JamesPa

To my mind splits have two unique advantages, namely that the condenser unit can be distant from the house with a much smaller penalty. They also don't have water outside. However if you don't need those advantages, then monoblocs are much more common in the UK (not necessarily the case elsewhere) which from me says monobloc if you can just because of industry familiarity.

A few questions is the house warm enough? Whats your heating pattern? is it a gas boiler, how old What does your EPC say your consumption is In the spreadsheet calculation, what are the assumptions about the fabric that you cant verify, particularly air changes per hour? Where in the country are you/whats the local design temperature Here are some facts about my house for you to compare. I use 18-20MWh/year and have monitored my smart meter readings for 2 years. I have a gas boiler which does space heating and DHW, and a gas hob. Never does the gas consumption go above 8.5kW (and this only happens for a couple of hours if I have turned the heating off at night, or if Im using all the gas rings) and most of the time its around 4kW. The house is warm and heated 24x7 in the very cold season, about 6am-11pm in the shoulder season. In the cold patch in late 2022, when it was -2 day and night for about 5 days, the consumption (I was watching it) was 7.5kW. Most of the time the smart meter reads 4kW. Its clear therefore that the actual demand at the design temp of -2 is about 7.5kW-8kW and I need a heat pump which does this but will modulate comfortably down to 4kW or preferably a bit less (unlike my hopelessly oversized (28kW) gas boiler which cycles a lot). To prove this to myself, this year I turned the gas boiler down to as low as it will go, 8.5kW, and it was never cold. Granted it was a fairly mild year, but note the measurements the previous year. The MCS spreadsheet gets to 10.5kW. Thats with default air change values and my best estimates of the construction (its a 1930s house with various fabric upgrades and some extension, so a very mixed fabric). However I can get it to 7.5kW if I assume 0.5 air changes per hour as opposed to the default of about 2.5. Two MCS surveyors, each after a 3 hr survey, have told me I need 16kW, which is obvious nonsense. They ignored the fabric upgrades that I made a special point of telling them about, double counted room to room losses and of course assumed ACH=2.5 per hour. Assuming a) I can get planning consent (don't ask) and b) I can persuade an MCS installer to cooperate I will be installing an 8kW ASHP, most likely the Vaillant badged as 7kW but actually good for over 8kW at 45C flow temp. Scaling my measurements to your 12MWh suggests you do indeed need about 5.5kW. You also need a heat pump that will comfortably modulate down to 3kW at say 10C OAT, preferably 2.5kW. Obviously this is subject to there not being surprises in the answers to the question above, and is not a recommendation just an observation. I wont be surprised if MCS installers recommend 10kW, which will most likely perform badly as it will probably be oversized.

Do you know how much energy you currently use to heat your house? How does it compare with the EPC value? If you do that will give you a clue. Have you got any way of monitoring it real time ( I watch my smart meter and download half hourly meter readings). If you can do this then you should be abke pretty much to nail it. Also how much of the 7.7kW is down to unknowns (the biggest possibly being ach). Again potentially bit more data ( this time on the uncertainty in the spreadsheet answer). Realistically all you probably need to know is whether to buy an 8kW or a 5-6kW pump. If your house really is 4.3kW, then most of the time it will likely be consuming 2.2, so you need the pump to work smoothly at that load. An 8kW one won't, a decent 5-6kW one should. On other hand if your house is really 7.5kW it will be consuming 4kW most of the time, which a decent 8kW model will modulate down to. Sometimes (not always) combining data may allow the answer to be deduced to a level which is, with a high degree of certainty, good enough.

I would trust this about as much as I trust the MCS GIGO spreadsheets - ie not at all. Personally I think that, for a retrofit which has had any 'history' of fabric changes since it was first built (ie most retrofits earlier than perhaps the 1980s), the only sane way is to measure it, whether from actual annual consumption (as opposed to the EPC estimated annual consumption) or more sophisticated smart meter data if you have it, or if you really want to go to town one of the companies that put in a calibrated heater. I do accept @JohnMo s comment about hugely oversized short cycling gas boilers. Most gas boilers are of course well oversized, so I would certainly not assume the >100% efficiency that is claimed for a condensing boiler unless there is evidence to support it. But a measurement of actual consumption is still better, surely, than pure guesswork by an MCS surveyor, whose only real concern is to ensure that the heat pump is well bigger than it needs to be in order to avoid call-outs and guarantee that he cant be challenged on the grant rules. At least with actual consumption you are unlikely to be more than about 20% out and, unless the gas boiler is perfectly set up and condensing properly (which most aren't judging from observation locally), this will be erring on oversizing not undersizing. Obviously for a new build you cant do this, but for a new build you know what the fabric is. For a retrofit the MCS guys will (at least in my personal experience) make some random assumptions which may be a long way from the actual truth. EPCs are even less rigorous than MCS GIGO, at least as far as I understand them.

Doubt it unless the manufacturers turn against mcs. Maybe flexi-orb will eventually get to the point of providing a competitive certification scheme.

In fairness electricity retailers are able to waive this requirement. Octopus announced thar they had waived it, but then backtracked. Not sure what the current position is. Personally I would favour the total disenfranchisement of mcs so that it has no special status at all. However it seems to have the ear of government so that's not going to happen.

I get FIT from panels we installed in 2010 and I agree. I suppose the problem is that a contract is a contract and once you break it confidence in current incentives vanishes.

Don't bother it will prove and change nothing. Instead remove the requirement for MCS from the rules for permitted development rights, shift green taxes from electricity to gas and open up the eligibility for the grant to any of the other registered bodies. Prices will come down and the grant can be progressively reduced or phased out. Possibly govt needs in addition to sponsor some training in system design for retrofit available only to people who can demonstrate the ability to think. It's pointless trying to fix a private monopoly, the solution is to eliminate it's monopoly position.

Personally I would deflect sideways or down in this situation, unless you know that the principal intake if from below in practice. You don't want to risk exhaust air recirculating back into the intake and cold air inevitably falls. Having said that Heat Geek rans some experiments with recirculation (there is a youtube video somewhere). The results were frankly amazing, basically they struggled to trigger any measurable degradation due to recirculation. Eventually they pretty much boxed in a heat pump on all sides, and it still had only a small negative effect.

Id be tempted to go that route too. Get a sparky to put in the power feed (which involves opening up the CU) and the rest is easy and largely safe DiY. Mine charged £180 inc parts to do the second fix power feed (ie I purchased and ran the cable in advance so he didn't have to) and fix up some poor earth bonding he noticed whilst there.

Maybe try breaking it down. Get an electrician to fit the 32A (or 16A) circuit. The rest is more complex but less dangerous and essentially suitable for DIY. Or alternatively get the plumber to ask his tame sparky (every plumber that does heating knows a sparky) to do everything except the FTC6, which is hardly any different to any system boiler. Or draw a diagram and tell them that's what you want (there is probably something suitable in the ecodan installation manual). It's fear of the unknown and reluctance to guarantee the unknown that's the problem, so if you can find creative ways around that may help.

Where is the flower bed in or relative to the photo?

... yet 'the industry' for the most part insists on replacing functional dhw cylinders with 'heat pump cylinders'. That's another £3k plus mate (more if they also replace 22mm primaries with 28mm). Thank goodness that MCS, with it's government sanctioned monopoly, is there to protect and ensure installation standards.

Interesting. So what happens with a non modulating heat pump, I wonder, if the tank hasn't reached temp -1C but the heat transfer to the cylinder is less than hp output. Is it the same as your atag - ie it cycles, or does the flow temp rise until there is sufficient heat transfer (obviously still at a ft less than the coolant temp). I guess this depends on the hysterisis allowed on the ft? Actually I think the last part of the extract you posted answers this. The hp cycles if flow temp goes above 60. So as long as there is sufficient heat transfer to the tank when ft=59 and tank is at the required temp, all should be well. That sets a minimum coil/heat exchanger size.

I assume you refer to the phe circulator pump. What's the advantage of modulating it?

You are right I did make that assumption because it seems obvious that they should and they already do so for space heating with a very simple feedback loop, so all the functionality is there. If they don't, why not is the obvious question? Perhaps those that aren't so fussy about coil sizes do modulate down?

All of which and more is why permitted development rights should not be linked to MCS so that the industry can move asap to the point where grants aren't necessary and the customer is what matters. The current situation is nothing short of deplorable. Costs will come down once MCS get out of the way and some innovators move on. Sadly I doubt it will happen because government is listening to MCS but not to informed of customers who don't have a voice.

Just to add there are afaik two considerations related to coil size. The first is reheat time, the second is cycling as you approach the target temperature. The latter will occur if the coil doesn't dissipate into the water at least the minimum output if the heat pump. Obviously this problem is worse if the ft is low, and more or less eliminated if the hp can operate at a high ft like the Vaillant and other R290 models. Here we get into the industry trap again...many suppliers oversize the heat pump by a large amount and as a result it won't modulate down as far as it could if correctly sized. This clobbers efficiency of space heating, and drives a requirement for large coils in dhw cylinders and even 28mm primary feeds to the cylinder, even if the customer doesn't care too much about reheat time. So then the cylinder has to be replaced and floors ripped up to substitute entirely unnecessary hardware for existing perfectly functional components. Of course it also enables the supplier to specify a pre-plumbed cylinder, which means that they can employ rookie plumbers whilst still charging top dollar for unnecessary work.

I spoke to them last year, if it wasn't connected to their heat pump they weren't interested and said it only designed for their heat pump. What on earth does 'designed for their heat pump' mean. It's just a tank with water in it and a heat exchanger for goodness sake. Do Warmflow heat pumps follow different laws of physics?

Agreed. And that ultimately is the problem. It seems that most of the industry can't think beyond what it says in their plumbing by numbers manual. It's disgraceful but sadly where we are. There are a few who can but they are definitely (judging by what we hear here, and personal experience) a small minority. This pushes up retrofit prices and disruption thus putting people off. I found only one supplier who will offer the obvious (in cases where an increase in heat capacity really is necessary) ie to retrofit a phe and pump to the existing cylinder. Thats Pete Miller of Conga. However they concentrate on a very limited geographical area.

I largely agree but there are a couple of matters to consider 1. Get the sizing right. The industry seems, from what we hear (and what I have experienced) tend towards gross oversizing to the detriment of performance for the majority of the season. 2. Decide on the design flow temperature and check the actual output of your candidate pumps at this and your design OAT. The actual output can be quite different (up or down) to the sticker capacity.

Agree with both of these, subject to getting some info on heat demand and anything that falls out of that. Best to KISS (keep it simple, stupid) in most cases.

Looks interesting but Im not sure I get what is going on from a plumbing perspective, can you clarify. Are you feeding water at 45C both the the rads and the UFH, however because the UFH flow rate is much lower the average UFH temperature is lower than the average rad temp. Or are you mixing 45C water down to 30C.

To my way of thinking all suitable roofs should be built like this.

Thanks. I was aware of this but thanks anyway. I'm wondering also about switching the charger ct in and out of circuit, which would be a safer option. If I use the secondary output from the immersion diverter as the signal source, the diverter should sort out the prioritization I think.