What is your heating system @BPLightlog ? 

Gas boiler .. 2years old

Interesting... 

Can you adjust the weather compensation curve? 

Is the weather compensation feature also using the internal temperature or just the external temperature? 

Do you have your heating on 24x7?

What flow temperature are you using without the weather compensation? 

Are you confident any radiators are correctly sized and balanced? 

Anything unusual about the piping in your home? 

I can adjust the start and end points but not the curve as such

There is an internal thermostat (smart stat), thermostatic radiator valves and another smart (controllable) stat in a small annexe - all fed from the same system

No - timed with various adjustments during each day. Individual days are individually adjustable

58 deg C but interestingly with the eco setting on, I can see at present it is 41.4 deg C

Yes - adjustments made when boiler was fitted and reviewed at annual service (last done in Jan)

Don’t know if it’s unusual but where we are due to the nature of the type of rocks, there is a requirement to have a void under the ground floor, so some of the pipework uses that void.

It was just that whilst I can see the advantages of external weather compensation, the occasional ‘lukewarm’ nature of the radiators feels odd and the house in general doesn’t feel right (It is just a feeling as the temperature does reach the set point quite well). The house is fairly open plan and so I have spent some time this winter ensuring the draft proofing is good. The house has cavity wall insulation and an insulated loft.

Then i think the issue may be the way the system is setup and running.

Just like a heat pump with weather compensation, you need to think differently. 

For example usually it is better to remove all the thermostats and TRVs and let the weather compensation system run your heating. The TRVs will throttle the flow through the radiators a bit at all times for example. This won't work well with the lower flow temperature at times of the water.

You have basically got multiple systems trying to manage the heating which always causes issues with efficiency. 

The lower flow temperatures at times with weather compensation may mean your radiators are undersized so you may need to experiment with the weather compensation settings. 

Some of the heat pump owners can probably give you some ideas to try and what you might need to think about changing. 

I understood that part L of building regulations made it mandatory for trv’s to be fitted to all radiators except where the thermostat is located. They were fitted as part of the boiler installation 

Weather compensation is simply a way to run the system at the lowest possible flow temperature. If you're getting better efficiency with it off you've got one of a number of issues 

1 - There's another control in your system that's moderating the flow temperature, which is setting lower flow temperatures.

2 - Other controls in your house (e.g. TRVs etc) are interfering with how the house warms, which can impact efficiency.

3 - The efficiency change with a gas boiler will be marginal so the difference might be due to external factors (solar gain, different numbers of people in the house etc).

Some people with Mitsubishi heat pumps have found that weather compensation isn't always the most efficient way to run their systems. Mitsubishi have an alternative optimisation system that uses an internal thermostat as well as the external temperature. It could be that your boiler has built in optimisation that is doing a better job than the weather compensation function.

Yep. But they don’t work well with weather compensation unfortunately.

That is the problem. The regs are designed for the old way of heating and haven't kept up. We have a mess of regulations, some of which are for a different era. 

At the very least open them fully and see if that helps. Just use them in the bedrooms.

TRVs aren't on/off, they gradually turn off, so they are throttling the water at the same time the weather compensation is throttling the water temperature. That isn't a great control as heat pump owners often find who are the more normal users of weather compensation. It is less commonly used for gas boilers so far at least.. 

It will probably take quite a bit of experimenting to find the optimal solution, but there are some common things to watch out for. 

As has been said, this appears to be another subject with not enough known to offer the best advice. After contacting my installer and system manufacturer (the smart stat and boiler are part of the same system) they both suggest that I should be using TRV’s alongside weather compensation. Even in my control system it gives me the following advice: 

I’m still experimenting with running my heat pump. I have weather compensation switched on and the room sensor also sends indoor temp info back to the controller which can modulate the radiator temp up or down, eg when there is solar gain. The radiators are large which means they can give out enough heat to keep the rooms at the target temp. They all have TVRs fully open. I’ve set that to 21 C daytime and 20 C night time, which was a huge stretch to begin with as I really wasn’t used to heating the house at night. In fact it nearly never comes on at night time except when it’s been sub zero. There is no on/off time switch like our old gas heating.

Whether that would be how to run a gas boiler with weather compensation I don’t know, but certainly the heat pump temps are low: eg today at 12 C outside the rads were 32 C, rooms were 21 C. Back in December when it was -1 C outside rads were 45 C.

If your radiators aren’t huge maybe the temp graph needs to give higher values?

That is often the challenge with the gas boiler industry looking to shift kit. 

So what this is doing is lowering the water temperature then also lowering the amount of water flowing through the radiators. 

That is tough to get right.

Then you may also have an on/off thermostat with the weather compensation depending on how it works with your setup.

We are not saying it won't work, it just has the potential of multiple control systems not working well together. 

Thanks for the feedback. I’ll continue with some tests as I like to be methodical so I won’t adjust the TRV’s until I’ve finished checking the weather settings. I’m building a series of steps to check so that I can see what affects what. 
Incidentally on heat pumps do you have a setback setting? This normally automatically adjusts the set point overnight or in some cases for winter or summer

Good plan. 

I suspect that as you are running your gas boiler anyway at a relatively low temperature (for gas boilers), so you are already running you setup more efficiently than the majority of homes. 

At the temperatures you are using without weather compensation , your boiler will also be correctly condensing, again improving efficiency. 

Your experiments will be interesting as you could look to mimic how a standard low temperature ashp might work with your pipes, radiators etc. 

I’ve programmed in the timing and temperature schedule for the setback on mine. Daikin Altherma 3

Can I ask what is meant by ‘large enough’ radiators? Most are around 5ft 6 x 2ft and often doubled ended in each room (apart from the smaller rooms). The house was built around 1963 and it looks like it originally had electric heaters. I’m replacing the radiators one by one to update the system but each year the whole system is cleaned, descaled and rebalanced. We had a small annexe built a couple of years ago with an extra bathroom, laundry room and home gym - this was coupled to the existing system with flexible pipes (they had to use the roof space to link the buildings). At around the same time we upgraded the boiler to match. 
 

Update: I’ve just found this fab btu calculator 

https://www.bestheating.com/btu-calculator

Your radiators are likely to have been specified for a higher flow temperature. If the water is hotter the radiator can be smaller and still emit enough heat. If you're running at a low flow temperature you might need bigger radiators.

I say "might" because few installers did heat loss calculations to work out the correct radiator size. They just installed whatever size radiator fitted on the wall so many houses have oversized radiators.

Might be worth sizing them for a heat pump so you don't need to change them in the future?

First test week completed without weather compensation and reviewing the data, including outside temperatures and general people movement from an average of ~60.7kwh previously used, the system now uses ~51.2kwh. I realise there will be some potential inaccuracy but given the data log (I have approx 2 years of data to make comparisons), I think this should be relatively low. 
For this next week I will remove the TRV’s and again review. For the following week, I will switch the weather compensation back on to check for this without TRV’s 

We have a similar setup, but only using a Bosch EasyControl.  It has been set up by ‘trial and error’ and a bit of internet help. There is plenty of room for error and very little help from Bosch.

We use a room influence of 2, found as recommended by Bosch engineer on internet,

living area day temperature 19 C,

and a night threshold of 15 C, we don’t like it too hot, but again Bosch recommend keeping the house ‘fabric’ at a minimum of 15C. Keeping the ‘fabric temperature’ up does seem to make the house more comfortable, presumably the effect of radiated heat.

The curve is still being tweaked, it’s working reasonably well, but have yet to test it at the extremes. 

I would not rely on day to day variation in consumption, there are far too many variables and lag in the system. I often see these effects, no system changes, but a warmer day using more energy than the previous cold day and this applied pre weather compensation. On our system we seem to see a gain when comparing equivalent monthly data, but as the boiler is already ‘smart’ we expect the gains are going to be small.

Good to hear from you @Taj. It’s interesting to compare. If I use similar settings to you, I get the water pump active almost all day and night. 
Our setback is 12deg C and the room with the smart thermostat is set as a high influence (3).

Reading various pieces of info and with the tests I’m doing (comparing similar days; temperature; solar gain; people movement), I’m not convinced that weather compensation works that well in my situation. 
We do have a small annexe which has programmed radiator valves but that part is useful with potential differences compared to the main building. Like I mentioned earlier, our room layout is rather open plan and so it is not easy to figure what’s best. 
Part way into testing without TRV’s (apart from the annexe) and it looks like they don’t help our system at all. 

Hello @BPLightlog,

as I understand it it the pump is designed to be permanently running. This went against the grain for me, but it eventually sank in that for my modulating boiler the pump was already ‘permanently’ running. The only time it shut off was during the night when the set temperature was 14.5C and the room temperature was above 15C.

Is your room influence a tad high? Zero is no influence, and it increases from there.  I would guess that at 3 you’ve got maximum influence so minimum effective weather compensation. I do wonder about the effectiveness of room thermostats, setting up weather compensation is problematic enough without the room stats getting involved.

You are right, every house is different and what serves me will not work for others. You’ll have to discover what works for you, but don’t throw out weather compensation if you can help it. Treat it as a learning curve for heat pumps, unless you are my age you will almost certainly have one eventually and it will involve some form of low temperature heat exchange for rooms.

Good discussion this, good to hear from you, @Taj - you’ve had 3 green tech badges added to your forum profile. Now we know what you have and that you might be able to share your learnings for someone following in your footsteps...

So I’m at the end of another test week and it has all been very interesting. 
Caveat: I appreciate that my findings won’t necessarily work in another situation. 
 

The system now runs effectively without TRV’s in play. Weather compensation is also off although the gas boiler works by adjusting flow temperature presumably by load feedback. 
I have compared similar days, temperatures, people movement as best as I can and this appears to be the clearest indication 

The green line is the latest set up with no TRV and weather compensation disabled. 
The pink line is a similar day but with weather compensation enabled. 
While there are similarities, it’s the area under the curve which gives the energy used (lhs scale is kWh) and as such there is a significant reduction in energy consumption without wc but achieving the same heat level through the house. 
Perhaps the boiler on its own is a good controller along with the smart stat which provides the programmed temperature required. 
I calculate that I save around 12% of fuel without weather compensation enabled. The one thing wc does help with is on very cold mornings, the house gets up to temperature more quickly. Perhaps I can look into a modified control to switch on wc just overnight if the outside temperature is below -2 deg? I’ve started looking at an IFTTT app for that

Thanks for posting.

I wonder if without WC, the boiler is simply operating at a fixed flow temperature, what you are seeing is the boiler switching on and off?

With WC on a cold morning, the WC would kick in with a high flow temperature before falling back as the outside temperature increased? 

How did the house feel, was the temperature stable? 

I don't know what control you have of the weather compensation "curve" but perhaps try adjusting it given what you see in the graph? 

If I look at the boiler during the day, I can see that it ranges the temperature, presumably depending on the load. So it does vary up to the max that I’ve set.

Yes - that’s my point about having WC on below certain outside temperatures for a start but then switching it out after the first hour or so.

It was .. I was a little dubious about my own interpretation given my hand on the controls as such but I asked other family and they commented that the house felt more comfortable.

I can only adjust the start and end point of the curve so am not sure how much control I can actually adjust.

It was just an interesting experiment in that most of the advice I’ve been given was weather compensation gives more cost savings but it is obviously dependant on the specific situation.