Solved

Does any of you OVOers have a Combi Boiler? Any good?

  • 25 October 2018
  • 7 replies
  • 646 views

Userlevel 1
Badge

Hi, fellow OVOers,


About 5wks we [my wife & I] got a new heating system installed. Our previous one was a Vented System [with a cold water cistern in the loft & a copper boiler in airing cupboard]

Our new heating system is a Combi Boiler [Vaillant ecoFIT sustain] Does any of you OVOers on here, have this system? If so I want to pick your brains, so to speak lol :)

What is the best 'Energy Efficient' temperature to have Boiler at and also to have Rads at & Hot Water at? We have a 2 bedroomed up/down house, smallish kitchen & bathroom, bottom hallway, stairs, top hallway & a back porch of the kitchen no need for rad there, we have 7 [various sizes] rads in our home

I've been doing some online research on this subject but didn't really yet any straightforward answers, too many variables. 😠

That's the boiler/rads temp.

That's the room temp but, we have the TRVs on the rads in the kitchen, bedrooms, bathroom, upstairs & hallways set at a lower temp than the living room.
icon

Best answer by Transparent 30 October 2018, 22:03

Oh wonderful! I do like reading other people's fridge-magnets @Crìstian :)

If your Combi boiler takes a while to fire up and deliver Domestic Hot Water (DHW) to a tap, I can recommend another proverb to add to your collection:


I don't have a Combi boiler, but the principles are familiar to me.

For DHW, I can't see why you'd want to have it set much above a temperature at which you'd just be able to keep your hand without scalding yourself. 50°C is typical.

Central heating is more tricky, not just because it's a Combi, but because it is only efficient whilst running in condensing mode. Once the return water from the radiators is above the condensing temperature, the boiler no longer operates at the SEDBUK-A specification of 90%. It will probably fall to around 65-70%.

The amount of heat which can be passed into the return water falls off significantly if that water is already "hot".

If you know your physics, this is a consequence of the First Law of Thermodynamics. I recommend the version of this most eruditely rendered by those geniuses, Flanders & Swan, under whom my thirst for science was awoken whilst spinning gently at 33rpm!

"You can't pass heat from a cooler to a hotter
Coz the cold in the cooler will get hotter as a rule... Oh!
"


Depending on how many radiators have their TRV's open, there might only be a couple of minutes before your boiler drops out of condensing mode. The higher you set the TRVs and/or room thermostats, the longer it will run in this low-efficiency mode.

The higher you set the outgoing water temperature from the boiler, the longer will be the time at which there is sufficient temp-differential to maintain condensing mode. But... the higher setting will use more gas to get it there in the first place!

The 60°C showing on your boiler photo seems too low to me. The return water will quickly reach that. A setting of 70-75°C is more typical.

If you have a thermometer probe which can clamp onto the copper return pipe to the boiler, then I could describe a better way to find the optimum setting for your boiler.

Hope that helps!
View original

7 replies

Userlevel 6
Badge +2
I can think of two superusers that might have some good advice for you @Crìstian

@Bumblebee is great on all things energy efficiency!

@Transparent knows his boilers!

Any advice Ninjas?
Userlevel 7
Badge +2
Oh wonderful! I do like reading other people's fridge-magnets @Crìstian :)

If your Combi boiler takes a while to fire up and deliver Domestic Hot Water (DHW) to a tap, I can recommend another proverb to add to your collection:


I don't have a Combi boiler, but the principles are familiar to me.

For DHW, I can't see why you'd want to have it set much above a temperature at which you'd just be able to keep your hand without scalding yourself. 50°C is typical.

Central heating is more tricky, not just because it's a Combi, but because it is only efficient whilst running in condensing mode. Once the return water from the radiators is above the condensing temperature, the boiler no longer operates at the SEDBUK-A specification of 90%. It will probably fall to around 65-70%.

The amount of heat which can be passed into the return water falls off significantly if that water is already "hot".

If you know your physics, this is a consequence of the First Law of Thermodynamics. I recommend the version of this most eruditely rendered by those geniuses, Flanders & Swan, under whom my thirst for science was awoken whilst spinning gently at 33rpm!

"You can't pass heat from a cooler to a hotter
Coz the cold in the cooler will get hotter as a rule... Oh!
"


Depending on how many radiators have their TRV's open, there might only be a couple of minutes before your boiler drops out of condensing mode. The higher you set the TRVs and/or room thermostats, the longer it will run in this low-efficiency mode.

The higher you set the outgoing water temperature from the boiler, the longer will be the time at which there is sufficient temp-differential to maintain condensing mode. But... the higher setting will use more gas to get it there in the first place!

The 60°C showing on your boiler photo seems too low to me. The return water will quickly reach that. A setting of 70-75°C is more typical.

If you have a thermometer probe which can clamp onto the copper return pipe to the boiler, then I could describe a better way to find the optimum setting for your boiler.

Hope that helps!
Userlevel 1
Badge
Thanks @Transparent and thanks for the Bib. reference. Although I may add, it doesn't apply to me either spiritually [I'm on Fire] 🙂 or practically [I get constant hot water within a few seconds of turning the tap on]. 🆒

Yeah, we just started putting [We took the magnets of the old boiler] the magnets back up [when the pic was taken] now you can't see the boiler for magnets lol] quote

Yes, I do have the DHW at 50°C I only have the living room TVR is fully open at '5', all the other TVRs are at '2' the kitchen is at '3' [as it's cold in the mornings, when the cooker is on and the kitchen is warmer we turn it down to '3' the bedrooms, bathroom & upstairs landing is also on '2' Apart from one rad that is always fully open. it's supposed to be, no TVR on it [Don't know tech term for this rad]

Condensing Wasn't too sure about this, I don't want to use more gas than I need, so I thought a happy medium of maybe 60°C would work keep rads heating nicely & not too much gas wasted in condensing, but if I turn boiler up to 70°C - 75°C although it would condense more, it uses more gas, costing me more.

I would be interested to hear/learn about the 'optimum setting for my boiler'. Thermometer probe? Do you mean the type used to find out the temp of cooking food?

As I'm not 100% which pipe is the 'return pipe' [I think it's the 2nd from the left but not sure]
I'll try to upload a photo of pipework in & out of the boiler, not sure if I'll get a good pic. as it dark so will use a torch, to give some more light.

Yesterday I turned the hot water pressure down as it was too powerful, as you can see from the pic I just took
Userlevel 7
Badge +2
Hi again @Crìstian,

So... responding to your points in quick succession:

1. The radiator with no TRV is generally called the "bypass" or "Over-run" because it needs to remain open when the boiler switches off and the pump runs for a seconds more to remove the very hot water from the boiler heat-exchanger.

This bypass radiator should be in the same room as your wall-thermostat.

b. Many C/H systems do not require a bypass radiator. If its preferred position did not coincide with the best position for the wall thermostat, then it was common practice to install a "bypass loop" within the pipework somewhere. You should have one or the other, but not both.

c. Most modern pumps do not require a bypass because they operate in modulating mode. They have a sensor which checks the flow in their output, varying as TRVs open/close. This uses a feedback loop to change the pump speed accordingly.


2. Condensing Mode: You only use "more gas" if you heat the C/H water so high that it retains most of its heat in the return rather than releasing it through the radiators.

Having the temperature set too low merely means that the boiler will simply fire up too often in an attempt to keep the house warm. (The hysteresis cycle is too great). This will induce extra wear & tear on the boiler, reducing its service life.

I still think 60°C seems low. If ambient temperature was steady for a few hours (an overcast, rainy day perhaps) you could time how many occasions the boiler fires up over an hour when set at 60°C and 70°C. But your experiment is obviously ruined if other factors kick in such as an external door being opened.

As you suggest - it's a matter of hitting a happy medium.

I once tried to hit a happy medium... but she kept ducking. That's clairvoyants for you! :P


3. A quick online check suggests that your CH/ feed is at extreme left, and the return is extreme right.


The second pipe from the left is the safety-valve relief pipe and should be empty. It's not shown at all in the above diagram.


4. I don't think you'd get adequate contact with a copper pipe using a cooking thermometer. You could try wrapping it parallel to the pipe with strips of aluminium-foil - basically encasing it within a thermally-stable environment, controlled only by the return water temperature. But its response time may just be too sluggish.

I have a thermometer probe as an accessory to my digital multimeter. This is small and responds very fast.

Hope that helps.
Userlevel 1
Badge
Thank You, for sharing your knowledge on boilers/CH/gas etc. I appreciate it. I read your replies a few times in case I missed something lol. Yes, you replies do help, I'm learning as I read lol.



BYPASS That's it I couldn't remember that term.

Yes, our 'bypass rad' is in the bottom hallway, rad is on side wall backing onto the living room, the thermometer is on the end wall backing onto the kitchen.

I will do that, correction; 'try' to do that :? ie. remember to count how many times the boiler fires up in an hour when the CH is on. I noticed if we just as much as fill a cup from the HW tap, the boiler fires up immediately, albeit for a few seconds.

Any chance you could share an image the 'thermometer probe' I did an online image search before I wrote my previous reply, and most of the images were of the cooking type, [£2 -£8] there were other types but at prices £50 plus, too steep for me.

If you could share a pic & how much it was. AND, if I could pick your brains even further, [you won't have any left with all my questions] Explain what I am to do with this thermometer [Keep it clean, no cussing me out lol 🙂 ] after I attach it to CH return pipe. etc. etc.

Thanks Again!!!
Userlevel 7
Badge +2
How's the counting going on the boiler firing @Crìstian?

Here's a photo of the multimeter type I use showing the temperature probe in the centre.


The last time I bought one of these, in 2016, I got it direct from China for about £14. But they're actually not much more than that now on Amazon.

If you tape the small sensor to your boiler return pipe, you'll get a reasonably accurate readout of the water temperature being returned from the radiator circuit. Once this reaches around 55-60ºC, the boiler will have dropped out of condensing mode. In cold weather you can sometimes get a visual indication of this by watching the white vapour leaving the flue outside.
Userlevel 1
Badge
Thanks!!! @Transparent

No, I haven't started counting the boiler firing, yet. [Procrastinating]

Thanks, for sharing imgs. of the probe, looks too technical for 'layman' me, think I might have to give the thermometer probe a miss :(

Yes, I've noticed some plume coming from the flue, looking from our kitchen window, just about 1-2 puffs when the CH is switched on in the morning.

Read a few online articles on Combi Boilers fuel efficiency

Boiler thermostat: this should be set at 65 degrees, to enable it to deliver water of 60 degrees (see above). While radiators will get hot quicker if the boiler thermostat is set at a higher temperature, the boiler may not condense, which will reduce its efficiency by 10-20%.

here isn't a single specific "right" flow temperature, but hopefully, the stuff below might help.

Generally, the return temperature needs to be as low as practicable for the required heat output from the rads. The reason for this is that it is the return temperature to the boiler that determines whether or not it runs in condensing mode - if the return temp is too high the boiler either won't condense or at least won't condense as efficiently. This can make a significant difference - the boiler efficiency rating will have been obtained with a fairly low return temperature.

Return temperature is determined by two things primarily, the flow temperature to the rads and the heat that the rads give out to the rooms. Increase the flow temperature and the heat output from the rooms increases as does the return temperature.

The most efficient way to set the boiler up is to use the lowest flow temperature you can whilst still getting adequate heat output from the rads. For example, at the moment I'm running my boiler at a 48 deg C flow temperature, rather than the 55 deg C I used to run it at, and am finding that the boiler burns for longer each time it fires, but runs with a lower modulation level (less gas being used). The return temperature is down around 35 to 38 deg C, so the boiler should be condensing very well and running at high efficiency.

If your radiators are undersized, then you may find that you have to increase the flow temperature to get the house warm enough. This will hit boiler efficiency quite hard, maybe by around 10% or more.


For me, this saves money on gas etc. by having the boiler at lower eg. 60°C, would be using less gas saving more money.

Reply

    Cookie policy

    We use cookies to enhance and personalize your experience. If you accept you agree to our full cookie policy. Learn more about our cookies.

    Accept cookies Cookie settings