowered. This is not uncommon as most of the gas safe engineers who have been slinging in these boilers have no heating engineering background.

I know your direction of travel is for an ASHP so the following is a bit academic.

First, you need to understand what the heat loss and hot water needs of your property is ( for example my property 5 bed detached is 6.5KW at -2°C outside temp).

Then you need to size the boiler according to the predicted heat load.

The boiler should have the ability to control its output independently for heating and for domestic hot water. It should have the ability to be ‘range rated’, controlling the heating side using weather compensation and have a good modulation ratio (low minimum firing rate). The latter is really important to prevent excessive cycling.

My example: Viessmann 200w, 19Kw condensing boiler. Range rated to 9Kw on the heating side. The minimum firing rate of the boiler is 1.9Kw. Weather compensation that controls the boiler output flow temperature flow temperature and is set so that at -2°c the flow temp is 45°C ( the other end of the curve is set at 20°C the flow temperature would be 20°C). Output is in proportion between these two points.

My boiler is always condensing in operation and below an ambient of 12°C will never ever cycle due to the low modulation performance. It may be interesting to know I have no TRVs on radiators or a room thermostat - the control is pure weather compensation only referencing Outside temperature, the set control curve and boiler flow temperature.

With an ASHP much of the above criteria apply - do not oversize because the modulation is around 30% of rated capacity which means more cycling in the shoulder seasons.