MCS calcs for heat pump skewed by a thermally separate conservatory - any solution?

[DuncanL]

Primarily for environmental reasons, I'd like to switch our 2009 built detached house from a mains gas boiler to air-source heat pump. Octopus has quoted £5.3k for a fully installed system including their Cosy 10 ASHP, and have completed a survey, calculating 9 kW heat loss at -2.4 degC ambient temperature.

 

We built a conservatory a few years ago, but kept the existing patio doors, etc, so the conservatory is thermally separate from the rest of the house. We added a new, separate zone to our existing ground floor underfloor (wet) heating for the conservatory, and we heat the conservatory a little during mild spring/autumn days, but don't heat it over winter - this would be environmentally and financially costly as the conservatory is quite large and obviously has high heat loss!

 

The survey report from Octopus doesn't include the conservatory in the heat loss calcs, but they've now said that MCS requires them to include the conservatory due to the underfloor heating connection, and that they would need to specify a heat pump capable of heating the entire house AND conservatory for 99.6% of the year (i.e. all through winter). I've pointed out that heating a standard conservatory all through winter would be silly. I've also pointed out that if we were to fit a 12-14 kW heat pump (based on these MCS requirement calculations), it would probably run terribly (very inefficient) when only heating the house through most of spring and autumn (i.e. milder weather).

 

Octopus has stated that the only way to satisfy MCS requirements would be to permanently disconnect the conservatory underfloor heating as part of the install. Is this really the case? Has anyone else experienced a similar situation (e.g. a radiator in the conservatory), and did you find a solution that satisfied MCS without fitting a heat pump that's too powerful for actual heating requirements? I've suggested Octopus sets the target temperature for the conservatory to be 5 degC, but they've said MCS requires a minimum of 16 degC for conservatories.

 

Please let me know if any further information would be helpful! Thanks for any advice!

 

In case it's relevant, smart meter readings over this winter show our highest gas use over any 24 hour period is 100 kWh (noting that we haven't had any super-cold days this winter). Dividing by 24 shows average heating power over this period is 4.2 kW (assuming close to 100% efficiency from the gas boiler). If we assume that on a very cold day, we might need 6 kW of continuous heating (instead of the 4.2 kW we've seen so far this winter), and that an "X kW" heat pump can only deliver about 70% of X kW at an ambient temperature of -5 degC (if I understand correctly, nominal power rating is typically based on an ambient of +7 degC), then it looks like an 8 kW heat pump would be about right for us. I can't find much data on the Cosy 10, and how well it modulates down to lower power levels, but it's clear that from the Octopus offerings (Cosy 6, Cosy 10), that the Cosy 6 wouldn't be sufficient.

[JohnMo]

Get other quotes, don't mention the conservatory is heated.

 

£7500 + £5300. For house that has UFH already? That's bonkers as normal. Or is it £5300 on total, so zero cost to you?

[DuncanL]

Good point, I wasn't clear on price! It's £5.3k for me to pay, so total "cost" is £12.8k with £7.5k from the heat pump grant.

[JohnMo]

19 minutes ago, DuncanL said:

£12.8k

What are you actually getting for this, if you already have UFH? Or am I missing something?

[Beelbeebub]

Just disconnect the flow & return where they connect to the conservatory manifold and cap them off with a speed fit stop end.

 

Remove the blending valve and pump.

 

Obviously you can't remove the pipes from the floor and it would make sense to leave the manifold in place.

 

Conservatory disconnected.

 

Of course, once everything is fitted and they have gone home some one might reconnect the flow and return to the manifold....

[Beelbeebub]

What are thry actually providing for the. Money.

 

Heap pump obviously

New cylinder?

Any new rads?

New pipework?

 

Anything weird like the heatpump miles from the house?

[DuncanL]

@Beelbeebub I like your thinking! It might be slightly more complex, in that the conservatory is a separate zone with programmable thermostat and control valve... I definitely need to retain the ability to NOT heat the conservatory whilst the rest of the house is heated (i.e. through winter). Could be a last resort if we can't come up with an official approach that satisfies MCS requirements.

[JohnMo]

9 minutes ago, DuncanL said:

slightly more complex

You just undo the pipes on the manifold that feed the loops on the conservatory. You then blank the manifold ports. Positive isolation sorted. Let them install ASHP, reconnect pipes later if you want. Thermostat and valves can remain in place, as they do nothing now.

[DuncanL]

What's included:

• Cosy 10 heat pump
• Daikin 300 litre hot water cylinder
• Volumiser
• One new radiator (basic k2 600*700 unit replacing a marginally smaller radiator)
• Controls
• Plumbing and electrical components, as required
• Labour

The install appears quite simple to me - the heat pump will be located on the outside of the house wall where the gas boiler currently is, which is about 1.5 m away from the electric meter. The pipes between the gas boiler and the cylinder (in the upstairs airing cupboard) are 22mm, so I understand need replacing with 28mm - I think this would probably be the most awkward bit.

 

Cost-wise, I was assuming the something along the lines of the following:

• Heat pump: £4.5k
• Cylinder: £2k
• Controls: £1k
• Volumiser: £500
• Pipes, valves, lagging, anti-freeze, filters, etc: £500
• Labour: 10 days total at £350 per day: £3500
• TOTAL: £12k (so not a million miles away from the £12.8k Octopus has worked out)

Any other quotes, indicated prices, etc that I've seen have been much, much higher than Octopus, but maybe I should look harder?

[Beelbeebub]

Do they really need.to swap the 22mm to the cylinder to 28mm?

 

28mm pipe can carry 10kw at heatpump dTs

 

And you are having a 10kw (nominal Max) heatpump. But reheating your cylinder at 10kw is going to be less efficient and noise as your HP will be going flat out.

 

But 22mm can carry 6kw.

 

Which is probably right where you want your 10kw HP to run for max efficency and lowest sound.

 

Recharge times for the cylinder will be slower, but is that a major issue?

 

Depending on how the pipe runs are, the repiping might be a grand or more of cost.

[sharpener]

1 hour ago, DuncanL said:

Has anyone else experienced a similar situation (e.g. a radiator in the conservatory)

 

Yes. Even years ago heating a conservatory outside the thermal envelope from yr central heating was not allowed under the BRs. You were supposed to fit electric UFH (which is of course more expensive to run and worse for CO2 emissions, makes no sense if you want the heat anyway).

So got the plumbers to drill through the wall and fit isolating valves. After the BI had been to inspect the new utility and loo off the conservatory I fitted the rads myself and connected up. Separate zone for the Honeywell Evohome, TRVs set to 16C just in the morning to take the chill off.

[JohnMo]

25 minutes ago, DuncanL said:

The pipes between the gas boiler and the cylinder (in the upstairs airing cupboard) are 22mm, so I understand need replacing with 28mm - I think this would probably be the most awkward bit

My routing was quite onerous, once in the house already had 22mm plastic pipe (12m each way), no easy way to replace. So just use a close coupled tee (28mm to existing 22mm) and additional pump. Pump is powered with diverter valve. Works well. Pump £100-150. Two 28 to 22mm tees.

 

6kW will heat cylinder in about 45mins.

[sharpener]

5 minutes ago, Beelbeebub said:

But 22mm can carry 6kw.

 

Which is probably right where you want your 10kw HP to run for max efficency and lowest sound.

 

Entirely, the problem might be in forcing the HP to run at that low output. Noise Reduction mode could be the solution.

[DuncanL]

Thanks all, good thoughts on possibly using the existing 22mm pipes for water heating, and also easier than I'd envisaged to temporarily disconnect, and then reconnect (after heat pump install) the conservatory underfloor heating.

If possible, the easiest option for me would of course be for it all to be sorted as part of the install, which it seems would require the MCS heat loss calculations to be sensible where the conservatory is concerned (i.e. not insisting that the heat pump be sized to fully heat an unused conservatory in the depths of winter)!

[Beelbeebub]

13 hours ago, DuncanL said:

Thanks all, good thoughts on possibly using the existing 22mm pipes for water heating, and also easier than I'd envisaged to temporarily disconnect, and then reconnect (after heat pump install) the conservatory underfloor heating.

If possible, the easiest option for me would of course be for it all to be sorted as part of the install, which it seems would require the MCS heat loss calculations to be sensible where the conservatory is concerned (i.e. not insisting that the heat pump be sized to fully heat an unused conservatory in the depths of winter)!

Local plumber should be able to disconnect the UFH for you.  IIRC most have shut off valves just before the manifold. Turn those. Then disconnect (after depressurising) the Manifold from the isolators. Put some temporary caps on the system) probably vaccum put the water from the UFH pipe to avoid freezing. Take the fuse out of the switched fuse spur.  

 

System deactivated. 

 

Then get the MCS lot in.

[joth]

19 hours ago, DuncanL said:

• Controls
•  

What controls are they providing, and what do you currently use to disable/enable the conservatory zone?

The one potential snag of a temporary "permanent" disconnect during the ASHP install (to reconnect after) is they disable whatever controls you have working now, and you need to redo some electrical work to get it going again. It's probably not that complicated but potentially bit of a faff for someone to have to figure out and reconnect after the fact .

 

Anyway, the temporary disconnect is exactly what I would do (if I wasn't doing the heat pump myself, or getting a more amenable installer to do it) 

[DuncanL]

Not much has been provided by way of detail on the controls front for the new system. A Madoka (Daikin control panel) is about all that's been specified.

 

At the moment, the conservatory has a digital programmable stat, which can trigger the boiler and open the conservatory-specific UFH diverter valve, as well as activating the pump for this circuit.

[JohnMo]

33 minutes ago, DuncanL said:

At the moment, the conservatory has a digital programmable stat, which can trigger the boiler and open the conservatory-specific UFH diverter valve, as well as activating the pump for this circuit.

Obviously your boiler link will be deleted following ASHP install. I would heat from borrowed house circulation water. Let thermostat open valve only but not call for heat. Or you will end up having a small zone dominating how the heat pump runs, which isn't good.