Renovating and extending 1970s house with warm air heating

[SteamyTea]

28 minutes ago, Gooman said:

 

According to the boiler manual, at peak the fan should be pushing out 0.27 m³ a second

I was working on 0.0875 m³.s^-1.

So should be no problem there.

27 minutes ago, Gooman said:

External air is only used for combustion, not for circulation

Makes life even easier.

 

Stick a 2 kW fan heater in and see what it does.

 

(it is always possible that I have made an error somewhere, so worth double checking)

[Gooman]

FYI, a classic temperature curve with warm-air heating ... you can clearly see how the system throttles back the heat demand as it warms up and even spot the cycling of the temperature to within 0.1 of a degree once it reaches temperature.

[SteamyTea]

52 minutes ago, Gooman said:

you can clearly see how the system throttles back the heat demand as it warms up

Or just follows the heating curve formula.

T = 1-(e^-kt)

 

 

[Gooman]

Yes indeed, but that's not the way that a traditional gas boiler+radiators wet system behaves.

 

This is what you get with a traditional on/off thermostat:

 

 

Opentherm improves things somewhat, removing the overshoot:

 

(both graphs from https://theevohomeshop.co.uk/content/21-What-is-OpenTherm-and-why-use-Honeywell-evohome-with-it)

 

But you'll notice in both cases that the temperature increase is linear. By contrast the rising curve on the warm air system changes gradient as it gets close to the target temperature.

 

[Gooman]

One other topic I'd be interested in any comments on ... in our previous home with a traditional wet system, our combi boiler was rated at 35kW output.

 

With our current - 50% larger - home and a warm air system, the boiler is rated at 14.5kW. OK, it doesn't heat water, but that's using a 3kW immersion heater.

 

Why is the energy output requirement so much lower? Is it because it's heating the air directly rather than heating water to heat the air? That's still a huge difference.

 

?

[SteamyTea]

9 minutes ago, Gooman said:

Why is the energy output requirement so much lower

You have answered this already.

10 minutes ago, Gooman said:

OK, it doesn't heat water

Air takes 1kJ.kg.^-1.K^-1 

Water takes 4.18kJ.kg.^-1.K^-1 

 

If you work out the mass of air that is heated every day, say 2 tonnes, multiply it by the temperature difference, say 15K, you get 30,000 kJ.

Do the same for water say 150 kg and 40 K, you get 25,000 kJ

A similar amount (an purely speculative).

The difference is that you want hot water quickly, so need a lot of power (kW), the house is heated constantly, so a lot less power is needed to deliver a similar amount of energy.

[Gooman]

The boiler in the old house was a combi, only heating what was used, so wouldn't use as much energy as a whole tank.

 

The immersion heater in the new house by contrast is heating the whole 140l hot water tank.

 

[SteamyTea]

Just now, Gooman said:

The immersion heater in the new house by contrast is heating the whole 140l hot water tank.

Yes, but not from the mains temperature each time, and any heat that escapes into the house, reduces the need for space heating.

 

I will say though, that I am a fan of forced air heating, to me it is the sensible way to heat a building, we live in the air, not the walls, floors, furniture etc.

We also like to breath in warm air, this reduces the the amount of body heating as we are filing about a quarter of our volume with something warm, several times a minute.

I get hiccups if I go out into the cold.

[Gooman]

Given that this house is 50% larger, and looking at our bills (before the recent price increases) I'm certain that the warm air system is no more expensive to run than a wet system. Might even be slightly cheaper.

 

But you're right, it's a lot more comfortable.

 

[SteamyTea]

8 minutes ago, Gooman said:

Might even be slightly cheaper.

Might be because it distributes the air more evenly.  Radiators, which are really convection heaters, allow much of the air to raise to ceiling level, which will be hotter than you want, while the floor will be colder.

This causes the mean room temperature to be higher than it needs to be.

Edited November 3, 2021 by SteamyTea

[Gooman]

I can believe that. All of our heating vents are at floor level, so the warmth starts at floor level and rises (rather than hitting the ceiling and then falling as it cools).

[Rjwlb]

On 30/07/2019 at 09:17, Gooman said:

Isn't Aerogel as expensive as VIPs?

 

There's definitely no asbestos in the vent system itself - the survey was outstandingly comprehensive and thorough. Only places it's present in the heating system are on the inside of the door of the boiler cupboard and in the cement flue in the loft. The cupboard door will be disposed of in one piece, and the flue in the loft will stay where it is.

Who did your asbestos survey? We’ve had one done in a property we’re hoping to get abs the asbestos survey said it’s in the door to the boiler cupboard and also seen visually in the lining of the ductwork.

 

Edit: oops didn’t see the reply. Sorry. See my next post in new to this site and clearly not grasping it. 

Edited November 25, 2021 by Rjwlb
Hadn’t seen their previous reply

[Rjwlb]

On 26/10/2021 at 09:43, Gooman said:

We had a specialist asbestos survey from these guys: https://safety-surveys.co.uk/. This involved them taking invasive samples and having lab tests done.

 

We decided to keep the warm air system in the end - and we're glad we did! It works really well! It's a Johnson & Starley J50. No rads and *very* fast to warm up. None of the fears that others raised were realised - no dust circulation or dry skin or dry eyes. In terms of costs vs a traditional wet system it's hard to judge ... you could only really judge after having ripped it out and replaced it! But based on the increase in room size over our previous property, we think the bills are in-line with a wet system.

 

Long term, we think our best replacement option is some form of electric heating into a heat exchanger that uses the same ductwork. Johnson & Starley make such a heat exchanger (called the Aquair) which works with electric boilers or air-to-water ASHPs. But the ASHP output temperature is a lot lower (35-40deg) rather than the 80deg of a gas boiler or regular electric boiler and so may not be viable unless the technology changes. We also briefly explored connecting an air-to-air ASHP indoor unit directly to the ducting, but it seems the existing ducting is too small in cross section (esp. that on the ground floor in the concrete). Upshot is that until gas becomes much more expensive than electricity, we're probably staying with the gas boiler.

 

We haven't insulated under the ground floors. The main insulation focus so far has been from the top down. Loft insulation was fine. As it's a chalet-style roof, we have eaves cupboards and eaves voids and these were entirely uninsulated! So there's been a ton of Celotex installed there. As the heating system blows warm air in directly, insulating the floor wouldn't provide a fast enough RoI to be worth the hassle and expense.

 

Our biggest sources of draughts have been dealt with (new front door, new kitchen extension and resealing around all windows and patio doors). One important one is the combustion air supply for the boiler. Some newer warm air boilers draw their combustion air down the outside of the flue from the roof terminal, but ours was a little older. The combustion air was previously supplied by a vent in the outside kitchen wall, as the boiler cupboard is in the kitchen. This clearly worked well as there was a fierce draught into the kitchen! After consulting the boiler manufacturer and our heating engineer, we now instead have a 5" circular plastic duct from the outside running between the ceiling and the floor above directly into the boiler cupboard. Depending on your boiler engineer's interpretation of the regs, this may also require ducting into the return air plenum in the boiler cupboard.

 

One more point on the boiler - the timer control is very old fashioned, just being a 24 hour mechanical rotary timer. I've augmented this with a Wifi remote control that's integrated into our home automation system, so I can program different times on different days, as well as having it automatically turn off when we leave and turn on when we come home inside programmed times.

 

We're planning to have the cavity walls insulated (yep, I know, we should have prioritised that). One other thing to watch on 70's builds is that the ground floor sockets are often dropped down from the first floor ring main, with the drops in the cavities (although not current practice this is apparently fine, but when you have cavity wall insulation done you need to mention it to your installer as they have to use blown fibres rather than polystyrene beads). This means that your sockets are backing into the cavity air, and are a source of draughts! We've plugged the holes in the back boxes with sealant to solve this.

 

I would advise you to definitely live with the warm air system a little before you decide what to do with it. We're vary glad we did! If you have any other questions on it, I'd be more than happy to help.

Just seen This sorry. Thank you for your reply. Our survey identified asbestos in the cupboard door and seen in the lining of the ductwork so we want the system out as I get that from that inspection it’s in all the ductwork. 
 

thabks for advice on cavity insulation as we will need to do that too. If we ever get the house. 
 

the sellers don’t want to move on the price even though we’ve been quoted £6k to remove asbestos from the door and all the vents. Then we’d have to install a new heating system. So hard to know if they’re being over cautious and saying there is asbestos there rot if we should trust that they’re the specialist and will know if there’s asbestos in the lining of the ducts. Just don’t want me or my family  to be getting cancer in 20 years time because we left it. 
 

thw roof also need replacing soon but again, sellers don’t want to negotiate. We’ve come to the conclusion that we may have to walk if they don’t compromise at all. 

[Gooman]

The door to our boiler cupboard had a brown asbestos panel on the back. Broadly speaking asbestos comes in three categories - white/grey (which is rarely a problem if you don't cut, drill or break it), brown (same as white/grey but a little more hazardous) and blue (very hazardous). We had the whole door removed and safely disposed of.

 

We also had soffits in white asbestos, white asbestos in Marley floor tiles (who'd have thought?) and white asbestos in the cement around the boiler flue in the loft. All left in place - again, if you don't mess with it, it's not a problem.

 

Was yours just a general survey or a specialist asbestos survey? I'd thoroughly recommend a specialist survey with a lab test on samples - that's what we had and it gave us great peace of mind. They'll put any old crap in a general survey just to cover themselves from you suing them later - quite often the stuff they come up with is total bs.

 

If you didn't have a specialist asbestos survey with lab tests there's no guarantee that the ducts do actually contain asbestos. Can't think why anyone would put it on the inside of the ducts - it would impede airflow as it's not smooth. If it was used anywhere it would be on the outside - but ours had neither.

 

If it is present, you don't need to actually remove the ductwork. As above, asbestos is generally safe if you don't disturb it. Just decommission the warm air heating and leave the ducts.

 

The best way to negotiate is to show them examples of what else you could get in the same area for the same price. If they see that and have had trouble selling they might realise that they're overpriced.

[Archer]

Hello all. Been following this thread with interest. We are in the process of buying a 1970's house with a warm air ducted heating system in place. We don't have much info yet because this is pre-survey etc but there is loads of useful information in this thread. Hadn't even considered the possibility of asbestos paper in the ducting. 

 

The house appears to be timber framed and we want to (over time) renew the first floor shingle cladding and add EWI. The ground floor looks trickier because it has a brick skin but we will probably add a thinner layer of IWI. If the suspended ground is uninsulated we are considering using a spray foam system like Q-Bot which would also encapsulate any heating ducts. 

 

It would seem that upgrading a warm air system, theoretically at least, gives us the option of introducing whole house cooling via an Aquair HIU with an ASHP as the primary heat input. It's also quite attractive that these seem to allow for summer night time ventilation without heating or cooling as a way of purging built up heat. 

 

@Gooman (or anyone else) - it seems like it should be possible to run the system with lower warm air temperatures if the house has higher levels of insulation. 80 degrees rules out heat pumps & even high temp models run inefficiently with a COP of 2 or less. Hybrid systems are not permissable under the govt Boiler Upgrade Scheme which rules them out unfortunately.  It looks like there are models now that do reasonably well in mild weather generating up to 55 degrees (ie. The Hitachi Yutaki S80 HT, COP 3.63 @ 55 degrees). 

 

Cutting to the chase, my question is whether there is any accurate way or working out how the insulation level could affect the required warm air temperature to avoid a situation where you specify an expensive system and then have to run the heat pump at an expensive and inefficient setting rather than sticking with gas which could be half the price?  Obviously the alternative for us would be to do the insulation work and address the heat pump down the line when electricity pricing is more competitive with gas. It just seems crazy from a policy and climate change perspective though...

[SteamyTea]

@Archer

First thing to do is create a thermal model of the place.

@DamonHD and myself both have older timber frame places. We have taken slightly different routes in upgrading the thermal properties of them. Damon has gone all high tech and I have just stopped the air leaking out.

Both have made a huge difference.

 

[Archer]

13 hours ago, SteamyTea said:

@Archer

First thing to do is create a thermal model of the place.

@DamonHD and myself both have older timber frame places. We have taken slightly different routes in upgrading the thermal properties of them. Damon has gone all high tech and I have just stopped the air leaking out.

Both have made a huge difference.

 

Thanks Steamy Tea. By a thermal model, do you mean with certain modeling software, or in more general terms?

 

I would like to do both of those approaches (ie. some upgrade of the fabric + some "tech" to update the heating and energy systems). Depending how far the money stretches, my instinct is fabric first but the govt £5k grant seems a good thing to grab while it is still available if possible. 

 

Could it work to use a thermal store or heat battery charged on off peak electricity from the ASHP? In which case a COP of 2.5 - 3 might be acceptable and basically equivalent to running off gas. Or an actual battery I suppose and run everything in the house off the E7 rates but the battery cost for this might be prohibitive. 

[SteamyTea]

3 minutes ago, Archer said:

By a thermal model, do you mean with certain modeling software, or in more general terms

General terms just to calculate he inputs and outputs.