Hi there,

About to start conversion of a Church into a 3/4 bed dwelling (~200m2) - it's grade II listed. I must decided on my heat source and method of delivery. The building may not be occupied all the time (we work away a lot), so I'd like a fairly responsive system (an hour or so to 'feel' the difference is OK). I want to use a heat pump to supply the heating (outside scope of this thread). We'll also have a 9kW double-sided wood stove for really cold periods.

We're doing our best with insulation - floors - 0.16W/m2K (may get to 0.12), roof - 0.2W/m2K (may end up slightly better), walls first 1.1m - 0.3W/m2K, but then 600mm solid stone up to first floor height. So we could exceed Regs in a few places, but those solid walls present a problem (decorative stone reveals with not much plaster depth ~10-20mm). Not bad for something built in 1896 though (I'm still thinking where I can make improvements - suggestions please?).

I'm deliberating my options (and have been for many months!). I've warmed (pardon the pun) to idea of warm air heating but as mentioned in the posts on this forum - there is little use of it in the UK (predominantly due to non-requirement for air-conditioning in the summer) - so makes making an informed decision tricky.

I must install a MHRV system as I cannot modify the existing components (e.g. stained lead windows with no openers) to satisfy Regs, so am considering coupling a heat pump coupled to this and utilising the ducting to supply my heating too. I see this as offering the same advant of UFH in terms of non-occupation of wall space (i.e. rads). I should be able to route the ducting within the new constructed elements.

Just wondering if anyone can offer some advice/observation/experience on this and what to look for/think about when specifying the system.

Any comments greatly appreciated. I do not want to chicken-out on a potentially better solution due simply to lack of confidence - but given the amount of money involved............

Cheers,

Phil

Cheers Tony. A few omissions - secondary glazing is also going in - oak frames with low E dg.

By 'feel' the difference, I mean a shift towards warm not stabilised warmth. The analogous situation/example in my house with my combi and wet rad. system would be that you 'feel' it starting to warm within about 10-15mins of the boiler starting up - admittedly it takes a good bit longer to get up to 'full' heat.

I'm still working on reducing U-values but with such an old building and the grading options are somewhat limited, 0.1 is just a dream for me!

Grain dryer is an interesting idea - but I'd specifically need/want a heat pump.

Phil

Thanks SteamyTea.

Will give it a go!

I had planned to do this to ensure my system will deliver what I need (my worst nightmare). I've done very rough stuff using online calculators - but realise the limitation of this (t'was more so to have an idea of the potential size of system required - ~17kW was the result - but I purposely made it the worst case scenario for our approach).

I have had a quote on a system. They've offered to perform some heat loss calcs - so it'll be good to see how the two figures compare!

Phil

I would be very dubious about trying to use warm air heating in this case. Your ground floor insulation levels are very poor, especially for the top half of the wall. I also suspect you will find it very hard to make the building airtight, resulting in significant loss there too. You absolutely need to do the calculations.

Air does not have a very high heat carrying capacity - much less than water - and to provide the level of heat you will need you're almost certainly going to need large amounts of air flow, resulting in very large ducts, big fans and almost certainly lots of air currents. This is unlikely to be easy or comfortable and could be noisy.

You also have huge thermal mass, so if you are going to allow the structure to cool down then you will need a lot of heat to start to warm it back up. Hot air is unlikely to do this quickly.

The air flow will be massively more than you need for normal ventilation, so I wouldn't link it to your MVHR system unless you want it to be very expensive. You would also need very large heat exchangers to get the heat into the air, which will introduce quite a bit of resistance to the air flow. You would need PassivHaus levels of insulation and airtightness to get away with this - and you look to be far from these levels.

UFH could be a good option, but the response time is not always fast, especially with your level of thermal mass. Your heat losses will probably make it prohibitively expensive to keep the whole house up to temperature all the time.

Your best bet is almost certainly a radiator based system, but this generally needs high water temperatures and is probably not compatible with a heat pump. Maybe oversized radiators running at a lower temperature, but this conflicts with your desire to not have too much wall space occupied. Perhaps skirting radiators?

I would be inclined to go for an even larger wood stove than you currently assume but this will depend on your heat loss calculations. You could consider a large boiler linked to your heating system as well - double sided boiler stoves do exist.

The last option is to hang some jumpers near the entrance to put on when you take your coats off!

Whatever you do you'll probably need a large heat source, so don't assume a small heat pump if you're wedded to this route.

OK, I bow to Paul's first hand experience. If it works for him under similar conditions and -23C then it'll work here.

Do note the size of the main air ducts though - these will have to be an integral part of the design, rather than an afterthought unless you want to go for the industrial look.

Posted By: SteamyTeahen it comes to cold weather and heat pumps Paul knows his onions, better at sums than me too

Thanks for the vote of confidence! Any system is only as good as the design and installation that's behind it - if the calculations are done properly, there's no reason why a forced-air heating system should not be quiet, effective and economical to run. One certainly doesn't need passivhaus-levels of insulation - though, of course, if you do, the heatload is reduced and hence the capital cost of the heating equipment can also be reduced (together with the running costs). Ironically, though, paying more for high-efficiency heating appliances in a low-load application can have the perverse effect of not being cost effective since the potential for savings (through the higher efficiency) is diminished because the absolute consumption figures per year are reduced. This is the same diminishing returns that extreme amounts of insulation also incur. It's always worth doing a proper cost-benefit analysis to see what the appropriate trade-off is. That's what engineering is all about :)

In my case, it wasn't cost effective to install a heatpump that would meet 100% of the worst-case winter heating load as the increased capital costs of the equipment and deeper borehole required would work out at more than the cost of running auxiliary straight resistance heat for the 2% of the time these would be required - the engineering compromise in my case was to size the heatpump at around 80% of the peak heatload - the breakeven point being around 25 years of operation (which is the typical lifespan of the heatpump itself). A good installer will certainly help quantify these trade-offs.

Paul in Montreal.

Phil

Where in the country are you? Have you been to the MET office and looked at your Temperature Regime?
Also look up how Degree Days are used, briefly they show the amount of time that the temperature goes below or above a set temperature. They are usually calculated using the Mode (most frequently occurring) as opposed to the Mean (numeric average) temperature as this gets rid of the outliers, or the extremes that serve little purpose when working out your heat load.
It is also worth remembering that during the day that there are other heat sources, mainly solar gain, that help to raise the temperature. Also things like fridges/freezers/cooking/hot water tanks/people also raise the temperature inside.

Insulating as much as you can and draught proofing will make a huge improvement. I had what seemed like a little draught from my back door, maybe a 2mm gap 150mm long and fixing that means I now do not need my storage heater on in the kitchen, the fridge does all my heating.

If you want to experiment to find out how a heat pump works, play around with an old fridge, try it at different settings and see how long the compressor is running, then stick it out in the sun (or in a hot room) and do the same thing. You should find that the compressor runs for longer as there is less of a temperature difference where the fridge gets rid of the heat. This means that the compressor runs for longer for a smaller temperature difference, or less heating for more energy use.

Good luck with the caravan, insulate it well and keep the draughts out.

Posted By: PhilbobagginsiiiHe said, we're looking at 4kW loss due to ventilation (therefore recovered by the HRV system - 95% eff.),

Your HRV will not recover this - this heat loss is due to uncontrolled ventilation, i.e. air leakage - this is above and beyond what your HRV will supply. Really, the only way to know these figures is to have a blower door test done. If your air leakage is greater than about 3ACH@50Pa, a HRV will not save you any energy at all.

Posted By: davidfreeboroughTo get the best efficiency from the GSHP you need to make sure it's flow/return temperatures are as low as possible. This in turn means the supply air heat exchanger/coil needs to be sized to deliver the heat load required with a relatively low temperature difference across it

Standard rule of thumb over here for forced air systems supplied by a heatpump is that you need a temperature difference of around 14-16C between the supply and return temperature. Any less than that and the supply air feels cool (and you'll notice drafts) - any higher than that and it will also feel drafty as there will be too much of a difference between the supply air and the room temperature. This is why many people complain that forced air systems are uncomfortable - if you heat the air with gas or oil, you get a much higher temperature rise and the difference with the room temperature is noticeable.

Posted By: davidfreeboroughAlternatively, have you looked at the ready integrated MVHR/ASHP systems from Genvex?

This will never work for the heatloads we're talking about here - the airflow is an order of magnitude too low.

Paul in Montreal.

If you have MHRV then you dont need or want trickle vents in the windows.

My MHRV works OK and does everything is quiet and is the only form of additional heating apart from light-bulbs that we have.

They can do and with low and very low heat demand MHVR can do it all -- mine does

I was warned that the ducts would be too small but my heat demand is extremely low

I can confirm that forced-air heating systems work well, in the right circumstances. My first house was fitted with a gas-fired one. I bought the house new about 1980. There was some noise but not enough to disturb me. The main benefit was that it made the house feel warm very quickly, precisely because it wasn't heating the structure first. I was living alone at the time so the house was empty most days.

Posted By: Paul in MontrealYour HRV will not recover this - this heat loss is due to uncontrolled ventilation, i.e. air leakage - this is above and beyond what your HRV will supply. Really, the only way to know these figures is to have a blower door test done. If your air leakage is greater than about 3ACH@50Pa, a HRV will not save you any energy at all.

I'd also second what Paul says about HRV and airtightness. At that level of airtightness MEV (mechanical extract ventilation) is more cost effective.

Posted By: dicksterWhat about cold feet?. Presuming that you have high ceilings, won't you experience cold air falling down the inner face of the external walls and flowing to the centre to rise up again? Underfloor heating?

I have high ceilings (10 feet) downstairs in my house and it doesn't seem to be a problem. The duct layout is such that the return air vents are situated away from where people would be sitting. Careful vent placement and airflow design is key to making a forced-air heating system comfortable. We get very good mixing of the air with very little stratification - this means that the floor temperature is close to ambient - even more so as the basement under the ground floor is also heated. I was surprised to find that, for example, when seated at the dining table, which is only a couple of feet from a supply vent, the candles on the table don't flicker when the heating system is running. Everyone who visits our house always comments on how comfortable it feels.

It can be quite a challenge to integrate underfloor heating and forced air as the control responses of the two systems have quite different time constants and so it can be an interesting problem to program these for comfort.

Paul in Montreal.

dream on!!!

What Mark said.

The key point is that a forced-air heating system recirculates air inside the house, so it can have much higher flow rates with no losses in a heat exchanger and no change in the humidity. It also has larger ducts than an MVHR system.