Boiler efficiency query

[deema]

It’s that time of year when the boiler is again starting to grown into life and warm the homestead which has led to considering how to ensure my boiler is working as efficiently as possible. So here’s the dilemma and my question:

Most home boilers are over specified and therefore too large for the property. To get the condensing boiler to work efficiently I need the return flow to be as low temperature as possible. I have thermostats in every room to switch the heating on and off, (it’s under floor heating throughout) so what I’m noticing is that although the heating is always enabled to allow it to switch on, ie the house is set to a constant temperature, the heating of each room is sporadic. So, usually only one room needs a burst of heat every now and again. As it drops colder the frequency just increases. Now, the return flow temperature I can achieve with the heating this way is too high, resulting in the boiler not operating efficiently in the condensing mode. The boiler is too large for the amount of heat that is required to heat just an odd room.

What I’m wondering is whether it’s better to go open flow circuit everywhere, so that when any room needs a burst of heat, all rooms receive heat. This way the boiler is operating efficiently ie in condensing mode, however, I’m not sure how to determine apart from doing it whether this is more efficient than running the boiler inefficiently heating up less water.

In my last house, which had radiators I switched that to run so that the house maintained a constant temperature and opened up all the TRV’s. I’m not sure what had the best effect, but I cut my heating costs down by a third and had a much warmer house as a consequence. It was however an old building, solid wall, that tried to dump as much heat into the atmosphere as possible…..it could be that by doing it I kept the walls drier and created better insulation as a consequence. My new house is very different and well insulated.

 

[Sideways]

My temptation would be to go open flow wth only the largest / most important space on the ground floor controlling the boiler. Then gradually reduce the boiler output over a few days to find a setting with a low return temp so you get longer run times at low power with good condensing.

Has the system been balanced with all the trv's open ?

When this is working, you experiment with turning down the TRVs in the least used rooms one at a time. This should nudge up your return temp and reduce the boiler on times. Where the best economy lies I don't know.

My own boiler is oversized too and if there aren't enough radiators open, it is constantly starting and stopping. Not great. You want decent long burns but at the lowest output you can get it to run at reliably.

 

[Ozi]

I know this doesn't help but that never stopped me before.

When as a student I worked for British Gas at their research center the people I worked with (I as the only person in the office without a doctorate these were smart fellows) all had two small boilers in their systems. Partly as they run most efficiently at maximum output so only one fired unless demand is high. Also if you get a problem with one your not stuck with no heating.

Noticed yesterday my condensing boiler producing steam so in the same boat.

 

[John Brown]

I know this doesn't help but that never stopped me before.

When as a student I worked for British Gas at their research center the people I worked with (I as the only person in the office without a doctorate these were smart fellows) all had two small boilers in their systems. Partly as they run most efficiently at maximum output so only one fired unless demand is high. Also if you get a problem with one your not stuck with no heating.

Noticed yesterday my condensing boiler producing steam so in the same boat.

It's my understanding that condescending gas boilers do not work most efficiently at maximum output. Are you old enough to have been an apprentice in the pre-condensing age?

 

[Bingy man]

Many many years ago I was sub contracted to b . Gas and as such were trained externally on new technologies. With older , big cast iron boilers with huge radiators and even longer pipe runs the replacement boiler was often oversized to take account of additional rads being installed and extensions to the home etc . The boilers could then be ( range rated ) to suit the load. We were taught that with condensing boilers the whole system should be replaced and the rads and pipe sized to give the optimal return temp at all times . Combi,s took this option away as the heating output was fixed .!many ignored the system and rads etc and just sold the customer a new shiny boiler . System boilers were replaced with combi,s with little or no understanding of the impact because they were easy to install- flow return hot and cold water and gas it up and away you went . Fast forward to the new generation of efficiency standards and it’s the opposite in new builds . The whole house is designed with efficiency in mind - better insulation for example and system boilers are back , so is the ability to range rate it . The rads are smaller and standard 15mm feeds to that rads have been reduced to 8 or 10mm . So it seems big and beautiful has been replaced by small and petite. So in a nutshell my guess is a system boiler or indeed boilers that can automatically range rate themselves to maintain optimum flow and return temperatures and maintain individual rooms or spaces with intelligent controls is where we will probably end up . The same argument can be applied to ground / air source heat pumps that the system and the property should be designed to achieve an effective and efficient system but I guess that’s another debate..

 

[deema]

@Bingy man The house has a modern self throttling boiler, whilst refurbishing the house last year I had all pipes replaced with a minimum 15mm off the main feeds. I have just about 100% UFH, and the house has been fully insulated. The boiler is slightly over sized, I’m comparing it with the size required for replacement air source heat pumps that would be required. I’ve had the system designed such that I can swap the boiler at any time to air source. What I’m finding, is that for most of the time, because the house is set at a constant temperature, which I understand is the best way of running UFH, heat is only required at one time for say 5~10% of the house. So the boiler is being asked to throttle back to 5~10% of its rating. The boiler set at 55C is still not having a flow return sufficiently low to achieve optimal condensing mode.

 

[Ozi]

It's my understanding that condescending gas boilers do not work most efficiently at maximum output. Are you old enough to have been an apprentice in the pre-condensing age?

Pre-condensing age how dare you Thomas Newcomen, why I barely remember him!

I think you are correct about condensing boilers and max load as already mentioned low return temperature is critical to ensure condensing and recovering latent heat that's the biggest consideration for efficiency. Getting a low return depends on the balance between output and load when the load is lower a smaller boiler is required so makes that easier to achieve, particularly with long burns as opposed to stop start running which also reduces the life of the boiler. Secondly all boilers have to by law draw an excess of air above their maximum burner capacity whatever load they are running at. That prevents CO issues but reduces the efficiency across the heat exchanger. IF I understand and that's not necessarily the case a boiler that is condensing correctly will gain further efficiency running at higher loads as long as it still condenses.

One thing I am sure of if a boiler has been off for any length of time you should always clean out any fluff etc. before it's run and a two uniy system makes that almost a certainty.

 

[Pedronicus]

Questions:
1. Can the boiler actually throttle down to 5~10% of it's output rating?
2. What are the actual flow (not set temp) & return temps?
3. What do you consider the delta T should be for optimal condensing?

 

[Spectric]

I have thermostats in every room to switch the heating on and off

You might have too many zones, but we do not know the size or layout of your system to provide any good suggestions.

The thing to bear in mind is that with a radiator hot water flows through the radiator and it loses heat before going back to the boiler, with underfloor heating it is a pumped set of closed loops that the hot water at a lower temperature is circulated round and when it reaches a setpoint the thermostatic valve opens to let more hot water in and let the cooled water return to the boiler, this ensures even heating for the given floor area.

 

[Sideways]

https://www.heatgeek.com/condensing-boilers-efficiency/

Can you get it work at 50C flow, 30C return, as a starter for 10 ?
40 / 20 ?
35 / 15 ?

 

[Fitzroy]

What a great conversation and a light-bulb moment for me! Idiotically I'm a chemical and process engineer by profession and in all my years of thinking about my heating bill I've never twigged to this issue. I now need to go an assess my system and see what my return temperature is and where it needs to be.

From a theory perspective you want the minimum flow rate and the minimum supply temperature that keeps your house warm. Ideally you want a controllable flow rate in the system as well as a controllable supply temperature.

CH pumps are I expect fixed speed devices so how much they circulate will vary based on the back pressure in the system. Running an open flow circuit will result in the maximum system flow, running with zones isolating themselves will result in a lower flow rate. However in a zoned system you are also isolating heat transfer so the opportunity to loose heat from the system is also reduced. In my mind I could easily design two systems which would be most efficient each of these operating modes, so I doubt one rule holds true for all domestic systems.

The one thing you can do is turn your supply temperate down to the minimum, which should help in all situations.

 

[deema]

The dichotomy I have is that to run the boiler at a lower temperature I increase the risk from legionella, since water should be stored at 60C. If my reading is correct legionalla dies at temps in excess of 50C. So I’ve decided to stick at for 55C. Im looking for a delta T of 20C, so my return needs to be down at c35C, but if I can get it down to 30C so much the better.

I had wet towel rads added to the bathrooms, so that when the hot water tank is being heated they would also be on. Nice warm towels and also somewhere for the boiler to dump heat in summer.

The house is due south facing, so I have a lot of solar gain in many rooms but equally rooms facing all points of the compass in a rural environment. I have every form of UFH there is available to resolve issues of retrofitting in a house, which I’m very pleased with. However, what seems to be happening with UFH is that once the floor is at temperature, the water inside the pipes reflects that temperature, so unlike rads, doesn’t ever get really cold, as we never let the house cool. So the UFH only ‘sips’ a bit of heat even when it’s in heating the floor, I’m also thinking that unlike rads, the return is running through a warm environment so again doesn’t lose that much heat. I have rooms that are up to c80m2 and some down at 5m2. The smaller rooms, typically bathrooms we would like warmer than the bedrooms. 19C in a bedroom is great for sleeping, but a little chilly stood in my birthday clothes in the bathroom in a morning!

The more I think about it the less I like the idea of using only one room to switch on and off heating. I have massive differences in solar gain around the house. The largest room needs the least heating as a consequence, and some of the smallest the most! What I ideally would like is a control system that monitors the return water temp and switches on heating in rooms that arn’t calling for it to dump the heat, even if these rooms are up to temp. So, let’s say dump heat into bathrooms and the hall as an example of areas where it being ‘too’ warm won’t be an issue.

 

[John Brown]

Worried about legionella in your UFH pipes? I can understand if you're worried about legionella in your hot water, but is it a problem in the heating?

 

[Spectric]

but is it a problem in the heating

They are sealed and pressurised .

 

[Fitzroy]

Worried about legionella in your UFH pipes? I can understand if you're worried about legionella in your hot water, but is it a problem in the heating?

I'm guessing the same system also heats a domestic hot water tank which is where the legionella risk occurs.

 

[John Brown]

I'm guessing the same system also heats a domestic hot water tank which is where the legionella risk occurs.

Indeed, but if you have a "proper" system boiler, is it not possible to set different temperatures for the DHW and CH?
Genuine question, as our system is a bit of a Frankenstein's monster - we have a combi, but the DHW side is blanked off, and there are external diverter valves. I'd always assumed a proper system boiler was different.

 

[woodieallen]

Excellent and informative thread.

 

[deema]

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.

 

[deema]

I understand that ‘heating systems’ should be designed such that all rooms heat up at the same rate, however, due to difference in thermal efficiency I think they will cool at different rates. Ie an external corner room with all windows with a north west aspect will cool at a different rate to south facing room with fewer windows. So, the heating system if run on full open cycle will over heat some rooms or alternatively under heat others. Since systems appear to be designed to meet the demands of the typical coolest days for a particular post code, it suggests to me that getting a boiler to run in condensing mode most of the time is going to be very difficult. Most of the time it’s not that cold in the UK, ie dipping to the minimum temp on only a few days each year. Therefore the delta T of the internal to outside of the house will (my best guess) be circa 10C smaller for most of the year ( I think my post code the minimum is -3C but in the winter it averages say 7 or 8C), which on thermal migration through walls / windows / roofs will have a big effect. Ie less heating is required due to losses than in the coldest days. Have I understood things correctly?

 

[Fitzroy]

I'm not a central heating designer but my expectation would be that room size and expected heat loss would be taken into consideration during system design and in a well designed system you get the right rate of heating, through the correct sized radiator or similar, into each space such that it all runs relatively harmoniously. I'd expect that this would apply for the average winter condition, and you may need to adjust something for the exceptional condition, for example increasing supply temperature during cold snaps. In room thermostatic valves would 'trim' to account for small differences between design and actual performance.

Putting a few things together in my mind I think it comes down to boiler size. If the minimum duty (heat energy into your house per hour) that a boiler can supply is more than the duty required (heat lost from your house per hour) then the system's only control option to to cycle on and off to reduce the total heat input to your house.

• By running with heavily zoned heating you are keeping each zone as close to it's required temperature as possible, this will minimize the overall heat input to your house, however it may result in the heaviest system cycling and least efficient boiler performance so a higher gas usage to supply the same heat input.
• Running the house open circuit will maximize the rate at which you put heat into the overall house, but you will get warm spots and cold spots. If you want every room to reach a minimum required temperature you will on average heat your house more than you needed to. However you would minimise your return temperature and this may take less gas to supply the same house heat input as the boiler is more efficient.