Air-to-air ASHP replacing warm air heating

[Gooman]

We have a 1970s 3-bed house with a Johnson Starley 15kW gas fired warm air system, with existing ducts throughout the house. Hot water is from an electric immersion heater.

 

Does anyone have any experience of replacing a similar heating system with air-to-air ASHP using the same ductwork?

 

I'm assuming you'd simply site the indoor unit in the same cupboard where the current boiler, connecting it to the existing ductwork (somehow).

 

Any thoughts, advice, guidance or warnings gratefully received!

[severnside]

Will follow this thread with interest as we have similar system (Aquair S20 with Oil Boiler). So far we have been advised to move to wet heating system as not many people understand warm air systems.

Johnson Starley literature for Aquair suggest it can be used with heatpumps, so it should be possible to replace the boiler with heatpump and run hot water pipes to Air handling unit.

Issues I see with that

- RHI payments are for wet system. Though this system is part wet, I am not sure if it will qualify

- In Low temperature mode air handling unit starts at 33C and stop at 20 C (compare to high temp mode 60 start and 42C stop). Air moving closer to 20 might feel cooler than it is. It should be similar effect as using a ceiling fan (though not same air movement)

- EPC rating people don't understand anything other than wet heating systems

[SteamyTea]

There is quite a bit of useful information on this site.

 

https://www.engineeringtoolbox.com/hvac-systems-t_23.html

[Gooman]

14 minutes ago, severnside said:

Will follow this thread with interest as we have similar system (Aquair S20 with Oil Boiler). So far we have been advised to move to wet heating system as not many people understand warm air systems.

Johnson Starley literature for Aquair suggest it can be used with heatpumps, so it should be possible to replace the boiler with heatpump and run hot water pipes to Air handling unit.

Issues I see with that

- RHI payments are for wet system. Though this system is part wet, I am not sure if it will qualify

- In Low temperature mode air handling unit starts at 33C and stop at 20 C (compare to high temp mode 60 start and 42C stop). Air moving closer to 20 might feel cooler than it is. It should be similar effect as using a ceiling fan (though not same air movement)

- EPC rating people don't understand anything other than wet heating systems

 

Strikes me that using the Aquair with a heat pump would end up as a hybrid of a new outdoor unit and the Aquair instead of a matching indoor unit. It could lead to a lack of accountability if you had any problems. Johnson Starley must surely be thinking of some kind of business strategy for the end of gas/oil boilers ... but there's nothing on their website to suggest what that might be.

 

Air-to-air doesn't seem to qualify for RHI, but this page suggests that the price of the units and installation is a lot cheaper than wet systems: https://www.greenmatch.co.uk/heat-pump/air-to-air-heat-pump

 

We're thinking of just replacing the entire Johnson Starley unit.

[severnside]

We found it easier to split responsibilities. There is a regular Oil Condensing Boiler which is called by thermostat when heating is needed. It works like it it's running regular wet Central heating. 

The air handling unit is controlled by water temperature. It starts when Water temp is 60 C and shut down at 42C. It also changes fan speed based on water temp. There is no external control to this unit other than power on Off.

We would also be interested in a ducted air conditioning unit if you find something suitable. If it can mix some fresh air then it will cover MVHR function as well along with heating/cooling.

Edited May 25, 2021 by severnside

[severnside]

I contacted Johnson Starley about heat output using ASHP. They provided this graph, so at 40C water temp it would mean upgrading the Aquair unit. But ASHP could provide option for mixing with DHW and wet heating in extention. I am not sure how ASHP will work with multiple Zones requesting heat. E. G. Warm air in main house and UFH in extention both running off ASHP. 

[SteamyTea]

4 hours ago, severnside said:

I am not sure how ASHP will work with multiple Zones requesting heat. E. G. Warm air in main house and UFH in extention both running off ASHP. 

Base the power calculation on the same flow temperature i.e. 40°C, then adjust floor insulation in the extension, along with pipe spacing, to suit. 

[severnside]

@Gooman Just checking if you find some solution for this. Probably some system like this could connect to existing ducts

https://www.orionairsales.co.uk/mitsubishi-electric-air-conditioning-pead-m100ja-ducted-concealed-inverter-heat-pump-10kw36000btu-r32-a-240v41550hz-9921-p.asp

 

Edited June 14, 2021 by severnside

[Gooman]

Nope, haven't found anything yet. It looks theoretically possible, but I've yet to find anyone who's done it, or anyone specifically offering a solution for interfacing with existing warm-air ducting.

[severnside]

Does anyone have experience of https://www.invisible.ac/ ?

This seems to cover ventilation, heating, cooling, humidity control and works with Air to Air, Air to Water Heat Pumps. It would be good to know if someone has experience of them or know about cost. Their air handler unit should work with existing warm air ducts

Edited August 3, 2021 by severnside

[Ommm]

I don't have any experience in this area, but just to note that this setup is common in the US where it's called 'central air conditioning'.  They use it for both cooling and heating - although some units eg in Texas only have resistive heating because they don't use it very much, they use heat pumps in places like Missouri where it gets cold and hot in different seasons:

https://www.consumerreports.org/cro/central-air-conditioning/buying-guide/index.htm

 

I can't recommend UK suppliers but I would imagine that it's more in the skillset of commercial A/C installers, where you might already have ducted air in say a hotel or office building.  I would talk to people who do commercial HVAC and see what they say.

[robin coles]

We have a blown air system through rigid metal ducting. It's a Creda Electricaire Night Storage heater which whilst it's still going I have been trying for ages to find a replacement heat source and have been wondering about air to air source heat pumps. 

 

I came across this company who seem to be offering it https://www.mittensheatpumps.co.uk/heat-pumps/ducted-vs-ductless/

 

It has to be doable; such a retrofit is common place in the US where blown air systems are much more common, there's a whole video about it on youtube. Also a colleague of mine is having a new build done in France and he's having a Mitsubishi air to air heat pump distributed through ducting to wall outlets. Each outlet is motorized and has it's own thermostat. 

 

The gear my mate in France is using is here.

 

https://www.orionairsales.co.uk/mitsubishi-electric-air-conditioning-pead-m100ja-ducted-concealed-inverter-heat-pump-10kw36000btu-r32-a-240v41550hz-9921-p.asp 

 

However, rather than the internal bit being a slimline ceiling void unit (as shown in that link) he's having a rectangular box type unit that sits in an internal cupboard and, I assume, receives the warm air from the external heat pump before sending it into the ducting. 

 

It's frustrating as that unit he's having would sit over my existing plenum chamber (the entry point the duct system) exactly where my current night storage based unit is; it seems so simple. 

 

I've enquired with the company that installed my solar panels and who do offer all renewables but they don't know anything about it. My feeling is that not many companies are going to or are going to simply say that as they don't want to get involved in something different. 

 

Unfortunately, mittenheatpumps have told I'm too far away for them to consider. Though I have asked them if they can provide me any more info about how and what they've installed so far.  

 

Hopefully some of this info is useful and maybe soon I'll find a solution

 

Robin

 

 

[Gooman]

@robin colesThat's interesting. It's at odds with what a local commercial and residential HVAC and ASHP specialist has told me. He said that sending the lower temperature of ASHP air (around 40 deg rather than the 80 degC output from our current boiler) down our current ducts just wouldn't work. He added that air-to-air ASHP systems are usually specified with much larger ductwork. As the ductwork from the boiler and all our ground floor ducts are buried in concrete, it's not easy to replace. I guess that's also why Johnson & Starley themselves have their Aquair heat exchanger, as the fan is specified appropriately to shift enough air.

 

There are some important caveats on the Mittens Heat Pumps site you linked to [with my notes in square brackets]:

• Obsolete ductwork techniques can influence efficiency levels [I'm guessing this means mainly small ducts and right-angle corners - both of which we have]

• They should be used in combination with good insulation and sealing [the lack of under-floor insulation is my biggest concern here]

• They are also perfect for smaller apartments and homes, or buildings where cooling/heating is needed only in certain spaces [although we're a 3-bed, we have a large ground floor area and so large rooms to heat]

I guess really it would be dependent on getting a survey to look at the specifics of each individual property. It's too early for us at the moment. We've got too many other things in our renovation project that need to take priority at the moment.

 

I did a new Google search based on some of the terms on the Mittens site and found these guys who claim to have nationwide coverage: https://www.heritageheatingandcooling.co.uk/blog/the-benefits-of-ducted-air-systems/ - you might want to try them?

 

[robin coles]

@Gooman thanks for your reply. In our case the ducting consists of a main rectangular trunk which comes directly away from beneath the plenum chamber, without measuring I'd guess at 50x30cm. There are then circular offshoots to each room, probably 20cm in diameter. The circular bits do leave the main trunk at right angles but that's it, it's then rounded 45's as required to get to the various room vents.

 

I see your concerns w.r.t your existing system. 

 

I would assume our all electric night storage system runs at lower temps than yours. I can certainly put my hand down that main trunk at about 3m from the source with no ill effect!

 

Thanks again for the reply ill get straight onto that company you found

[robin coles]

@Gooman Heritage heating and cooling got back to me - they don't do domestic. I've been going down this road for years. Seems like it's going to take a few more.

[Gooman]

21 minutes ago, robin coles said:

I've been going down this road for years. Seems like it's going to take a few more.

 

Yep. There really doesn't seem to be anyone seriously addressing the case of using existing forced-air ducting for air-to-air ASHP systems. I would guess that part of the issue is that air-to-air doesn't qualify for the RHI grant, whereas air-to-water does. So all the installers are focusing on air-to-water.

 

As other posters have noted, air-to-air ASHP is common in other countries. I found this comprehensive Canadian guide and workbook to sizing air-to-air ASHP systems - which includes the case of using existing ducting: https://www.nrcan.gc.ca/sites/nrcan/files/canmetenergy/pdf/ASHP Sizing and Selection Guide (EN).pdf

 

Unhelpfully, the duct sizing guidance suggests "measuring the cross-sectional area of the supply trunk(s) near the equipment, before any branch take-offs". In my case, that's underneath the (huge) Johnson & Starley boiler. So that's not anything I'm about to do myself. I might ask our heating engineer about the sizing when he comes to do the service.

[ProDave]

Reading this I see some scope for some DIY if you fancy?

 

The Electricaire unit is as you know a dirty great big storage heater and blown air ducting system.  The fan and ducting all works.

 

If you were to strip out the big storage heater stack of bricks from the top (warning beware may contain asbestos?) you would be left with an empry box with a fan that circulates air around the house.

 

Now all you need is a duct heat exchanger unit that will fit within the space where the storage bricks came out.  Feed that from an air to water heat pump and you have got what you want.

 

Do some measuring and find the size of the brick stack and see if you can get a duct heater(s) that will fit the space and deliver enough heat into the airstream.

[SteamyTea]

7 minutes ago, ProDave said:

Reading this I see some scope for some DIY if you fancy

I would be tempted to go that route.

8 minutes ago, ProDave said:

see if you can get a duct heater(s) that will fit the space and deliver enough heat into the airstream

Yes, I can't see why you cannot have ore than 1 'heater'.

Even a series of small, normal radiators, stacked sided by side, would work.

 

What is the heat load of the house?

[saveasteading]

All I can add, in case it helps,  is that we put air-source to air-ducted space heating into our big office. A USA system.

The internal circulation was simply a plenum box with a 'hot' radiator in it over which the air was forced. The distribution pipes were about 100 dia then splitting into  50mm tendrils. I think the principle was that all the ducting was much the same length to keep all the flows the same. Worked fine, and when the outdoor temperature was silly low (v cold winters sometimes in SE England) and the four ashp units were struggling, an electric heater kicked in at the plenum.

Much easier in an open plan with a single recovery duct than with multiple rooms, I think.

[robin coles]

Thanks to all for the informative replies and sorry for my slow response (just picked up 4G heading into port from the N.Sea!)

 

@Gooman - that pdf looks really good. I will consult that if I take this idea any further.

 

@ProDave - yes, you are correct that model does indeed contain asbestos in the insulation. Can I ask why you say to consider an air to water heat pump and not air to air?

 

@SteamyTea I've done a rough heat load calculation is an online calculator (dimplex website). The default desired inside temperature was 18°C for all rooms bar the living room which was 21°C. I thought that was a bit low so upped it to 22°C all round. This comes out at 20.04kW or 68379 btu's. I would have to do a more accurate calculation and cross check across different websites. 

 

Thanks again to all for the replies

[ProDave]

3 hours ago, robin coles said:

oDave - yes, you are correct that model does indeed contain asbestos in the insulation. Can I ask why you say to consider an air to water heat pump and not air to air?

Because that would mean burying an A2A HP inside your Electricair box, and I doubt you will find one single unit with enough output, and the pipework from that to the outside unit is regrigerant gas piping.

[robin coles]

@ProDave - ok thanks. I did have a look at some air to air source heat pump specs and I see what you mean w.r.t the outputs. 

[Jeff V]

Tagging on here, because we are in a similar situation.

Our 50 year old warm air furnace finally died and the parts are no longer available. It could be welded, but it's time for a new system.

There are two stage models available from the US - the air is first heated with heat from a heat pump then topped up with gas heating. It's not ideal in environmental terms, but it's better than all gas. To me, the biggest issue is that these are still gas units, and finding engineers to work on them is like finding hens with teeth. 

So I would very much prefer to  divorce the heating from heat distribution, using something like a Johnson & Starley Aquair, but that doesn't allow for two stage heating without another separate unit. And then the controls are not OpenTherm based, so modulating the blower may be a problem. But this does allow sophisticated boilers to be used - so it may be possible to find a two stage boiler ... where water is first heated using the heat pump, then topped up with gas to reach 80C, which is the sort of temperature needed by the warm air.

I'd also quite like automated  registers/vents - like these https://smarterhomeguide.com/smart-vents/ - but that adds another layer of problem.

[severnside]

I can provide some update here. We discussed our situation with a commercial AC company. They have looked at ducting and it's insulation condition. They think a ducted AC unit can work in our situation. We are going to get it installed next month and see how well it performs. We are getting this unit to replace Johnson & Starley Aquair (with Grant Oil Boiler)

https://www.orionairsales.co.uk/mitsubishi-heavy-industries-air-conditioning-fdum140vh-ducted-ceiling-concealed-14kw48000btu-r32-a-15031-p.asp

 

AC Company will make some custom Plenum to connect to existing Warm Air ducting. This will also provide option to add some fresh air.

[Gooman]

17 minutes ago, Jeff V said:

Tagging on here, because we are in a similar situation.

Our 50 year old warm air furnace finally died and the parts are no longer available. It could be welded, but it's time for a new system.

There are two stage models available from the US - the air is first heated with heat from a heat pump then topped up with gas heating. It's not ideal in environmental terms, but it's better than all gas. To me, the biggest issue is that these are still gas units, and finding engineers to work on them is like finding hens with teeth. 

So I would very much prefer to  divorce the heating from heat distribution, using something like a Johnson & Starley Aquair, but that doesn't allow for two stage heating without another separate unit. And then the controls are not OpenTherm based, so modulating the blower may be a problem. But this does allow sophisticated boilers to be used - so it may be possible to find a two stage boiler ... where water is first heated using the heat pump, then topped up with gas to reach 80C, which is the sort of temperature needed by the warm air.

I'd also quite like automated  registers/vents - like these https://smarterhomeguide.com/smart-vents/ - but that adds another layer of problem.

 

It's true that current heat pumps don't produce a high enough temperature airflow. But that is likely to change - there are some higher temperature models that are available now and more due to hit UK shores sometime soon (it's just a shame your unit has decided to go t*ts up at this point!):

• https://www.homebuilding.co.uk/advice/high-temperature-heat-pumps
• https://www.theheatinghub.co.uk/articles/high-temperature-heat-pumps

It should be possible to link these with a Johnson & Starley Aquair - but you might want to check with J&S yourself. I have had informal conversations with them that suggest this should be possible, but we didn't get into specifics.