Condensing boiler, flow temp vs gas use

[DrFfybes]

Yesterday I put the heating on with the boiler flow temp set to 68C. Return temp is generally 45-50C.After a couple of hours, the chill was off the house despite being about -3 outside. The lounge hit a balmy 17C.

Reading the meter before and after showed gas use was a lot lower than feared, certainly lower than the 30kWh the boiler is rated at, and came in at roughly £1.85/hour. It was so much lower than I expected I checked the boiler and looked up the model no and GC no to check. As we have microbore pipes and the flow is poor, but heat loss from the pipes embedded in the wall is massive, presumably the boiler cycled. The room stat definitely didn't click as it was set well above ambient.

So today we tried it with the flow set to 50C, which the boiler spec sheet says is most efficient of the temps tested. Return temp was (according to Draper's finest IR thermometer and some tape on the pipe) the predicted 30C once the heating had been on a couple of hours. As MrsF was WFH we splashed out and turned the rads on in her office, but turned the bedroom TRVs right down. Checkin at 18:30 the hourly gas use was right down, about £13.20 for the 8.5 hours since it was turned down, plus 2 hours earlier and one last night before bed, all at 65C, and 30 min of hot water. Tonght we';; use the immesrion.

Now here's the rub, the information you read says that running the boiler cooler saves gas, and this is correct. They also say your home will still be hot but take longer to get there, this is not correct. Because despite closing all the curtains at 4pm, and having had the heat on all day and the TRV open, the lounge is still 15C. And now the gas fire is on. I've just turned the flow temp up to 55C.

Our lounge is 6x4m, with 3 external wall, and because it was designed for the views it has 11m2 of glazing, and solid floor. The radiator is 1400x700 and double fluted, so at 11000 BTU at delta T65 (the difference between rad temp and room temp) is it well over the 8800 BTU the room requires. Except that at Delta T50 that drops to 8000 BTU, and at Delta T30 (which is about what we had today, the rad was 38C in the middle) the output is only 4000 BTU. And I expect doubling the radiators would increase the gas use.

Paul

[GrahamPlatt]

Insulate.

[DrFfybes]

Insulate.

Care to elaborate?

We've had a quote of nearly £100k for the roof, and £30k for the floors.

Then there's the main house with the 200 year old solid walls so that means exterior render or major internal disruption and smaller rooms afterwards. then there's the consequent issues with damp course (or lack of it).

The cavities in the newer bits are full of debris so apparently not suitable for cavity insulation unless we remove a lot of bricks to clean it out, and the house is very exposed (views to the hills 30 miles away) so quite a risk of damp penetration if not cone properly.

But thanks for your extremely well thought out and constructive one word answer.

[servodude]

Now here's the rub, the information you read says that running the boiler cooler saves gas, and this is correct. They also say your home will still be hot but take longer to get there, this is not correct.

In your post you haven't mentioned what the indoor temp was at when the heating went on.

It's the increase in temperature needed (to get to your target for a given space) that determines how long your heating (at a given output) needs to be running for.

It's also complicated by the fact that the room thermostat (or the part of the space it occupies) is just one point - and most people move about, and feel a bit differently about stuff than a bi-metalic strip

[Itsallaguess]

Now here's the rub, the information you read says that running the boiler cooler saves gas, and this is correct.

They also say your home will still be hot but take longer to get there, this is not correct.

Because despite closing all the curtains at 4pm, and having had the heat on all day and the TRV open, the lounge is still 15C.

And now the gas fire is on.

We're in a similar position to you, with a poorly insulated home in a relatively exposed location, and like you, we've struggled to maintain a 'good' level of comfort this week without expending relatively large amounts of gas and electricity.

Beyond doing what we can at a 'personal comfort' level, with more local heat sources etc., I think sometimes in our situation you've just got to take the hit over these colder weeks and simply turn things up a bit, and also perhaps turn things on a little earlier in the day, if you're not finding the 'normal running' situation delivers enough warmth and comfort in the home.

The natural heat-loss from homes in our position is simply too high for a low level of heat-input during weeks like this, and whilst there's likely to be some level of graded home-improvements that might generate benefits in that area, there comes a point quite quickly I think, where the cost/hassle vs benefit ratio spikes quite rapidly, and so we're likely to be left looking at the input side and simply having to make a decision, which is to ask ourselves what's more important during these really colder periods - cash in the bank or warmth in the house...

The good news is that from tomorrow, general temperatures seem to be forecast to rise quite a bit, and so if we were to consider the number of really cold weeks like this over the course of a full year, and perhaps mentally amortise any spike in energy-costs during these colder weeks back across the whole 12-month period, I do sometimes ask myself if making ourselves more comfortable in these colder periods is likely to make that much difference, in the grand scheme of things...

On a slightly tangential note, one thing I am kicking myself for is in discovering towards the end of the summer that we could have our electric shower on it's 'half-power' setting during the warmer months, with no huge loss in performance. As the weather has got cooler, and the input water a lot colder, that opportunity has now gone for a while, and we're back to using full power, but next spring when the weather warms up, I'll be making sure that we're all back to using the shower on it's lowest setting again, and with three of us in the house taking showers every day, I expect that longer-term solution will generate a lot of benefit to my electricity usage from now on.

I only mention that because I think where we're making savings like that in one area, I think it's much easier to justify some level of pay-back in other areas, such as a slightly higher level of gas usage during some periods, so the full 100% of 'savings' from one modified-aspect doesn't necessarily all need to come off the bills - and perhaps some of it can be used to deliver improved comfort in other areas, and especially during these colder snaps...

Cheers,

Itsallaguess

[Urbandreamer]

We / You don't know enough about the conditions to draw the conclusions.

Extreme low temperatures outside may mean that a boiler can't keep up. While comparatively warmer conditions may lead to a belief that what you adjust reduces gas usage.

OK, here is the theory of what you are doing.

With a "standard" central heating system a thermostat in the coldest room turns the pump on or off. Normally this means that the pump runs most of the time when you want the house to be warm.
The boiler turns on when the returning water drops below the point that you are setting.
The water temperature drops as it releases heat in the radiators OR THE PIPES.
Lower water temperatures improve the efficiency of condensing boilers by using the cooler water to extract the latent heat of vaporization from the fumes*.

So with a "standard" system, there are lower losses while the thermostatic radiator valves are closed by reducing the return flow switch point.

*fumes: burning gas or other hydrocarbons is a chemical reaction combining hydrogen with oxygen producing steam, and other stuff.
"standard central heating", are gradually becoming less standard. For example it is possible to have a thermostat in each and every room via smart radiator valves and only turn the pump and boiler on when one or more rooms need heat. Adjusting the flow temperature on such systems will have have little effect as there will be less losses.

[Dod101]

Yesterday I put the heating on with the boiler flow temp set to 68C. Return temp is generally 45-50C.After a couple of hours, the chill was off the house despite being about -3 outside. The lounge hit a balmy 17C.

Reading the meter before and after showed gas use was a lot lower than feared, certainly lower than the 30kWh the boiler is rated at, and came in at roughly £1.85/hour. It was so much lower than I expected I checked the boiler and looked up the model no and GC no to check. As we have microbore pipes and the flow is poor, but heat loss from the pipes embedded in the wall is massive, presumably the boiler cycled. The room stat definitely didn't click as it was set well above ambient.

So today we tried it with the flow set to 50C, which the boiler spec sheet says is most efficient of the temps tested. Return temp was (according to Draper's finest IR thermometer and some tape on the pipe) the predicted 30C once the heating had been on a couple of hours. As MrsF was WFH we splashed out and turned the rads on in her office, but turned the bedroom TRVs right down. Checkin at 18:30 the hourly gas use was right down, about £13.20 for the 8.5 hours since it was turned down, plus 2 hours earlier and one last night before bed, all at 65C, and 30 min of hot water. Tonght we';; use the immesrion.

Now here's the rub, the information you read says that running the boiler cooler saves gas, and this is correct. They also say your home will still be hot but take longer to get there, this is not correct. Because despite closing all the curtains at 4pm, and having had the heat on all day and the TRV open, the lounge is still 15C. And now the gas fire is on. I've just turned the flow temp up to 55C.

Our lounge is 6x4m, with 3 external wall, and because it was designed for the views it has 11m2 of glazing, and solid floor. The radiator is 1400x700 and double fluted, so at 11000 BTU at delta T65 (the difference between rad temp and room temp) is it well over the 8800 BTU the room requires. Except that at Delta T50 that drops to 8000 BTU, and at Delta T30 (which is about what we had today, the rad was 38C in the middle) the output is only 4000 BTU. And I expect doubling the radiators would increase the gas use.

Paul

I cannot comment on gas usage as I have a bog standard oil fired boiler but Dr F has indicated on another thread that he is not poor and if I were him, I would turn up the boiler to get a comfortable temperature. Unless that is he is in to measuring and testing the effect of different settings. Like many I am sure, I used very little oil during the very mild Autumn and currently (it is I think a bit less cold this morning) I just use my boiler as required to keep comfortable, whilst doing sensible stuff like closing curtains and so on. At my stage of life I will not be cold if I can help it.

Dod

[GrahamPlatt]

Insulate.

Care to elaborate?

We've had a quote of nearly £100k for the roof, and £30k for the floors.

Then there's the main house with the 200 year old solid walls so that means exterior render or major internal disruption and smaller rooms afterwards. then there's the consequent issues with damp course (or lack of it).

The cavities in the newer bits are full of debris so apparently not suitable for cavity insulation unless we remove a lot of bricks to clean it out, and the house is very exposed (views to the hills 30 miles away) so quite a risk of damp penetration if not cone properly.

But thanks for your extremely well thought out and constructive one word answer.

Sorry about that. Admittedly it was not too helpful. However, I believe that insulating as much as possible is probably the most long-term cost-effective strategy. I have just upgraded my house insulation. Here’s what I wrote (slightly modified) to a friend these past couple of days:

13th December…

“I’ve been trying to optimise the energy consumption here lately. Trying different heating profiles (times the heating comes on and at what temperature level is demanded throughout the day). Started with a “mostly on” profile (20C from 7am to 10pm, 15C overnight). This was with the (newish 35kW condensing gas) boiler set to provide water at 65C to the radiators. Lovely, but costing £30 a day (assuming 15p/kWh). I then dropped to an “economy mode” profile; 19C 7-9am, 17C to 5pm, 19C 5-10pm. 12C overnight. In fact, the 12C setting’s irrelevant as the temp only drops to 15C by the time the heating resumes at 7am. This is actually fine, liveable. So then I dropped the water temp at the boiler to 60C… this helps with the condensing aspect of its function and it runs a lot more efficiently. This has reduced the cost of operation to £15-20 per day. But the external temperature has now dropped to -5C today and while the room thermostat is requesting 19C, we’re only getting 17.5C in the room. So it’s either put the boiler temp back up, or turn the fire on.

I have spent some effort making the place draught-free (to good effect) and have a lad in today, fitting the loft insulation I bought back in July.

14th December

Yesterday evening I had the gas fire on from 8:30 to 10:30 to maintain the room temp at 19C. Today, (admittedly only -2C outside) since the insulation’s been mostly installed, we’re comfortably at 19C without the fire. That’s still on the economy profile and boiler at 60C. The lad’s still up there though. He fitted 20 rolls yesterday, which wasn’t enough so I went and bought 5 more. He tells me it’s still not going to be enough; 2 more required.
Thing is, I got the original 20 on a deal, bu from a firm that was miles away and in any case is now out of business. So I’ve had to buy locally at ~50% over the odds. Should mention, these rolls are 12cm thick “rockwool” and 4.5 sqm each.

[DrFfybes]

Now here's the rub, the information you read says that running the boiler cooler saves gas, and this is correct. They also say your home will still be hot but take longer to get there, this is not correct.

In your post you haven't mentioned what the indoor temp was at when the heating went on.

It's the increase in temperature needed (to get to your target for a given space) that determines how long your heating (at a given output) needs to be running for.

It's also complicated by the fact that the room thermostat (or the part of the space it occupies) is just one point - and most people move about, and feel a bit differently about stuff than a bi-metalic strip

Yeah - the hall 'stat is set to 12C, although playng with it the room hasn't got above 8 or 9 for a week. We don't heat the core as it goes straight upstairs and out of the roof.

I did mention, the lounge started at 17C, but after turning the flow temp down it gradually cooled once the sun went down. I turned the flow up to 65C for this morning, then down to 55 at 10am as the sun started to warm up the outside. I'm guessing this is what Mike4 means by "Weather compensation"

Paul

[chas49]

Now here's the rub, the information you read says that running the boiler cooler saves gas, and this is correct. They also say your home will still be hot but take longer to get there, this is not correct.

In your post you haven't mentioned what the indoor temp was at when the heating went on.

It's the increase in temperature needed (to get to your target for a given space) that determines how long your heating (at a given output) needs to be running for.

It's also complicated by the fact that the room thermostat (or the part of the space it occupies) is just one point - and most people move about, and feel a bit differently about stuff than a bi-metalic strip

Yeah - the hall 'stat is set to 12C, although playng with it the room hasn't got above 8 or 9 for a week. We don't heat the core as it goes straight upstairs and out of the roof.

I did mention, the lounge started at 17C, but after turning the flow temp down it gradually cooled once the sun went down. I turned the flow up to 65C for this morning, then down to 55 at 10am as the sun started to warm up the outside. I'm guessing this is what Mike4 means by "Weather compensation"

Paul

When you say "hall 'stat " do you mean the main thermostat that switches the heating on or off? I'm assuming not but it's not clear from your description. If that is set lower than the individual TRVs, it'll cut out before the rads get to target. Can't imagine that's your issue but I had to ask