How to maximise efficiency of a condensing boiler

I've been wondering about ways to control the return water temperature on my Worcester 24Ri boiler. The front panel control sets flow temperature leaving the return temperature to its own devices.

Would installing a blending valve be worthwhile to keep the return temperature down to optimum condensing temperature be a good idea?

The aim would be under normal demand conditions to keep the flow temperature as low as possible so condensing mode would be achieved most of the time with a typical 20 degree delta T across the rads, but when in the depths of an arctic blast, I could raise the flow temp setting and yet keep the boiler in condensing mode.

Any thoughts?

TIA

Bob

Reply to
Bob Minchin
Loading thread data ...

In article , Bob Minchin writes

I'm afraid it's a system design issue, the only way to get a greater drop is to increase the size of the heatsink, ie the radiators.

However, it doesn't really matter how you get your 40-50degC return temp, if you set the flow to 50 and drop only 10 then you still get an uber efficient 40 degree return.

Don't be tempted by to increase drop by throttling the rad flow though, systems should be balanced, not throttled.

Reply to
fred

Not sure that blending will help much. It lets you lower the flow temperature a tad and may be useful for UFH, but its not really going to do much for return temp.

Modulating the pump speed lower might force the boiler to modulate its power input down lower for a given set flow temperature.

Weather compensation is the normal solution to this problem. It in effects takes control of the flow temperature for you and sets it to the lowest possible that will still get the job done.

For example, I fitted weather compensation to my system, and in recent weeks when it has still been chilly enough the heating has kicked in now and then according to the demands of the prog stat. However since the external temperature sensor was reading 10 degrees or so, the demanded flow temperature was only around 40 degrees.

Reply to
John Rumm

The only way is bigger radiators/heat emitter without reducing the boiler output/temperature. You can only reduce output in mild weather or if the boiler is over sized anyway. (They quite often are)

So, without making any physical changes (To increased radiator sizes) all you can do is turn the boiler thermostat down as low as you can get away with.

If you measure the temperature of the exhaust gas, as long as it is less than 100degC all the water is being condensed and all latent heat recovered. Beyond that point (ie cooler), the gain is relatively small.

This measurement should be taken after the system has stabilised, ie been running for several hours continuously at full capacity ie in cold weather. Usually there is somewhere inside the boiler cabinet where this can be done. From outside is not so good though a thermocouple (ifyou have one) can be poked down the exhaust flue.

The main problem to do with efficiency is that most of these boilers are oversized. Even when not oversized they run in less than full load conditions so making them inefficient. It would be better to have an undersized boiler and use supplimentary heat on the few occasions required.

Reply to
harry

My idea was to put the blending valve in the return ie only allow water back to the boiler once it was at or below the setting, otherwise it would be sent round again to cool some more. Hopefully this would allow greater flow temperature from the boiler without the usual elevation of the return as would be the case for a nominally fixed delta T system. I don't have any UFH.

Reply to
Bob Minchin

if by any chance *all* of your existing radiators are single panel with no convector fins, you could replace them all with identically sized double panel, double convector fin ones. That would allow you to reduce the flow temperature as the new radiators will now be at least double the heat rating of the old ones.

Reply to
Stephen H

In article , Bob Minchin writes

That is pretty much creating a throttled system, reducing the flow through the boiler which will cause it to drop out or create a low flow alarm.

You just can't dissipate more heat (with the effect of reducing the return temperature) without increasing the size of the heatsink.

Reply to
fred

I can understand what you are saying - maybe I will just have to keep the flow temp as low as I can for effective heating. The boiler does not have an external way of adjusting the flow temperature from a weather compensation controller (yet!) I will try and get the circuit somehow. I'm very tempted to try harry's suggestion (for once!) and put a thermocouple in the exhaust gas analyser port to keep an eye on the flue temperature.

Reply to
Bob Minchin

Hope I didn't sound dismissive, it's just that I have a very grubby T-shirt from when I went through a similar loop.

I see it's just a pot that controls the temp, if you're feeling adventurous you could clip it out of circuit and replace it with something electronically variable (can't remember if you're one of the electronically handy group).

On my Keston I added a signal relay that overrides the control pot setting when there is a heat demand for the h/w cylinder but it voided my warranty of course.

I had plans for automatic climate control but in the end I just set the control manually, cooler flow in summer and warmer in winter but still as cool as it will go and still get a decent comfort level.

Beware of setting the flow temp too low, if you do then you will drop below the lowest modulation point that the boiler can maintain resulting in losses in efficiency from excessive boiler cycling.

Good luck!

Reply to
fred

Any idea what that might be in an average house? I have my flow temperature set to 60 on the basis I would expect a loss of at least

10 degrees when heating our house in cold weather.

Jonathan

Reply to
Jonathan

It will be system dependent but you can find the sensible minimum set point by observation. Lower the flow temperature control in stages until you notice the boiler cycling whilst the room thermostat demand is still active, that suggests that the control may be on the low side so set it a bit higher and mark the dial as a minimum set point.

Since I posted to Bob I noticed his boiler only has a single temperature set point for both c/h and h/w demands which will effectively limit the minimum flow set point for his heating as it will need to be hot enough under h/w demand to transfer heat to h/w cylinder and keep it at a safe legionella killing temperature (65degC cyl stat setting in my case so flow needs to be higher to transfer heat).

My boiler is similarly limited but I overcame the restriction using the previously described relay mod which sets the flow temp to max during h/w demand.

Reply to
fred

Why shouldn't I use a temperature controlled valve to throttle back flow when the return temperature is above a certain level?

I'm assuming here that the control is sufficiently progressive that the boiler won't be damaged from overheating from too low a flow rate.

Reply to
Fredxx

EEPots are quite handy for this sort of app... they sit on a single wire bus like I2C and can be adjusted electronically. If you don't talk to them, they just stay at whatever they were last commanded to do (and keep the setting in EE ram). Would be quite a simple Pi or Arduino job to do it...

As long as it has pump overrun, the loss from cycling is far less than the nominal worst case with an old cast iron lump and no overrun.

Reply to
John Rumm

Boilers don't like low flow so you need to be careful when adding devices that could limit flow.

The problem arises when the system approaches design temperature and TRVs start to shut down, flow will be reduced in rads so the ability to lose heat from those rads is reduced too. The result is reduced flow and reduced system heat loss giving an increased return temperature. The clever throttle sees the increased return temperature and attempts to solve the problem by more throttling. The result is boiler shutdown due to insufficient flow.

If you attempt to resolve the low flow with automatic bypass then you increase the return temperature which is what we are trying to avoid.

Given the potential vicious circle cited above I don't think the assumption follows but if you can make it work I will buy one ;-)

Reply to
fred

Oh balls, now I need to code and do electronics just to make my boiler work ;-)

Thanks for the pointer though.

Some analogue switches or signal relays with resistors might offer a coarser but acceptable alternative for a mid-tech solution.

Ok, not a cast iron lump but:

I think the most efficient my (condensing) system ran was before I got round to installing the controls (TRVs excepted). I set the demand from the boiler stat to control the room temp and it tickled along on low mod virtually all the time and barely cycled at all. If it was too hot I just turned the boiler stat down a bit.

When I put in a Honeywell proportional stat (I'm sure you've seen my rants about them before so I wont go into details) it was balls-out-off-balls-out-off all the time so I couldn''t view it as the most efficient usage.

I'm back to a limit based room stat now with a low circulation temp and everything seems so much smoother, hourly cycles with even ramps.

Another system I look after has some issues with distant rads being slow to reach temp so moderate duration cycles result in lots of pipe heating/cooling with attendant losses and not a lot of heat gain at the affected destination rads. Again, another reason to reduce that kind of cycling.

Reply to
fred

Are you suggesting that boilers can sense a low flow rate and shut down. If so then what is the problem?

Reply to
Fredxx

Routine and repeated recovery from beyond limit fault states is not a healthy way to operate a boiler or any other piece of equipment.

If you ever successfully develop and implement a control that operates as you describe and meets your aims I will be happy to hear your results.

Until then I will leave you to it.

Reply to
fred

All these gizmos are irrelevant for what you want. Energywise, what goes into the boiler must come out. (Either as hot water or wasted in the flue gases). The only way to improve efficiency of the system is by running the water cooler. The only way to achieve this is by reducing the input and/or increasing the input/output heat exchanger size.

The problem with the former is the limited turndown ratio all boilers have.

formatting link
After it is at minimum, you are down to switching the boiler on and off which in itself creates inefficiency. So, if your boiler is oversized and has a poor/no turndown ratio, you are on a hiding to nothing. Unless you boiler burner can be reset/readjusted to a lower level, but most can't. Check instruction book. Usually done by resetting the gas pressure and then adjusting the air supply to suit.

And many boilers are oversized due to the wankers that installing them have no clue about boiler sizing. And the limited range of sizes of boilers available And the cover-my-arse-syndrome.

There is one other expensive way forward and that is use of a heat store (usually water). This can be heated up using the boiler operating at best parameters, the boiler is then shut down and the heat from the store used over period. But this is a poor bodge more usually used for saving/storing some form of waste/intermittant heat.

Reply to
harry

Thanks for your inputs Fred and John. Yes I am an electronics dabbler and so willing to have a go. EEpots sound interesting. I already have two arduino processors on the thermal store. One programmed as a solar PV dump controller and the other acting as a whole store calorimeter with the ultimate aim of assessing the volume of available DHW at the taps and using that to decide when and if to fire the boiler to top up the store. The store will also be fed with heat from an array of solar thermal tubes on the roof. The calorimeter processor will tie all this lot together to only fire the boiler when desperate need for DHW. After yesterdays sun, the calorimeter is showing about 300litres of available DHW at 52C.

For the purposes of warranty preservation, I could fit a stepper motor to the boiler flow temp control?? As a very minimum that could elevate the setting when there is call for DHW boost and set it back when that has been satisfied but also give scope for all sorts of schemes! The other way would be to fit some sort of shunt across or in series with the flow temp sensor - it has spade tags on it so any mods could be removed invisibly for warranty purposes.

Bob

Reply to
Bob Minchin

I accept your position. Have you looked at using flow sensors, to independently turn off central heating demand?

My thought was that the boiler could be set to a higher temperature than

60 degs so could then be used to heat hot water to a sensible temperature quickly, but also then have the condensing efficiency for heating. I also note that some wet floor heating have a temperature sensitive flow device in the return.
Reply to
Fredxx

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.