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Thread: Modulating boiler or ASHP?

  1. #1
    Automated Home Jr Member
    Join Date
    Feb 2018

    Default Modulating boiler or ASHP?

    Long time lurker, first time poster!

    We're about to start on a huge renovation project, and after reading this forum I'm pretty much convinced about the modulating boilers, and it makes so much sense: you don't drive a car by 'pulsing' the accelerator to max and off again! But I wanted to raise a question on air source heat pumps?

    I already use evohome in a non-zoned setup (long story short: sold big old flat before Brexit, bought a bigger house on both probate and post Brexit for a dime and using savings to do it up and rent a tiny flat in the meantime). The new house will be a 3-storey end of terrace with a side return kitchen and loft conversion done. In total it'll be 12 zones, 11 rads and UFH in the kitchen. I've already ensured the rads will be sufficiently oversized to handle radiator temps of 35C for easy wins and 45C where not so easy, to make the most of the modulating condenser.

    Some generic questions first: For evohome is given there is only 1 UFH zone, is a BDR and DT92 sufficient or do I have to get an HCC? Also due to the loft conversion, gravity system is out and unvented cylinder is in, so am I right that S-Plan is best? Would additional zone valves for UFH and/or upstairs be necessary, and how do many different zone temps play with modulation?

    In regards to modulation, one thing that hasn't been mentioned is a comparison of energy usage prior to installing a modulating boiler vs an on/off boiler (factoring out evohome). Is there anyone who had evohome before and after that can share details of consumption?

    The alternative to a modulating boiler is the Daikin Altherma Hybrid has piqued my interest. This is an 'out of the box' bivalent ASHP and boiler system, and the only extra consideration is the outdoor unit. Gas is so cheap today that ASHPs have a big problem 'competing' with them, so it makes sense to use a bivalent system that flips between the fuel source (gas or electricity) and based on current conditions. The Daikin Hybrid is very interesting because it's a rebadged Intergas boiler with the hydrobox behind it, but it only does 10% modulation either side of the Weather Compensation curve. The temperatures also influence whether the Hybrid chooses to use the heatpump or the boiler based on actual energy rates. Given this, it'd be a perfect scenario for an Opentherm control to request a specific temperature and have the box decide which energy source to use to provide that, but there's no sign of it being that clever (and I think LWT's need to be fixed for RHI?). If anyone has any ideas of how to bridge evohome's OT to Daikin's systems so this modulates properly that would be epic. Reading the hybrid's installation manual is in itself quite interesting.

    Regarding the maths, the cost of the system is ~4k compared to ~1k for a normal boiler, and the running costs are increased by an estimated 136.40/year for me due to the increased electricity consumption over a boiler, however with RHI for ASHPs being increased to 10.18p/kWh the BEIS calculator yields ~1400/year for any radiator temperature under 50C. At 35C radiators, it's 1500/year, so not a big incentive to go for super low rad temps. So 3k extra capex up front over a boiler and 1k additional fuel expense over 7 years with 9.8k returned by RHI, in theory you get a new installation for free after 7 years, so the maths does make sense, assuming the govt doesn't yank RHI. The other unknown (I haven't got quotes yet) is how much more it costs to install (like I said, radiator sizing is already factored in with a modulating boiler). If it actually reduced my bills and wasn't much more involved to install, it'd be a complete no brainer, and this is the basis of my hesitation.

    The Hybrid's smarts on its WC bivalent strategy also got me thinking about evohome monitoring energy and using its smarts to choose fuel source - I wonder if Honeywell, given that it has a finger in the smart meter pie via Elster meters, whether they will make another evohome module that will be a CAD (Consumer Access Device) to read both the main meters and the RHI meters, and control normal boilers and immersion heaters as well using a bivalent strategy like the hybrid. At first glance it seems silly, but when you think about Solar Immersuns and Economy 7 it's not so outrageous... but even if evohome just replaces the smart meter 'IHD', this would be much better.

    Thoughts / answers greatly appreciated!

  2. #2
    Automated Home Jr Member
    Join Date
    Mar 2016


    Are you sure you have the RHI payments correct. In winter it looks like your system would be using gas (no RHI) in summer ASHP through meter(RHI) but requirement for heat reduced due to warmer weather. I feel your expectations are high

  3. #3
    Automated Home Guru
    Join Date
    Aug 2015


    My warning would be that you are introducing *significant* additional complexity which will lead to numerous possible outcomes: greater chance of failure, higher cost of maintenance, harder to find someone to install and maintain (limiting your choices and therefore likely costing you more), more difficult to troubleshoot, etc. Even something as simple as finding information on a forum will be far more difficult due to the (probably) extremely limited number of people running a system similar to yours.

    To put my point of view in perspective, after reading months of posts about OpenTherm, I'm very far from convinced that all the additional complexity, incompatibility and unpredictability of outcome is in any way worth any possible (entirely theoretical) savings. Simple is better, unless there is a very compelling reason to go for a more complex solution.

    Wouldn't it be simpler and more predictable to go for a solar hot water component and a solar electrical (with battery) component? Each of them can be completely independent of the main (heating and electrical) systems, and provide known/understood outcomes.

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