Most efficient way to run modulating boiler with UFH to prevent cycling

[tom23432]

I have a Vaillant ecoTec plus 618 boiler. The minimum heat output of the boiler is 4kw.

I have UFH downstairs with 5 zones. Some of these zones are very small, like the utility room and the study.

I think the UFH manifold is set to around 32/35°C.

The house is a well insulated new build and loses very little heat.

I work from home and typically don't need all the UFH zones on, so I have been just running the study zone with the room stat set to 21°C. This room is small so the UFH requirement will be lower than the boiler min output. This causes the boiler to go into anti cycling frequently.

I have looked into the possibility of a buffer tank between the boiler and UFH, but the tanks look kind of expensive, unless anyone has suggestions on which one I might look at. The full downstairs UFH area is around 70m2.

Currently to stop the boiler cycling so much I have set the heating partial load setting on the boiler to the minimum which is 4kw. Then I have set the flow temp to 40°C and turned on three of the UFH zones. Study at 21°C, then the hallway and lounge at 19°C. The study heats up faster due to body warmth and the computer.

This leads the boiler to burn much longer without cycling. However I am wonding if this is actually more efficient than just running the one UFH zone and letting it cycle.

If I leave the heating partial load setting on the boiler to the default auto mode then it uses a bit more than 4kw, probably closer to 6kw and it heats up far too quickly leading to cycling.

So my questions are:

1. Is it more efficient to heat multiple UFH zones rather than just one to make use of the minimum heat output of the boiler to help prevent cycling and wasted energy.

2. Should I really be looking into installing a buffer tank between the boiler and UFH if I want to run small UFH zones efficiently? The 100L buffer tanks I have seen online are all around £500 without even considering installation costs. Doubt I would ever make my money back on that by running the boiler and UFH a bit more efficiently. Are there cheaper options or alternative solutions to my problem of the boiler cycling due to the min heat output of the boiler not being met by small UFH zones?

Additional info: Upstairs have 5 radiators, 2 towel radiators and a hot water cylinder. I turn the heating partial load and flow temp back up at the end of the day so when the hot water cylinder kicks in early morning it can get up to temp. Looking at new controls to allow different flow temps for central heating and water cylinder.

 

[JonathanM]

How short are the burn cycles?

 

[tom23432]

How short are the burn cycles?

With my current settings for the heating partial load, flow temp and three UFH zones active the burn times can be hours before it cycles.

With just my study UFH zone active with the same heating partial load and flow temps set it can cycle initially after 30mins, then every 10mins perhaps.

I think that's what you mean by burn cycles anyway. Let me know if I have misunderstood.

 

[JonathanM]

I think that's what you mean by burn cycles anyway. Let me know if I have misunderstood.

I meant how long does the boiler come on and burn for, before turning off. You seem to be saying, with just the study zone active, that once the boiler is up to temperature, it burns for ten minutes before cutting out? That sounds like a decent burn. Have you seen this graph from heatgeek?

 

[tom23432]

Thanks yeah I have been over all the Heatgeek articles.

I will measure the burn times accurately tomorrow and see how long it actually burns before cycling. 10mins was a rough guess from what I have been able to observe so far going back and forth to the boiler between working at my desk.

 

[JonathanM]

I will measure the burn times accurately tomorrow and see how long it actually burns before cycling

I would also measure how long it takes to come back on again.

 

[thetman]

Why not during the day put all the zones on a set back and get an electric heater to top up your study, will be far cheaper in the long run and a lot less messing around

 

[tom23432]

Why not during the day put all the zones on a set back and get an electric heater to top up your study, will be far cheaper in the long run and a lot less messing around

I do have a decent electric oil radiator that I can use. I'll compare the energy usage against the boiler over a couple of weeks and see which is more cost effective. Would prefer to use the UFH as it is a more comfortable heat and it keeps the temperature at a more steady level. But if it works out more cost effective to use an electric oil radiator and heated blanket them I'm not against that.

We do have Solar on the roof so that will offset some of the cost for the electric oil radiator, but obviously this becomes less relevant when we get in to the darker months.

 

[Johntheo5]

A few sums will give a worst case but use your own numbers.
If you assume your minimum heating at 2kwh for say 8hrs/day for 365 days/annum then total reduced requirement is 5840kww/annum.
Gas consumption at 88% efficiency = 6636kwh
Gas consumption at 83% efficiency = 7036kwh (assumes a efficiency degradation of 5%)
Extra Gas consumption = 400kwh
Extra cost @ 10p/kwh = £40/annum

Some worry about wear & tear but a combi boiler cycles for up to dozens of times/day, every time a HW tap is opened, so?.
Vaillant's are one of the worst boilers in firing up on the first refire after a cycle due to remaining at ignition settings for 60secs before releasing the burner to modulation causing burner shutdown at SP+5C, especially if your running with very low flow temperatures.