lookseehear Posted June 6 Posted June 6 You may have read my previous thread: I really need to make my mind up and I'm still struggling. As a refresher: We have an 18-25kW external condensing oil boiler and 1200L oil tank in the garden The house is 5 beds and detached and we're putting the living space on the first floor and bedrooms on the ground floor We're replumbing and rewiring the whole house Two extensions will be insulated to modern standards, as will the new roof The older parts of the house are harder to insulate substantially, but we will be putting in a thin layer of carefully installed and well detailed internal wall insulation and significantly improving the airtightness We're replacing all windows and doors with triple glazing/modern equivalents I'd estimate we would be averaging 50-70W/m2 peak heating requirement when all done but it's hard to tell because at this point we're chipping away at the project bit by bit Option 1: Stick with oil, new radiators everywhere (oversized) Pros Cheapest at the outset Easy to control room by room temp with TRVs Reduced oil usage compared to high flow temps and small rads Cons We commit to keeping burning oil No weather compensation The boiler is (I believe) oversized for what we need, so would need to make changes (buffer tank/TS?) to the installation or swap out for a smaller boiler We have to fit large radiators in each room Option 2: ASHP and radiators Pros Renewable factor and compatible with solar PV/battery Means we can remove the big, unsightly oil tank from the garden Can utilise the BUS to get the grant from the government Option of FCUs for cooling Cons Risk of high running costs due to high flow temperatures required running rads off an ASHP We have to fit large radiators in each room Less controllable room temperatures due to the time it takes to warm up a room using low flow temperatures, and counter-productive nature of using TRVs to throttle flow in an 'open loop' system Option 3: ASHP and UFH Pros Renewable factor and compatible with solar PV/battery Means we can remove the big, unsightly oil tank from the garden Can utilise the BUS to get the grant from the government No need to find space for radiators 'comfort factor' of having warmer floors than using radiators Likely lower running costs than Option 2 due to lower flow temps meaning higher SCOP Option of FCUs for cooling Cons Least controllable of all the options Requirement to find the headroom for some insulation (or not based on this thread) Most disruptive to install Likely most expensive up-front Most risks of getting a poor quality install (maybe?) A few other thoughts on UFH and the complexity of the specification Heat output is obviously restricted by some floor finishes, and we'd likely have carpet in bedrooms, but I think it can be mitigated if the pipes are within a chemical screed, heating is run continuously and small pipe spacing. I had thought an overlay system would work, but I realise that overlay systems just can't get the same heat output Some rooms downstairs will be occupied just at bedtimes and will be fine at 18-19 degrees, but our kids bedrooms will likely be occupied a lot more as they get into their teenage years, so those rooms will probably need to be 20-21 degrees. The above is just a brain dump really, but I'm really tearing my hair out trying to work out what the right option is, when we'll need to live with it for the next 20+ years. Any thoughts? How can I bring some clarity to the decision?
Roger440 Posted June 6 Posted June 6 Option 4 Keep oil boiler and fit UFH. Will be your cheapest option, and with UFH will be lovely. If you didnt already have a boiler, then maybe a different option. But you do. If you decide to throw it away, please let me know and ill come and take it away! (distance dependant!) 1
JohnMo Posted June 6 Posted June 6 Option 5 Keep oversized oil boiler, feed to a thermal store, then boiler being oversized doesn't make much difference. Will run happily not cycling. Then take heating system water and DHW from store. DHW via a plate exchanger. Then you can heat big rads at lower temp or UFH if you want. Or leave as is. 20 minutes ago, lookseehear said: Option 2 and : ASHP.... Cons Less controllable room temperatures due to the time it takes to warm up a room using low flow temperatures, and counter-productive nature of using TRVs to throttle flow in an 'open loop' system With ASHP the sensible option is to run low and slow 24/7. On off operations cause the heat pump to at higher loads playing catch up. 1
lookseehear Posted June 6 Author Posted June 6 Ok this is definitely helping. The reason I was moving away from the idea of a thermal store is because of some comments on here suggesting that you get roughly half the amount of hot water storage in one compared to an UVC of the same size. That point aside I like the idea of using cheap electricity or solar PV to heat thermal store to contribute to either space heating or DHW. We have a 200l UVC currently and don’t use all that much of it, but the kids are still young and we are going to end up with four bath/shower rooms. Do I therefore need to be looking at 400l thermal stores? separately, if I’m thinking of UFH and can’t put much insulation in the refurb areas, should I be thinking about the no insulation option or would putting 25-40mm PIR be better. Assume for now that comfort is more important than heating costs.
JohnMo Posted June 7 Posted June 7 Two schools of thought. One insulation and lots of it, 150mm PIR minimum. Or none. Lots of insulation is always best, results in least energy consumption. The or none version - you run the UFH at a low and slow temperature 24/7. The ground below the house buffers heat. To get it to work you need to screed over the pipes so everything transfers heat Well. If you switch the heating off the heat moves away and you start the buffering process again wasting loads of energy. Thin insulated floor, the heat still goes down (slightly slower) but cannot come back, so is not good force energy consumption. You mentioned FCU as an option for ASHP and UFH. UFH alone can do cooling with a cooling enabled heat pump. Or use FCU for heat and cool, without the bother of UFH.
lookseehear Posted June 7 Author Posted June 7 I understand the theory, but I assume that only works with ASHP rather than with oil boiler and thermal store because of how much more efficient UFH would run at constant low flow temps. If I’m using oil to run UFH via a thermal store it’s no more efficient than running radiators, so the ‘no insulation’ scenario means burning a lot more oil.
JohnMo Posted June 7 Posted June 7 Sorry just reread your first post 14 hours ago, lookseehear said: living space on the first floor and bedrooms on the ground floor I really would not have UFH on ground floor at all. Do all the major heating upstairs, bedroom needs to be cooler than a living space. Any heat that escapes downwards from upstairs UFH is going to heat downstairs anyway. No idea if you need to cool downstairs if you do, use fan coils to bedrooms. Use the UFH upstairs as heating and cooling - If going heat pump. You need to decide on heat pump or boiler then go from there. Maybe ask for a heat pump quote, it may make the decision for you.
lookseehear Posted June 7 Author Posted June 7 This was my line of thinking too, but on the ground floor will also be a home office and kids bedrooms which will likely be occupied a lot more as they get into their teens, and hence will need to be able to be heated to 20-21 degrees to be comfortable. Here’s my new plan: Keep the boiler, because I don’t feel that I can economically run radiators off an ASHP Put in a thermal store (size?) and have radiators downstairs and ufh upstairs use some of the savings from not getting ASHP to relocate the oil tank or put in a smaller one Put in as much insulation in the floor downstairs as I can fit consider a separate air con unit for the living space upstairs. Even poorly insulated as it is, downstairs rarely overheats even in heat waves, so I don’t see any requirement for cooling downstairs electric UFH mats in bathrooms on timers/switches for comfort Does this seem sensible? I need to understand the thermal store bit a bit more though.
JohnMo Posted June 7 Posted June 7 Can't help with thermal store sizing, but generally bigger rather than smaller. Two types for DHW, one is a coil and the other a plate heat exchanger and pump. Believe the plate heat exchanger yields the most DHW for a given size of cylinder. So plate exchanger will allow a smaller cylinder for same DHW delivered. Other thing to think about is boiler run time, you are best to get a reasonable run time and then a decent gap between runs. So things you need to know, to size cylinder. No of people in the house - DHW sizing. Heat loss calculation for whole house. This will give you a likely time between boiler runs in winter. A note: UFH will draw warm water from cylinder, so if cylinder hot there will be a mixer, it will add a little bit of hot water to return water, and send it around the circulation loop again. Radiators if oversized will do similar. But if small hot radiators these may deplete cylinder quickly to fire up boiler again. So the hotter the heating system the more likely you are to have a big cylinder, this could be wrong? 1
JamesPa Posted June 7 Posted June 7 (edited) 5 hours ago, lookseehear said: Keep the boiler, because I don’t feel that I can economically run radiators off an ASHP You should question that assumption. Obviously it depends on the achievable rad size. I sized my rads for ft45C at the design temp of -2 , am actually running at ft42C with an ashp which is costing roughly 20% less like for like than my gas boiler. None of the rads are intrusive in any way. The house, now heated throughout 24*7, has never been more comfortable. Even I have been surprised. My only regret is not adding a couple more fancoils (I fitted just 1 where it was absolutely impossible to fit a sensible size passive radiator). They are great for light cooling. Edited June 7 by JamesPa
scottishjohn Posted June 7 Posted June 7 my thoughts based on having UFH runing from lpg and then ASHP in same house the main difference was that the ASHP -cost alot less to run than the LPG your first and critical job is the insulation --what ever system you go for I can also confrim that from experince with my new house --nearly moved in --next week is permanent move in it is a1730 granite house with a TF built inside it 150mm pir insulation concrete sub floor then insulation and UFH pipes -then screed stuck down LVT flooring although i have been running both houses at same base internal temp the new refurbed/ old house feels much warmer cos there are NO DRAFTS so what i am advising with which ever way you go is concentrate and spend most attention on insulation and draft prevention my new/rebuilt old house is 400sqms of floor space and is same or cheaper to heat then the old one which is less than half that size I have had ashp running on this house since november last year while the builders were doing things and the temp was raised somewhat to dry out the floor for the stuck down flooring as for running ASHP and rads it should not be more expensive to run than gas and all you need is to make rads larger basically twice the size as water temp will be nearly half of boiler temp -- and run them at alow tempwith moore surface area if size of house precludes that then stay with boiler My feelings are that you cannot over stress tthe importance of the air loss and insulation
lookseehear Posted June 7 Author Posted June 7 @JohnMo thanks for all your input here, I really appreciate you taking the time. 13 minutes ago, JamesPa said: You should question that assumption. Obviously it depends on the achievable rad size. I sized my rads for ft45C at the design temp of -2 , am actually running at ft42C with an ashp which is costing roughly 20% less like for like than my gas boiler. The house, now heated throughout 24*7, has never been more comfortable. Even I have been surprised. My only regret is not adding a couple more fancoils (I fitted just 1 where it was absolutely impossible to fit a passive radiator). They are great for light cooling. How big are the rads? If we’re going to have them in bedrooms I’d probably go for columns to save wall space. Is it feasible to run rads and ufh both from an ASHP in an open loop configuration? I assume the ufh would need to be mixed down to a lower temp. I’m also assuming that bedroom temperatures can still be controlled with TRVs on the radiators? From recommendations in my previous thread I want to keep things simple controls wise. One issue I can see is that we will get quite a lot of solar gain upstairs from roof lights, sliding doors being one open plan space which will have exposure throughout the day, whereas we won’t get a lot downstairs due to smaller windows and smaller spaces which only get sunlight at certain times of day. On a cold winter day with lots of sun, heating load upstairs will be reduced, but downstairs won’t be impacted as much, so any reduction in flow temperature overall could result in cold bedrooms.
scottishjohn Posted June 7 Posted June 7 (edited) 6 minutes ago, lookseehear said: How big are the rads? If we’re going to have them in bedrooms I’d probably go for columns to save wall space. Is it feasible to run rads and ufh both from an ASHP in an open loop configuration? I assume the ufh would need to be mixed down to a lower temp. yes but balancing them -so water does not just take easiest route eg going round the rads --will be the main problem and it sounds like you do not want to have zone controls and individual stats . both my UFH systems have each room on its own control and is the right way to do it no good UFH system will just have thermostatic valves they will have individual room stat and programmers Edited June 7 by scottishjohn
JamesPa Posted June 7 Posted June 7 (edited) 1 hour ago, lookseehear said: How big are the rads? If we’re going to have them in bedrooms I’d probably go for columns to save wall space. Is it feasible to run rads and ufh both from an ASHP in an open loop configuration? I assume the ufh would need to be mixed down to a lower temp The rads all fit comfortably under the windows (with a gap either side for curtains) and are either type 21 or type 22. There are just two exceptions: I fitted a type 33 by mistake, at a point in an extended process when I thought the heat loss was much bigger than it actually is. 22 or 21 would have done. I have one fancoil where a sensible sized rad just wouldn't fit. As I say my only regret is not fitting a couple more, just because of the fact you can use them for cooling. You can indeed mix down for UFH but from an efficiency point of view it means you are running your ASHP at the higher temp demanded by the rads, so if you are going to do this rads may be no less efficient. Of course UFH has its own advantages. 1 hour ago, lookseehear said: I’m also assuming that bedroom temperatures can still be controlled with TRVs on the radiators? Yes you can. After some experimentation I now do that in one bedroom only (used as a guest room) but not the others. The others are simply 'balanced' down on the LSVs to a slightly lower temperature 24*7. 1 hour ago, lookseehear said: From recommendations in my previous thread I want to keep things simple controls wise. One issue I can see is that we will get quite a lot of solar gain upstairs from roof lights, sliding doors being one open plan space which will have exposure throughout the day, whereas we won’t get a lot downstairs due to smaller windows and smaller spaces which only get sunlight at certain times of day. On a cold winter day with lots of sun, heating load upstairs will be reduced, but downstairs won’t be impacted as much, so any reduction in flow temperature overall could result in cold bedrooms. The best controls, particularly for an ASHP, are simple controls, and the low and slow mode from ASHP (or UFH) anyway facilitates and works best with simple controls; low temperature heating has a significant element of self regulation (if the room warms up the output of the emitters decreases and vice versa). I also get significant solar gain. The room with the most solar gain has the fancoil, so it simply switches itself off. All but one of the other rooms is run with TRVs disabled, ie permanently on (they remain fitted to the rads for aesthetic reasons!). I did a bit of balancing on the LSVs, but but rooms share heat much more than the manufacturers of fancy controls like us to believe Most of the year I run on pure weather compensation with no room influence at all, In April, when solar gain became significant in comparison to the base heat demand, I switched to what Vaillant call 'expanded' mode, which basically turns the ASHP off altogether when the room in which the room sensor is located gets more than about half a degree above the set temperature. I will switch back once the heating season restarts. So essentially its left on pure Weather compensation for most of t he year, with TRVs inactive. The result is much more stable than I ever achieved with the fancy controls and a gas boiler, and without the fiddling previously needed. The key is that the fabric heats up properly and serves to moderate any temperature variations. The house is 1930s 200sqm, originally with solid walls. IWI and a couple of modern extensions mean that the walls are now ~80% insulated, but 20% remain solid. Double glazing and 300mm loft insulation completes the picture. Floors (a mix of suspended and concrete) are uninsulated. 7kW loss, Vaillant '7kW' heat pump. Why did it work well? No system separation (ie no buffer tank, plate heat exchanger or low loss header between ASHP and emitters) ASHP capacity correctly matched to house loss Design for a reasonably but not excessively low flow temperature - radiators have to fit!, simple controls ie the controls native to the ASHP only Operate 24*7 choose a sensible tarrif (in my case essentially economy 7 ie 7 hrs cheap leccy in return for about a 10% uplift during the day. ie Nothing 'clever', just keep it simple but sensible. I could probably do a bit better still if I operated a 'set forward' at night to take better advantage of the cheap night-time leccy, but I cant currently be bothered! Edited June 7 by JamesPa 1
marshian Posted June 17 Posted June 17 On 06/06/2025 at 20:49, lookseehear said: I really need to make my mind up and I'm still struggling. As a refresher: We have an 18-25kW external condensing oil boiler and 1200L oil tank in the garden The house is 5 beds and detached and we're putting the living space on the first floor and bedrooms on the ground floor We're replumbing and rewiring the whole house Two extensions will be insulated to modern standards, as will the new roof The older parts of the house are harder to insulate substantially, but we will be putting in a thin layer of carefully installed and well detailed internal wall insulation and significantly improving the airtightness We're replacing all windows and doors with triple glazing/modern equivalents I'd estimate we would be averaging 50-70W/m2 peak heating requirement when all done but it's hard to tell because at this point we're chipping away at the project bit by bit Option 1: Stick with oil, new radiators everywhere (oversized) Pros Cheapest at the outset Easy to control room by room temp with TRVs Reduced oil usage compared to high flow temps and small rads Cons We commit to keeping burning oil No weather compensation The boiler is (I believe) oversized for what we need, so would need to make changes (buffer tank/TS?) to the installation or swap out for a smaller boiler We have to fit large radiators in each room My question as far as you wanting to run at "low flow temps" are concerned is just how low will your oil boiler go down in terms of flow temp? I think this is an important point as you need to size the rads for the lowest realistic flow temp that also keeps the boiler happy I heat with gas and previously had a glow worm condensing boiler that was oversized for the house 24kW - I upgraded all my rads to match the heat loss at a flow temp of 40 deg C but realistically 48 deg C flow temp was as low as the boiler would run without excessive cycling and I ended up running it slightly higher and controlling room temps with TRV intervention (which isn't great for cycling). Moving to a new gas boiler (I wasn't brave enough to go ASHP at the time) it happily runs in from mid 20's to low 30's (Weather compensated flow temp) - only had to change one more rad to balance the house warm up (one room took way longer to get to temp) most TRV's are set to elevated target temps that can't be achieved in order to minimise circuit shrink due to TRV's shutting down. All Rads are fed with the flow they need for the WC flow temp although I am planning to improve this area as the Drayton EB4 TRV Bodies only have 6 flow settings and I could do with a slightly finer adjustment on a few rooms so I'll be changing a few for Danfoss RAS-B2 TRV bodies as they have a finer adjustment for flow rates House is heated 24/7 no set backs - it's way more comfortable at a lower room temps because there are no temp swings, In the heating season the boiler cycles once per hour max - the length of the cycle determined by the heat loss If the old room comfort target was 22 it's now 19 to 20 max House is 4 bed detached - extended with a 2 storey ext - is a stupid T shape with several rooms having 3 external walls - some insulation improvements - floor area total across two floors is 113 m2 and peak heating at -2.5 is 39W/m2
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