severnside

It's not easy to compare the heating costs as they house has been extended and changed quite a lot. But as a reference today the heating oil is about 70p per litre which means about 7p per kwh equivalent energy. I am running Heatpump on Octopus Cosy tarrif which is 17p per kwh, assuming a COP of 3 I would get about 6p per kwh of heating energy. So like for like cost will be in similar range. One area though which I think could be improved is the duct insulation. Warm air ducts are quite big so big surface area and if they are in cold space then that would mean more heat loss. Wall mounted units have very thin coper pipes coming in which are easy to move to thermal envelope of house and less surface area so they have less heat loss. The warm air duct system is super quite compared to wall mounted Air to Air units which is big plus in bedrooms.

Now we have been in the house since July so I can post some feedback of system for heating. We usually run the ASHP for about 11 hours on normal days (minimum temperature above 0C) and it maintains temperature around 20C (set by us). On super cold weeks where minimum temperature was -3/4 C, the system does manage to reach 19/20C but it seems a struggle. We also don't turn off the system in night on such days instead set the temperature to 18C overnight. (Overall system run for about 19 hours on cold days) Under 0C we do see lot of defrost cycles (almost every 30 mins), so system is heating for lesser duration. We still have some insulation to finish in the house but the system does seem sufficient and slightly worse feeling compared to previous oil boiler. As another reference point we have wall mounted A2A units in new part of the house, they heat the rooms quickly to the desired temperature. Their performance seems better than Warm air retrofit.

Thanks JohnMo, it's sorted now. Plumber signed up with OFTEC skilled person scheme and he self certified it.

That should be 5.0kw to 14.5kw so approx 3:1 modulation Our house is quite far from neighbours so we didn't face this problem. Also the two units we put are in different locations (about 30m apart). MHI units are slightly noisier on paper than Mitsubishi Ecodan but in reality we didn't find them that noisy. Also the Max noise figures are at max output which should not be very often. Does your house need 29kW heating input? If 14kW is sufficient then they should be able to put 2 indoor units and one 14kW outdoor unit. We haven't upgraded the filter but as mentioned previously we haven't used the system much. At the moment we have old floor diffusers with dampers, but we never used them. We did keep couple of flooring offcuts next to diffusers to cover unused rooms if needed but never used.

@Archer The controls are basic but sufficient. These are similar to commercial units you might see in offices. You can set the temperature and turn on / off. It feels different as it's always running on low setting once temperature is achieved instead of on/off like radiators. The way I analysed it if it was sufficient was to compare it with existing system. Our exisiting unit was Johnson Aquair S20 which can push 1224m3/h. Mitsubishi FDUM 140 can push 2880m3/h (Max) and 1320m3/h (Min). We found that it's better than previous unit in pushing air to furthest outlets and much quiter. For heating capacity analysis our previous boiler attached to Aquair S20 was 20kW, which was not running all the time. I did a basic heat calculation using the spreadsheet available on the forum and asked the installer to install one size bigger than calculations. AC installers always over spec and use rule of thumb. Also they calculate based on cooling load. AC unit heating capacity is higher than cooling, so if you spec for cooling it would be over spec for heating. These units do modulate a lot so overspec is not bad, e.g. our 14kW Mitsubishi can modulate 5.0 to 14.5kw We haven't used heating much on the unit since install. We have just moved back in the house after renovation, we turned the heating on yesterday to heat the empty house and it was working well. We are still to test in full winter

In our renovation we are using A2A for Heating/Cooling and Direct Unvented Cylinder for Hot Water. The plumber who did all plumbing and Cylinder has G3 Qualification and also on Gas Safe Register. However he told me that he can't self certify using Gas Safe as they have changed the rules and only allow Cylinders connected to Gas Appliances. Gas Safe seems most popular scheme for plumbers so I am not sure what to look for in another plumber or service provider. Did anyone else faced similar issues and how did you resolve it? Thanks

We put 3 wall mounted split AC units connected to an additional outdoor units. So we have 2 outdoor units, one for existing Warm Air ducting and other for 3 additional split ACs. We did look into ducted for new rooms but these will be used occasionally so having 3 separate units provide more flexible control.

Hi Robin, If you see the image above, this is picture of old warm air system. Top part is return duct and bottom part is supply duct. There is no individual room level control other than vent louveres. The 4 pipes you see are connected to return (3 return and 1 fresh air if needed) to provide volume. There are 3 further pipes not visible in photo connected to supply ducts. These old warm air ducts start as one duct and then branch off into individual rooms. So it should be possible to use exisitng ducts in your case. We also built an extention and it wasn't possible to extend ducts there. We put 3 Air to Air split units in new part connected to a separate outdoor unit. Hot water is direct electric immersion connected to solar panel diverter.

We are using MVHR, A2A (Part Warm heating, Part Split ACs) and Direct electric Hot water. We still haven't finished renovation but It should work. If you have loft then a ducted unit is also possible instead of lots of wall units.

@earlsfieldwe are still out of house and build work is ongoing. We will be able to test it next winter. I have received lots of private messages around this topic, so I will do a follow up detailed post once I test it properly.

@RenewableNeilUnfortunately we are in middle of renovation and we had to move out. So we couldn't test in winter season. We did test it in summer and air flow was better and quiter than previous system. The unit is not very heavy, check Mitsubishi specifications. It should be possible to mount it to joists. Do check noise specs from Mitsubishi if it will be indoor for you, ours in separate Garage

I searched the Internet and can't find anything relevant. Additional question, should I not use Hep2o under cold suspended floor and use copper for ground floor

We are at first fix stage now and I am sold on manifold idea with Hep2o pipes. I wanted to check best practice for Hep2o pipe routing on Ground floor. We have very thin joists and we are planning to fill 70mm PIR in them. Do people keep pipes in first floor joists and chase down the wall or put the pipes above insulation on suspended floor. Also do I need anything to protect Hep2o from PIR or vice versa

Both of them suggested by SE but it took him 2 minutes to answer as it's very standard stuff. The RSJ with steel plate for a different reason as we had another RSJ linking into it otherwise it could have been a simpler solution.

We had similar situation in couple of walls one 300mm and another about 400mm with stone face. For 300mm wall we had RSJ with 8mm steel plate welded on underside. For 400mm we tried L shaped catnic Lintel but when we put stones on it (200mm) it bent slightly forward so we had to remove it. Now we are using 200 x 200 x 16 EA for front stone face wall.