Air-to-air ASHP replacing warm air heating

[Archer]

@severnside thanks a lot for your detailed reply above, that's mega helpful. I'm waiting on the technical details of the exact unit(s) proposed but I'm pretty sure it is the Mitsubishi Heavy Industries FDUM model so likely to be similar to your system. Hope you don't mind me asking a few more questions... This will be a huge financial outlay for us and I'm finding the whole decision quite stressful because of the scarcity of information available online. 

 

On 01/08/2023 at 12:37, severnside said:

These units do modulate a lot so overspec is not bad, e.g. our 14kW Mitsubishi can modulate 5.0 to 14.5kw

 

Do you mean that it can modulate by a ratio of 5:1 ie. a 14kw could modulate down to 2.8kw? Reading other posts about ASHP's it seems that the modulation ratios are normally quite narrow (ie. 2:1). A 5:1 ratio would be brilliant. 

On 01/08/2023 at 12:37, severnside said:

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

 

This is brilliant advice, thanks I'll try this tack with the engineer and see what he says. One of the reasons that I'm worried about oversizing the system is because of the noise of the outside units it would be 2x double fan units which is pretty large and ugly but also may be loud with our neighbors around 5m away. I'm now thinking about specifying an acoustic enclosure to try and address any issues. Have you had any problems with noise at all? 

 

Also, did you upgrade the filters, and are you ok to adjust the registers / dampers in each room to tweak the airflow when the system is up and running? 

 

Sorry, I'll stop spamming all the questions after this, promise!!! 

[severnside]

12 hours ago, Archer said:

Do you mean that it can modulate by a ratio of 5:1 ie. a 14kw could modulate down to 2.8kw? Reading other posts about ASHP's it seems that the modulation ratios are normally quite narrow (ie. 2:1). A 5:1 ratio would be brilliant.

That should be 5.0kw to 14.5kw so approx 3:1 modulation

 

12 hours ago, Archer said:

This is brilliant advice, thanks I'll try this tack with the engineer and see what he says. One of the reasons that I'm worried about oversizing the system is because of the noise of the outside units it would be 2x double fan units which is pretty large and ugly but also may be loud with our neighbors around 5m away. I'm now thinking about specifying an acoustic enclosure to try and address any issues. Have you had any problems with noise at all? 

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.

 

12 hours ago, Archer said:

Also, did you upgrade the filters, and are you ok to adjust the registers / dampers in each room to tweak the airflow when the system is up and running? 

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.

 

Edited August 3, 2023 by severnside

[DanDee]

13 hours ago, Archer said:

@severnside thanks a lot for your detailed reply above, that's mega helpful. I'm waiting on the technical details of the exact unit(s) proposed but I'm pretty sure it is the Mitsubishi Heavy Industries FDUM model so likely to be similar to your system. Hope you don't mind me asking a few more questions... This will be a huge financial outlay for us and I'm finding the whole decision quite stressful because of the scarcity of information available online. 

 

 

Do you mean that it can modulate by a ratio of 5:1 ie. a 14kw could modulate down to 2.8kw? Reading other posts about ASHP's it seems that the modulation ratios are normally quite narrow (ie. 2:1). A 5:1 ratio would be brilliant. 

 

This is brilliant advice, thanks I'll try this tack with the engineer and see what he says. One of the reasons that I'm worried about oversizing the system is because of the noise of the outside units it would be 2x double fan units which is pretty large and ugly but also may be loud with our neighbors around 5m away. I'm now thinking about specifying an acoustic enclosure to try and address any issues. Have you had any problems with noise at all? 

 

Also, did you upgrade the filters, and are you ok to adjust the registers / dampers in each room to tweak the airflow when the system is up and running? 

 

Sorry, I'll stop spamming all the questions after this, promise!!! 

Check the data book

https://www.hrponline.co.uk/media/pdf/a5/7f/f2/20_PAC_DB_350_Data_book.pdf

 

Keep in mind the 2:1 modulation you read about is from the air/water hp topics, the A/A has to deal only with 20C air in which the refrigerant has to condense, whilst the A/W refrigerant has to condense in range of temperatures from 20 to 50 ish. Thus make it impossible for the A/W to use the whole range modulation of the compressor/refrigerant pressure.

 

The 10-12-14 kW A/A discussed here have the same compressor, basically being the same double fan unit. The data sheet says it's minimum can be 3kW.

 

 

Before anything else you should focus only on a proper professional heat loss calculation, only then look for quotes. Without that it's like trying to put a V8 in a Mercedes A class. So what do you have A, C, E, S class?

Edited August 3, 2023 by DanDee

[Archer]

On 03/08/2023 at 10:32, DanDee said:

Keep in mind the 2:1 modulation you read about is from the air/water hp topics, the A/A has to deal only with 20C air in which the refrigerant has to condense, whilst the A/W refrigerant has to condense in range of temperatures from 20 to 50 ish. Thus make it impossible for the A/W to use the whole range modulation of the compressor/refrigerant pressure.

 

The 10-12-14 kW A/A discussed here have the same compressor, basically being the same double fan unit. The data sheet says it's minimum can be 3kW.

 

I didn't know that there was such a difference between the A2A and A2W units... so am I reading it correctly that the larger units (ie. the 14kw) can effectively modulate to a greater degree than the smaller ones (ie. because the minimum output is the same on them both) - That would be pretty neat giving a ratio of 6:1 in cooling mode for the 14kw? It sort of supports what my engineer has been saying - that there are less downsides to oversizing than undersizing the ducted A2A unit in this case. Appreciate that a full set of room heat loss calcs is the proper way to do it, but AC engineers don't seem to use them / seem to go off a rule of thumb approach

 

On 03/08/2023 at 10:32, DanDee said:

Before anything else you should focus only on a proper professional heat loss calculation, only then look for quotes. Without that it's like trying to put a V8 in a Mercedes A class. So what do you have A, C, E, S class?

 

Getting quotes from ANYONE has been so painful. I don't know if this is others experiences, but we've approached so many engineers and just found it really hard. The 2 quotes we now have actually both seem very good, both guys seem to really know their stuff (but not so much the MCS side - it's more commercial air con). I'm sort of torn because by having two different ducted zones - the upstairs and downstairs (now after talking to him again this will be a 12kw downstairs and 8kw upstairs) - there should be some ability to further control the overall energy use by just turning on one zone - so cooling the upstairs only or heating the downstairs only for example. 

 

I did have a stab at trying to complete @Jeremy Harris's Heat Loss Spreadsheet - I've inputted for just the ground floor - I've made an assumption at 5 ACH (no idea if this is right, but it's not going to be a very airtight building); using pessimistic assumptions - ie. running at -5 degrees with a 20 degree indoor temp... that gives a Total Energy Required per day of 259kw/h. I don't really understand what I'm doing, but have assumed that a 12kw pump x 24 hrs gives a total output of 288kw/h - therefore is sized about correctly for the floor, given that it can modulate down as well in milder weather? If anyone wants to jump in and check that spreadsheet or my basic logic above that would be appreciated. 

 

@DanDee - the other interesting thing from the HRP manual you posted above  - which I hadn't fully grasped - is that the output for cooling and heating is rated differently. Do you know which of the output's is used to "label" the  unit model? ie. would they call it a 12kw unit because it delivers max 12kw of cooling (but it would deliver max 14kw of heating)?

Heat loss calculator - GroundFloor.xlsx

Edited August 4, 2023 by Archer

[DanDee]

6 hours ago, Archer said:

 

I didn't know that there was such a difference between the A2A and A2W units... so am I reading it correctly that the larger units (ie. the 14kw) can effectively modulate to a greater degree than the smaller ones (ie. because the minimum output is the same on them both) - That would be pretty neat giving a ratio of 6:1 in cooling mode for the 14kw? It sort of supports what my engineer has been saying - that there are less downsides to oversizing than undersizing the ducted A2A unit in this case. Appreciate that a full set of room heat loss calcs is the proper way to do it, but AC engineers don't seem to use them / seem to go off a rule of thumb approach

 

Keep in mind, the A/W albeit limited in modulation, it runs in a range where the COP is at its best. It might even be designed as such.

The A/A is free to use the whole range, but the portion toward the maximum is at a lower COP that rated(10kW).

Always look at the output data of the unit for the -5..-7..-10C it is lower than rated a 7C.

 

 

This is an example from the Vaillant A/W of two sizes using the same unit, with the extra compressor speed modulation at the top, putting out the extra power.

 

 

6 hours ago, Archer said:

 

@DanDee - the other interesting thing from the HRP manual you posted above  - which I hadn't fully grasped - is that the output for cooling and heating is rated differently. Do you know which of the output's is used to "label" the  unit model? ie. would they call it a 12kw unit because it delivers max 12kw of cooling (but it would deliver max 14kw of heating)?

 

Yes with A/A is always like that, the rated  label matches the cooling rated capacity

 

 

[Archer]

Have I got the basics of the heat loss spreadsheet right - ie. pick a given Delta T (outside & inside temperatures) and then compare the "Total required energy per day" to the output of the pump x 24(ie. 24 hours)?

 

I've put in all the correct u-values, window sizes etc. 

[DanDee]

1 hour ago, Archer said:

Have I got the basics of the heat loss spreadsheet right - ie. pick a given Delta T (outside & inside temperatures) and then compare the "Total required energy per day" to the output of the pump x 24(ie. 24 hours)?

 

I've put in all the correct u-values, window sizes etc. 

 

9 hours ago, Archer said:

I've made an assumption at 5 ACH (no idea if this is right, but it's not going to be a very airtight building)

 

I don't have experience with this in order to give you a proper answer.

 

From what I've seen around, the wrong number(ACH) can drive up(double) the required capacity. The general number(if rule of thumb) of ACH is quite high vs the reality. Just to give you my understanding of 5ACH=1air change per 12 minutes, I doubt your house goes from 20 to whatever temp is outside in 12 minutes, with all windows closed and no active heating.

 

https://trystanlea.org.uk/roombyroomheatloss2

https://openenergymonitor.org/heatlossjs/

I've changed the ACH to 5 in this table and it doubled the requirement

 

An example from twitter of a discussion on the topic of the right ACH. From my understanding 1.5-2ACH and 3(bathroom only) should be your max rule of thumb without actual testing.

 

 

 

 

 

 

[Gooman]

Just saw this really interesting development in my news feed - it's an ASHP (air-to-air I think) with an indoor unit in a tower format, specifically designed to replace a forced-air furnace: https://www.electricair.io/

Notably, it also cools and dehumidifies and includes a HEPA filter for air purification.

 

Not available until next winter - and likely in the US only, at least at first. But promising nevertheless!

[Gooman]

PS - If I was going to try and persuade them that the UK is a worthwhile market, does anyone have any stats on how many domestic warm air heating systems are in use in the UK?

[Gooman]

If anyone is interested, please use the Contact section on their website to let them know ... it might galvanize a response!

[Archer]

On 08/12/2023 at 10:10, Gooman said:

Just saw this really interesting development in my news feed - it's an ASHP (air-to-air I think) with an indoor unit in a tower format, specifically designed to replace a forced-air furnace: https://www.electricair.io/

Notably, it also cools and dehumidifies and includes a HEPA filter for air purification.

 

Not available until next winter - and likely in the US only, at least at first. But promising nevertheless!

This unit does look perfect as a direct replacement. You still have an outdoor and indoor unit by the looks of things but it's very design led. The wall units are some of the nicest I've seen. 

 

Having gone through the mill on this with the current gas warm air providers (including J&S etc), I'd be a little wary of an unproven American import. The warranties all seem quite weak and you wouldn't get great aftercare support etc. 

 

The commercial A2A products are a bit niche, but Daiken, Mitsubishi etc do sell products with decent warranties and engineers that know what they are doing with them. Government should look again at A2A systems, they are not getting the support that A2W does and it makes no sense. HVAC has additional benefits to air quality etc that could really improve uptake imo

[severnside]

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.

[Archer]

That's helpful @severnside and also a bit worrying from my perspective. We are still waiting on planning (looooonnnnngggg story) but will be retro-fitting warm air in a similar way to you. 

 

How has the cost compared with your old oil fired system out of interest? 

[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.

 

 

[Matthewr]

Hi

I'm joining this forum rather late in the day but have read through with interest as I have the same challenge as others who've contributed, i.e. a gas powered, ducted warm air heating system that I'd like to convert to using an ASHP. 

 

The current system is based on a J&S WarmCair warm air unit.  This has a conventional condensing gas boiler that currently heats water to 70degC.  The heated water passes through a water to air heat exchanger over which a fan blows air, providing ducted warm air for space heating.  The heated water also passes through a heating coil in a hot water cylinder to provide domestic hot water.

 

I appreciate that the system installed by @severnside uses an air-to-air heat pump but, as I live not far from Bristol, I wondered, would you be able to share the name of the installer you used so that I could explore whether they could work to convert my system as well?

[gravelld]

Really interesting thread. We're looking at a renovation project at the moment with a late 70s forced air heating system. First priority would be upgrading the building fabric (and likely removing a lot of asbestos).

 

But after that, it would be interesting to use the existing ducting.

 

If we achieve good levels of airtightness, would it be possible to use an exhaust air heat pump to also combine ventilation?

[SteamyTea]

59 minutes ago, gravelld said:

If we achieve good levels of airtightness, would it be possible to use an exhaust air heat pump to also combine ventilation

Yes, I assume this would be to just heat the DHW, then you would use something else A2A maybe, for space heating.

 

[gravelld]

Interesting idea. I want to make it as simple as possible. Although exhaust air heat pumps were originally about DHW I think they now also cover space heating. But am I trying to pump to much energy through the ducts when the "air cannae take it".

 

I guess it's down to the heat loss eventually...

[JohnMo]

17 hours ago, gravelld said:

would it be possible to use an exhaust air heat pump to also combine ventilation

What happens when the heat pump is idle, assume the ventilation stops? Or would you have MVHR/MEV and the outlet air feeding the exhaust air hp?

 

 

[Gooman]

17 hours ago, gravelld said:

First priority would be upgrading the building fabric (and likely removing a lot of asbestos).

 

Just on that point ... we had assumed the same as well, but after a very thorough asbestos survey (which I would recommend you get) we found there was very little that needed to be done.

The only asbestos in the heating system was a brown asbestos panel on the back of the furnace door (which was dangerous, and was removed for specialist disposal), and asbestos cement around the flue. The recommendation on that (as with most white asbestos) was to leave it in situ, with a suitable warning tag in the area not to disturb it.

If your property is, like ours, a chalet-style roof (see pic) one of the most awkward parts of upgrading the building fabric was in properly insulating all the eaves spaces. Some of them were accessible as eaves cupboards, but some were sealed up. Existing insulation was 10mm fibreboard between rafters and on internal walls and a little (very little) loft insulation between floor joists.

[gravelld]

I have my fabric first retrofit checklist and rooms in the roof makes for a low score! 😉  Best way of fixing those types of roof structures is to take the covering off, imo. Has to be done to make EWI continuous with loft insulation anyway (in a cold roof).

 

Thanks!

[gravelld]

4 hours ago, JohnMo said:

What happens when the heat pump is idle, assume the ventilation stops? Or would you have MVHR/MEV and the outlet air feeding the exhaust air hp?

 

Well... I don't know. But I do know that Nibe et al sell their exhaust air HPs as a combined solution. It might be the ventilation levels are not enough to satisy a tight build.

 

Yeah, I assumed the ventilation must be continually provided whether or not the compressor is running. A bit like in a A2W HP the CH pump and the pump between the HP exchangers can keep running if the compressor is off... (AIUI)

[JohnMo]

19 minutes ago, gravelld said:

Nibe

I did look at those, when I was planning, then decided not to put heating and ventilation in the same package, so kept ventilation and heating as completely separate. I think they can end up as normal ventilation with heat pump off and additional boost ventilation when the heat pump needs more energy. So robbing house air (with attached energy) to satisfy heating demands - so robbing heat to provide heat

[gravelld]

Hmmm. Ok, that rings a bell. Looked at them many years ago.

[Gone West]

3 hours ago, gravelld said:

I do know that Nibe et al sell their exhaust air HPs as a combined solution.

We had a Genvex Combi 185 at our last house which incorporated an EASHP to heat the DHW and the ventilation air, if required. It only works in an extremely well insulated house. We heated our house to 23C with just three electric towel rails and the Genvex.

 

https://www.genvex.com/en/products/air-ventilation---water-heat-pump/combi-185-bp-and-combi-flex