Help with oil boiler efficiency with Hive

Beanbag_35

Hi all

We moved house earlier this year into a 4 bed detached 1970's house.  We had the (ancient) central heating system completely overhauled - new rads, new boiler, new hot water cylinder and we installed Hive to control it all.  We moved in in February so had some experience with it last winter, but I never got to grips with using it efficiently and I'd appreciate any thoughts or experience as I start to set schedules this winter.

We're on oil so the boiler doesn't modulate.  I monitored oil usage as best I could last winter with the Watchman device, working out how many hours of heating we used each time the monitor moved down a bar.  Not an exact science, but applied consistently.

The main issue is what I would call "short cycling" - the Hive is obviously aiming to keep the temperature as we've set it but this means the boiler turns on and off a huge number of times.  When we moved in it was freezing outside and each hour of heating used c2.9 litres of oil.  As it got a bit milder outside the efficiency dropped massively to each hour using around 3.7 litres of oil - c25% less efficient.  I noticed that the daily switch ons had gone from an average of 6 per day to an average of 15 (and up to 24 times in a day).  

Once I clocked this I took better control of the heating schedule, but it's a pain not feeling able to set a schedule and leave it to do its (expensive!) stuff.  We work from home so are here all day.

Any ideas or experience on how I can best make this work?  I have thought about setting the schedule so it aims for a temperature it will never reach for shorter periods throughout the day and play about with that to ensure we stay comfortable but we minimise the switch-ons.  However, it's very easy to use 25% more hours that way and wipe out any efficiency gains.

Would you just set the schedule and live with the inefficiency?  I'm really struggling with that.

We have approx 16 radiators, a Grant Vortex 26/46 condensing boiler in the attached garage and a 180l indirect unvented hot water cylinder in the upstairs airing cupboard.  Water temp on the cylinder set at c50'C.

All ideas gratefully received - many thanks.

MikeJXE

I’m a low user on gas but the principles are the same 

My flat is set at 18 degrees 24/7 

You need to remember you must heat up the fabric of the home each time your temperature drops low

I find little difference than when I had it on 18 daytime and 16 at night 

vic_sf49

 WB Boiler + Hive + gas. (Combi boiler, but mine has a fixed temp for domestic hot water.) 

After running flow temp experiments one winter, I understood how my boiler worked better. Once it has run flat out for a bit, it's more efficient in the 2nd half hour, and more so in the 3rd and so on, so I like to run it for more than 1 hour. 

Like you, I hate that on / off / on / off /on /off, of the Hive keeping it at temp. 

Morning schedule will move earlier, when it starts to get really cold, and may need to run longer. 

Later evening, if I feel cold, I tend to just use Boost, as I find that need is too variable, to bother with another schedule. 

If it does get up to temp quickly within a heating period, I turn the temp down, so it stops doing the on/off thing. 

These are my rough timings.  I find it cools down enough between heating periods, for the boiler to get a decent run when called on again. 

Please note, these temps are NOT set properly yet. I'm currently just using Boost if I get cold.... 

QrizB

Increasing your system volume would lengthen the cycle. Volumiser tanks are more often used with heat pumps (for the exact same reason, to prevent short cyling) but one could be added to your system for the same purpose.

It would mean some plumbing, and finding room for a tank somewhere.

Lorian

This is down to lack of hysteresis settings in the Hive I think. Check you have anti-cycling turned on in the boiler if it's offered and moving the Hive to a room with no draught and best heat retention may help a bit 

Beanbag_35

Thank you for the comments everyone.  vic_sf49 - that's pretty much where we are at the moment - set to come on first thing then we use boost as needed during the day.  Interesting on your findings on your boiler efficiency, an oil boiler may well have the same efficiency gains the longer it's on.  

I've checked for an anti-cycling feature and sadly there isn't one.

I have contacted the plumber who installed the system to ask about volumiser tanks and whether he thinks that could help in our situation - thank you for the suggestion QrizB.

QrizB

We look forward to hearing how you get on!

vic_sf49

I'll check to see if I still have the figures from running the boiler flow temp experiment. I should still have them on a spreadsheet somewhere. 

It's easier for me, as I could take the half hour gas usage figures from Bright app. Then you can at least see the general usage figures of a/ny boiler. 

lohr500

Your setup is not dissimilar to ours.
We have a 4 year old Grant Vortex Blue flame 26/36 condensing oil boiler and 17 radiators.

Through the winter we have our Hive schedule set to 13c overnight (22:00 to 07:00), 16.5C from 07:00 to 16:45, 18.5C from 16:45 to 22:00.

The boiler also gives a short 1.5 hour boost to the hot water through the winter between 20:00 and 21:30 to supplement the  Economy 7 electric heating of the immersion tank overnight.Immersion tank thermostat set to 50C

In the depths of winter we are using 15 litres of oil a day to support the above schedule, dropping to +/-9 litres per day as the conditions get milder, before switching off the boiler in the spring.

I can measure the consumption fairly accurately because we have an in line fuel recorder on the line from the tank to the boiler.


Have you got any thermometers on the flow and return pipes to the boiler?

I have to say I am no expert on this, but here are my thoughts. Happy for anyone with a better understanding to chip in and correct anything I'm suggesting.

Firstly, the key to efficient condensing is to get the return temperature below 40C.

I played around a lot with the built in boiler thermostat temperature, the central heating pump speed and the flow valves on each radiator, so our system runs with a 55C flow and 40C return through the winter. I bought a couple of Brannan clip on pipe thermometers to monitor the flow and return temperatures.

Once you have the thermometers in place, you can play around with the boiler's own thermostat to set the outgoing flow temperature at 55C.

Then through a combination of adjusting the circulation pump speed (if its a reasonably modern pump, it should be possible to do this) and balancing the flow across all the radiators using the second valve on each, try to control the water flow so it returns at 40C once the system is fully warmed up.

Also its worth considering any thermostatic radiator valves. If you have lots of these and many are all set at a lower temperature than the Hive thermostat, then this can cause cycling. The Hive calls for more heat but the radiators are all closed by their TRV, so the boiler short cycles. Start trying to get the flow and return temps set correctly with any radiator TRV fully open.

I have to say again though that I am no expert and it was more a case of trial and error setting our boiler up to work as efficiently as I could!!!!

vic_sf49

Boiler flow temp experiment... 

- Dec 2023 - Feb 2024.

- Various times and dates.

- Boiler flow temp dial set between 3.5 - 5.0. (Cba checking what they equate to). 

- Inside starting temps varied from 14.6 to 18.0. (Hive thermostat).

- Outside temps ranged from +6 to -7. 

(feels like temps from Met Office app).

- Gas usage taken from Bright app. 

I have taken an average of 25 results. 

1st 30 mins, usage = 7.31kwh

2nd 30 mins, usage = 4.88kwh

3rd 30 mins, usage = 4.72kwh

I thought I'd remembered a bigger drop between the 2nd and 3rd 30 min time frame, but I was wrong. 

You can see how it runs at full pelt, in that first 30 mins though. 

I clearly don't have enough hobbies in my life. Sigh.