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Hi all,

I’ve paid them for a heat pump install, I was going to do it myself but their quote is cheaper than the parts.

If you like, I could be swayed to the Cosy 8 and we can monitor it, rather than the Daikin. I like daikin though. I’m expecting them to offer the 8kw unit.

My house is pretty interesting, both with what it has now and thermally.

Its a 2020 newbuild, but was provided with a 30kw system boiler that modulates down to 7.9kw minimum, originally set a static flow 80C. Rads for the system are all actually specified for 35C flow, however they have all been installed on 10mm drops regardless of the rad size.

I have since WC’ed it and done some basic monitoring of flow and return; but its minimum output is way over the heat loss of the house at -3… So the monitoring and gas use is wild as it seems to ramp to 100% to light up, dump all heat out the flue then repeat indefinitely. Its annoying because they make a 11kw model of this boiler than modulates to 3.1kw.

Its a 5 Bed 5 bath detached 230m2 house, I have calculated (over estimated) the heatloss at about 7.4kW. Which seems to track with my gut feeling - its hard to say because the boiler has never not cycled. I think its run for more than 10 minutes maybe 3 times a year. Uniquely there is a single room on the ground floor, everything else starts on the first floor and the rear garden is also at that level.

Roof has 500mm joist insulation, walls are 100mm full fill cavity, All double glazed, ground floor is just 50mm PIR concrete beam.

It should be a very easy install for them, the electrical is already in, the pad is done and the pump can go straight through the wall into plant room where current cylinder and boiler is.

Original EPC is nonsense.

We have 9kwp of solar and 15kwh storage on site. Oh and an Eddi but i’ve never used it since export price is over gas price.

Ill keep you updated.

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Octopus thought mine would be an easy install. Outdoor unit back to back with the tank, half a dozen radiator upgrades and no floor boards to lift.

Octopus sent two plumbers who were joined later by an electrician. They were hoping to be done in 3 days.

Unfortunately they were not well prepared for my solid stone walls and it took over one man day alone to drill the holes for the primary pipes. The guy was exhausted by the end of it.

eventually completed by lunchtime on day 5.

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No such issues here :sob: it’s all soft block.

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Have you got records of daily gas consumption on really cold days? That can help determine likely heat loss.

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Yeah! But even at -3C temps my boiler was still cycling as its minimum output is 6.7kw.

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If I decrease the WC curve lower it cycles even more, or hits a overheat error. You can see here it steady state fires about twice. It runs at 100% on every re-light, so any actual efficiency is never achieved, I can tell it dumps alot of heat out the flue for these cycles, because its steady state running for the HW coil at 80C is OK and quite good consumption for the energy output.

So rather than battle with this stupid 30kw boiler, that doesnt even have opentherm (they faked it), ill just swap it out. Even a burner + PCB change to make the boiler into the 11kw version would cost more than the HP.

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If you take the total gas consumption for that day (kWh), divide by 24 hours and multiply by boiler efficiency (say 85%), that’ll give you the average heat required (kW) for the property at that temperature. The fact it’s cycling doesn’t matter.

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As the cycle are so short that they radically drop the boiler efficiency, we have no way of knowing what the efficiency is. It maybe closer to 50% then 85%!

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Yeah having watched and looked at the boilers operation its no way possible it’s anywhere near 80%, my gut feeling tells me it’s closer to 50%. So basically any gas use estimates are going to be wrong and really hard to get right.

I did think of putting a buffer in, but again why go to the cost of this when we can just electrify the heat source.

Luckily I have a very detailed knowledge of the fabric of the house.

Its probably wrong to call this cycling, it’s almost like an error state caused by stupid selection of boiler for the required needs. I think it was chosen purely for cashback into the installers pockets.

You can imagine what it’s like when it’s 12C outside instead of -3.

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Is there a short cycling award? This is what happens if I reduce the WC curve.

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If the flow temperature is too low, the radiators can’t emit the heat quickly enough, so boiler has to cycle. Larger radiators or higher flow temperature will reduce the need for cycling, or rely on the room thermostat to cycle the heating on/off.

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I also note that the Return Temperature (green) goes higher after 8am, which suggests some radiators have turned off, perhaps with TRVs?

A condensing boiler is most efficient when return temp is low, and will also cycle less often.

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I think you are assuming the system design and install is actually correct.

The boiler is never reaching a modulation state, it fires at 100% as part of its turn on, begins to ramp down to the target flow temp, but it cuts out because the return temp gets too high compared to it’s target within those 5 seconds of turn on.

It’s not even true short cycling because of this. The boiler literally cannot start properly. This is what happens if your boiler is massively massively oversized.

It’s a system boiler, not a combi, so has no pwm control of the pump.